builder_spirv.rs

// HACK(eddyb) avoids rewriting all of the imports (see `lib.rs` and `build.rs`).
use crate::maybe_pqp_cg_ssa as rustc_codegen_ssa;

use crate::builder;
use crate::codegen_cx::CodegenCx;
use crate::spirv_type::SpirvType;
use crate::symbols::Symbols;
use crate::target::SpirvTarget;
use crate::target_feature::TargetFeature;
use rspirv::dr::{Builder, Instruction, Module, Operand};
use rspirv::spirv::{
    AddressingModel, Capability, MemoryModel, Op, SourceLanguage, StorageClass, Word,
};
use rspirv::{binary::Assemble, binary::Disassemble};
use rustc_abi::Size;
use rustc_arena::DroplessArena;
use rustc_codegen_ssa::traits::ConstCodegenMethods as _;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_middle::bug;
use rustc_middle::mir::interpret::ConstAllocation;
use rustc_middle::ty::TyCtxt;
use rustc_span::source_map::SourceMap;
use rustc_span::symbol::Symbol;
use rustc_span::{DUMMY_SP, FileName, FileNameDisplayPreference, SourceFile, Span};
use std::assert_matches::assert_matches;
use std::cell::{RefCell, RefMut};
use std::hash::{Hash, Hasher};
use std::iter;
use std::ops::Range;
use std::str;
use std::sync::Arc;
use std::{fs::File, io::Write, path::Path};

#[derive(Copy, Clone, Debug, Ord, PartialOrd, Eq, PartialEq, Hash)]
pub enum SpirvValueKind {
    Def(Word),

    /// The ID of a global instruction matching a `SpirvConst`, but which cannot
    /// pass validation. Used to error (or attach zombie spans), at the usesites
    /// of such constants, instead of where they're generated (and cached).
    IllegalConst(Word),

    /// This can only happen in one specific case - which is as a result of
    /// `codegen_buffer_store_intrinsic`, that function is supposed to return
    /// `OpTypeVoid`, however because it gets inline by the compiler it can't.
    /// Instead we return this, and trigger an error if we ever end up using the
    /// result of this function call (which we can't).
    IllegalTypeUsed(Word),

    // FIXME(eddyb) this shouldn't be needed, but `rustc_codegen_ssa` still relies
    // on converting `Function`s to `Value`s even for direct calls, the `Builder`
    // should just have direct and indirect `call` variants (or a `Callee` enum).
    FnAddr {
        function: Word,
    },

    /// Deferred pointer cast, for the `Logical` addressing model (which doesn't
    /// really support raw pointers in the way Rust expects to be able to use).
    ///
    /// The cast's target pointer type is the `ty` of the `SpirvValue` that has
    /// `LogicalPtrCast` as its `kind`, as it would be redundant to have it here.
    LogicalPtrCast {
        /// Pointer value being cast.
        original_ptr: Word,

        /// Pointer type of `original_ptr`.
        original_ptr_ty: Word,

        /// Result ID for the `OpBitcast` instruction representing the cast,
        /// to attach zombies to.
        //
        // HACK(eddyb) having an `OpBitcast` only works by being DCE'd away,
        // or by being replaced with a noop in `qptr::lower`.
        bitcast_result_id: Word,
    },
}

#[derive(Copy, Clone, Debug, Ord, PartialOrd, Eq, PartialEq, Hash)]
pub struct SpirvValue {
    pub kind: SpirvValueKind,
    pub ty: Word,
}

impl SpirvValue {
    pub fn strip_ptrcasts(self) -> Self {
        match self.kind {
            SpirvValueKind::LogicalPtrCast {
                original_ptr,
                original_ptr_ty,
                bitcast_result_id: _,
            } => original_ptr.with_type(original_ptr_ty),

            _ => self,
        }
    }

    pub fn const_fold_load(self, cx: &CodegenCx<'_>) -> Option<Self> {
        match self.kind {
            SpirvValueKind::Def(id) | SpirvValueKind::IllegalConst(id) => {
                let &entry = cx.builder.id_to_const.borrow().get(&id)?;
                match entry.val {
                    SpirvConst::PtrTo { pointee } => {
                        let ty = match cx.lookup_type(self.ty) {
                            SpirvType::Pointer { pointee } => pointee,
                            ty => bug!("load called on value that wasn't a pointer: {:?}", ty),
                        };
                        // FIXME(eddyb) deduplicate this `if`-`else` and its other copies.
                        let kind = if entry.legal.is_ok() {
                            SpirvValueKind::Def(pointee)
                        } else {
                            SpirvValueKind::IllegalConst(pointee)
                        };
                        Some(SpirvValue { kind, ty })
                    }
                    _ => None,
                }
            }

            _ => None,
        }
    }

    // Important: we *cannot* use bx.emit() here, because this is called in
    // contexts where the emitter is already locked. Doing so may cause subtle
    // rare bugs.
    pub fn def(self, bx: &builder::Builder<'_, '_>) -> Word {
        self.def_with_span(bx, bx.span())
    }

    // def and def_cx are separated, because Builder has a span associated with
    // what it's currently emitting.
    pub fn def_cx(self, cx: &CodegenCx<'_>) -> Word {
        self.def_with_span(cx, DUMMY_SP)
    }

    pub fn def_with_span(self, cx: &CodegenCx<'_>, span: Span) -> Word {
        match self.kind {
            SpirvValueKind::Def(id) => id,

            SpirvValueKind::IllegalConst(id) => {
                let entry = &cx.builder.id_to_const.borrow()[&id];
                let msg = match entry.legal.unwrap_err() {
                    IllegalConst::Shallow(cause) => {
                        if let (
                            LeafIllegalConst::CompositeContainsPtrTo,
                            SpirvConst::Composite(_fields),
                        ) = (cause, &entry.val)
                        {
                            // FIXME(eddyb) materialize this at runtime, using
                            // `OpCompositeConstruct` (transitively, i.e. after
                            // putting every field through `SpirvValue::def`),
                            // if we have a `Builder` to do that in.
                            // FIXME(eddyb) this isn't possible right now, as
                            // the builder would be dynamically "locked" anyway
                            // (i.e. attempting to do `bx.emit()` would panic).
                        }

                        cause.message()
                    }

                    IllegalConst::Indirect(cause) => cause.message(),
                };

                cx.zombie_with_span(id, span, msg);

                id
            }

            SpirvValueKind::IllegalTypeUsed(id) => {
                cx.tcx
                    .dcx()
                    .struct_span_err(span, "Can't use type as a value")
                    .with_note(format!("Type: *{}", cx.debug_type(id)))
                    .emit();

                id
            }

            SpirvValueKind::FnAddr { .. } => {
                cx.builder
                    .const_to_id
                    .borrow()
                    .get(&WithType {
                        ty: self.ty,
                        val: SpirvConst::ZombieUndefForFnAddr,
                    })
                    .expect("FnAddr didn't go through proper undef registration")
                    .val
            }

            SpirvValueKind::LogicalPtrCast {
                original_ptr: _,
                original_ptr_ty,
                bitcast_result_id,
            } => {
                cx.zombie_with_span(
                    bitcast_result_id,
                    span,
                    &format!(
                        "cannot cast between pointer types\
                         \nfrom `{}`\
                         \n  to `{}`",
                        cx.debug_type(original_ptr_ty),
                        cx.debug_type(self.ty)
                    ),
                );

                bitcast_result_id
            }
        }
    }
}

pub trait SpirvValueExt {
    fn with_type(self, ty: Word) -> SpirvValue;
}

impl SpirvValueExt for Word {
    fn with_type(self, ty: Word) -> SpirvValue {
        SpirvValue {
            kind: SpirvValueKind::Def(self),
            ty,
        }
    }
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum SpirvConst<'a, 'tcx> {
    /// Constants of boolean, integer or floating-point type (up to 128-bit).
    Scalar(u128),

    Null,
    Undef,

    /// Like `Undef`, but cached separately to avoid `FnAddr` zombies accidentally
    /// applying to non-zombie `Undef`s of the same types.
    // FIXME(eddyb) include the function ID so that multiple `fn` pointers to
    // different functions, but of the same type, don't overlap their zombies.
    ZombieUndefForFnAddr,

    Composite(&'a [Word]),

    /// Pointer to constant data, i.e. `&pointee`, represented as an `OpVariable`
    /// in the `Private` storage class, and with `pointee` as its initializer.
    PtrTo {
        pointee: Word,
    },

    /// Symbolic result for the `const_data_from_alloc` method, to allow deferring
    /// the actual value generation until after a pointer to this value is cast
    /// to its final type (e.g. that will be loaded as).
    //
    // FIXME(eddyb) replace this with `qptr` handling of constant data.
    ConstDataFromAlloc(ConstAllocation<'tcx>),
}

impl<'tcx> SpirvConst<'_, 'tcx> {
    /// Replace `&[T]` fields with `&'tcx [T]` ones produced by calling
    /// `tcx.arena.dropless.alloc_slice(...)` - this is done late for two reasons:
    /// 1. it avoids allocating in the arena when the cache would be hit anyway,
    ///    which would create "garbage" (as in, unreachable allocations)
    ///    (ideally these would also be interned, but that's even more refactors)
    /// 2. an empty slice is disallowed (as it's usually handled as a special
    ///    case elsewhere, e.g. `rustc`'s `ty::List` - sadly we can't use that)
    fn tcx_arena_alloc_slices(self, cx: &CodegenCx<'tcx>) -> SpirvConst<'tcx, 'tcx> {
        fn arena_alloc_slice<'tcx, T: Copy>(cx: &CodegenCx<'tcx>, xs: &[T]) -> &'tcx [T] {
            if xs.is_empty() {
                &[]
            } else {
                cx.tcx.arena.dropless.alloc_slice(xs)
            }
        }

        match self {
            // FIXME(eddyb) these are all noop cases, could they be automated?
            SpirvConst::Scalar(v) => SpirvConst::Scalar(v),
            SpirvConst::Null => SpirvConst::Null,
            SpirvConst::Undef => SpirvConst::Undef,
            SpirvConst::ZombieUndefForFnAddr => SpirvConst::ZombieUndefForFnAddr,
            SpirvConst::PtrTo { pointee } => SpirvConst::PtrTo { pointee },

            SpirvConst::Composite(fields) => SpirvConst::Composite(arena_alloc_slice(cx, fields)),

            SpirvConst::ConstDataFromAlloc(alloc) => SpirvConst::ConstDataFromAlloc(alloc),
        }
    }
}

#[derive(Debug, Clone, Ord, PartialOrd, Eq, PartialEq, Hash)]
struct WithType<V> {
    ty: Word,
    val: V,
}

/// Primary causes for a `SpirvConst` to be deemed illegal.
#[derive(Copy, Clone, Debug)]
enum LeafIllegalConst {
    /// `SpirvConst::Composite` containing a `SpirvConst::PtrTo` as a field.
    /// This is illegal because `OpConstantComposite` must have other constants
    /// as its operands, and `OpVariable`s are never considered constant.
    // FIXME(eddyb) figure out if this is an accidental omission in SPIR-V.
    CompositeContainsPtrTo,

    /// `ConstDataFromAlloc` constant, which cannot currently be materialized
    /// to SPIR-V (and requires to be wrapped in `PtrTo` and bitcast, first).
    //
    // FIXME(eddyb) replace this with `qptr` handling of constant data.
    UntypedConstDataFromAlloc,
}

impl LeafIllegalConst {
    fn message(&self) -> &'static str {
        match *self {
            Self::CompositeContainsPtrTo => {
                "constant arrays/structs cannot contain pointers to other constants"
            }
            Self::UntypedConstDataFromAlloc => {
                "`const_data_from_alloc` result wasn't passed through `static_addr_of`, \
                 then `const_bitcast` (which would've given it a type)"
            }
        }
    }
}

#[derive(Copy, Clone, Debug)]
enum IllegalConst {
    /// This `SpirvConst` is (or contains) a "leaf" illegal constant. As there
    /// is no indirection, some of these could still be materialized at runtime,
    /// using e.g. `OpCompositeConstruct` instead of `OpConstantComposite`.
    Shallow(LeafIllegalConst),

    /// This `SpirvConst` is (or contains/points to) a `PtrTo` which points to
    /// a "leaf" illegal constant. As the data would have to live for `'static`,
    /// there is no way to materialize it as a pointer in SPIR-V. However, it
    /// could still be legalized during codegen by e.g. folding loads from it.
    Indirect(LeafIllegalConst),
}

#[derive(Copy, Clone, Debug)]
struct WithConstLegality<V> {
    val: V,
    legal: Result<(), IllegalConst>,
}

/// `HashMap` key type (for `debug_file_cache` in `BuilderSpirv`), which is
/// equivalent to a whole `rustc` `SourceFile`, but has O(1) `Eq` and `Hash`
/// implementations (i.e. not involving the path or the contents of the file).
///
/// This is possible because we can compare `Arc<SourceFile>`s by equality, as
/// `rustc`'s `SourceMap` already ensures that only one `SourceFile` will be
/// allocated for some given file. For hashing, we could hash the address, or
///
struct DebugFileKey(Arc<SourceFile>);

impl PartialEq for DebugFileKey {
    fn eq(&self, other: &Self) -> bool {
        let (Self(self_sf), Self(other_sf)) = (self, other);
        Arc::ptr_eq(self_sf, other_sf)
    }
}
impl Eq for DebugFileKey {}

impl Hash for DebugFileKey {
    fn hash<H: Hasher>(&self, state: &mut H) {
        let Self(sf) = self;
        sf.stable_id.hash(state);
        sf.src_hash.hash(state);
    }
}

#[derive(Copy, Clone)]
pub struct DebugFileSpirv<'tcx> {
    pub file_name: &'tcx str,

    /// The SPIR-V ID for the result of the `OpString` instruction containing
    /// `file_name` - this is what e.g. `OpLine` uses to identify the file.
    ///
    /// All other details about the file are also attached to this ID, using
    /// other instructions that don't produce their own IDs (e.g. `OpSource`).
    pub file_name_op_string_id: Word,
}

// HACK(eddyb) unlike a raw SPIR-V ID (or `SpirvValue`), this allows random-access.
#[derive(Copy, Clone, Debug)]
pub struct SpirvFunctionCursor {
    pub ty: Word,
    pub id: Word,
    pub index_in_builder: usize,
}

// HACK(eddyb) unlike a raw SPIR-V ID, this allows random-access.
#[derive(Copy, Clone, Debug)]
pub struct SpirvBlockCursor {
    pub parent_fn: SpirvFunctionCursor,
    pub id: Word,
    pub index_in_builder: usize,
}

/// Cursor system:
///
/// The LLVM module builder model (and therefore `codegen_ssa`) assumes that there is a central
/// module object, then, builder objects are created pointing at that central module object (e.g.
/// for adding instructions to a basic block).  Several of these builder objects can be live at the
/// same time, mutating the central module object all at once.  Unfortunately, rspirv doesn't work
/// like that. Instead, there is a single builder object, which owns a module and a "cursor". This
/// cursor indicates to the builder where to append instructions when an instruction is added -
/// e.g. if `add()` is called, then `OpAdd` is appended to the basic block pointed to by the cursor.
///
/// So! We emulate the LLVM system by treating the rspirv Builder as the "central module object",
/// then, when a "builder object" is created, we store a reference to a `RefCell<rspirv builder>`,
/// *as well as* a copy of the cursor for that particular builder. Whenever the `RefCell` is
/// borrowed, then we stomp over the rspirv cursor with our copy, causing the duration of that
/// `RefCell` borrow to use that cursor.
///
/// So, if you're writing code inside `crate::builder::Builder`, then `self.emit()` will use
/// `self.cursor` (the current basic block) as that "stomp-over" cursor and return a mutable
/// reference to the rspirv builder. If you're writing code elsewhere (`codegen_cx::CodegenCx`),
/// then `self.emit_global()` will use the generic "global cursor" and return a mutable reference
/// to the rspirv builder with no basic block nor function selected, i.e. any instructions emitted
/// will be in the global section.
//
// FIXME(eddyb) try updating documentation like the above.
// FIXME(eddyb) figure out how to replace `BuilderCursor` with something like
// `Option<SpirvBlockCursor>`, but that can't handle "in function outside BB".
#[derive(Debug, Default, Copy, Clone)]
#[must_use = "BuilderCursor should usually be assigned to the Builder.cursor field"]
struct BuilderCursor {
    fn_id_and_idx: Option<(Word, usize)>,
    block_id_and_idx: Option<(Word, usize)>,
}

pub struct BuilderSpirv<'tcx> {
    source_map: &'tcx SourceMap,
    dropless_arena: &'tcx DroplessArena,

    builder: RefCell<Builder>,

    // Bidirectional maps between `SpirvConst` and the ID of the defined global
    // (e.g. `OpConstant...`) instruction.
    // NOTE(eddyb) both maps have `WithConstLegality` around their keys, which
    // allows getting that legality information without additional lookups.
    const_to_id: RefCell<FxHashMap<WithType<SpirvConst<'tcx, 'tcx>>, WithConstLegality<Word>>>,
    id_to_const: RefCell<FxHashMap<Word, WithConstLegality<SpirvConst<'tcx, 'tcx>>>>,

    debug_file_cache: RefCell<FxHashMap<DebugFileKey, DebugFileSpirv<'tcx>>>,

    enabled_capabilities: FxHashSet<Capability>,
}

impl<'tcx> BuilderSpirv<'tcx> {
    pub fn new(
        tcx: TyCtxt<'tcx>,
        sym: &Symbols,
        target: &SpirvTarget,
        features: &[TargetFeature],
    ) -> Self {
        let version = target.spirv_version();
        let memory_model = target.memory_model();

        let mut builder = Builder::new();
        builder.set_version(version.0, version.1);
        builder.module_mut().header.as_mut().unwrap().generator = 0x001B_0000;

        let mut enabled_capabilities = FxHashSet::default();

        fn add_cap(
            builder: &mut Builder,
            enabled_capabilities: &mut FxHashSet<Capability>,
            cap: Capability,
        ) {
            // This should be the only callsite of Builder::capability (aside from tests), to make
            // sure the hashset stays in sync.
            builder.capability(cap);
            enabled_capabilities.insert(cap);
        }
        fn add_ext(builder: &mut Builder, ext: Symbol) {
            // This should be the only callsite of Builder::extension (aside from tests), to make
            // sure the hashset stays in sync.
            builder.extension(ext.as_str());
        }

        for feature in features {
            match *feature {
                TargetFeature::Capability(cap) => {
                    add_cap(&mut builder, &mut enabled_capabilities, cap);
                }
                TargetFeature::Extension(ext) => {
                    add_ext(&mut builder, ext);
                }
            }
        }

        add_cap(&mut builder, &mut enabled_capabilities, Capability::Shader);
        if memory_model == MemoryModel::Vulkan {
            if version < (1, 5) {
                add_ext(&mut builder, sym.spv_khr_vulkan_memory_model);
            }
            add_cap(
                &mut builder,
                &mut enabled_capabilities,
                Capability::VulkanMemoryModel,
            );
        }

        // The linker will always be ran on this module
        add_cap(&mut builder, &mut enabled_capabilities, Capability::Linkage);

        builder.memory_model(AddressingModel::Logical, memory_model);

        Self {
            source_map: tcx.sess.source_map(),
            dropless_arena: &tcx.arena.dropless,
            builder: RefCell::new(builder),
            const_to_id: Default::default(),
            id_to_const: Default::default(),
            debug_file_cache: Default::default(),
            enabled_capabilities,
        }
    }

    pub fn finalize(self) -> Module {
        self.builder.into_inner().module()
    }

    pub fn dump_module_str(&self) -> String {
        self.builder.borrow().module_ref().disassemble()
    }

    /// Helper function useful to place right before a crash, to debug the module state.
    pub fn dump_module(&self, path: impl AsRef<Path>) {
        let module = self.builder.borrow().module_ref().assemble();
        File::create(path)
            .unwrap()
            .write_all(spirv_tools::binary::from_binary(&module))
            .unwrap();
    }

    pub fn has_capability(&self, capability: Capability) -> bool {
        self.enabled_capabilities.contains(&capability)
    }

    /// See comment on `BuilderCursor`
    fn builder(&self, cursor: BuilderCursor) -> RefMut<'_, Builder> {
        let mut builder = self.builder.borrow_mut();

        let [maybe_fn_idx, maybe_block_idx] = [cursor.fn_id_and_idx, cursor.block_id_and_idx]
            .map(|id_and_idx| id_and_idx.map(|(_, idx)| idx));

        let fn_changed = builder.selected_function() != maybe_fn_idx;
        if fn_changed {
            builder.select_function(maybe_fn_idx).unwrap();
        }

        // Only check the function/block IDs if either of their indices changed.
        if let Some((fn_id, fn_idx)) = cursor.fn_id_and_idx
            && (fn_changed || builder.selected_block() != maybe_block_idx)
        {
            builder.select_block(maybe_block_idx).unwrap();

            let function = &builder.module_ref().functions[fn_idx];
            if fn_changed {
                assert_eq!(function.def_id(), Some(fn_id));
            }
            if let Some((block_id, block_idx)) = cursor.block_id_and_idx {
                assert_eq!(function.blocks[block_idx].label_id(), Some(block_id));
            }
        }

        builder
    }

    /// See comment on `BuilderCursor`
    pub fn global_builder(&self) -> RefMut<'_, Builder> {
        self.builder(BuilderCursor::default())
    }

    /// See comment on `BuilderCursor`
    pub fn builder_for_fn(&self, func: SpirvFunctionCursor) -> RefMut<'_, Builder> {
        self.builder(BuilderCursor {
            fn_id_and_idx: Some((func.id, func.index_in_builder)),
            block_id_and_idx: None,
        })
    }

    /// See comment on `BuilderCursor`
    pub fn builder_for_block(&self, block: SpirvBlockCursor) -> RefMut<'_, Builder> {
        self.builder(BuilderCursor {
            fn_id_and_idx: Some((block.parent_fn.id, block.parent_fn.index_in_builder)),
            block_id_and_idx: Some((block.id, block.index_in_builder)),
        })
    }

    pub(crate) fn def_constant_cx(
        &self,
        ty: Word,
        val: SpirvConst<'_, 'tcx>,
        cx: &CodegenCx<'tcx>,
    ) -> SpirvValue {
        let scalar_ty = match val {
            SpirvConst::Scalar(_) => Some(cx.lookup_type(ty)),
            _ => None,
        };

        // HACK(eddyb) this is done so late (just before interning `val`) to
        // minimize any potential misuse from direct `def_constant` calls.
        let val = match (val, scalar_ty) {
            (SpirvConst::Scalar(val), Some(SpirvType::Integer(bits, signed))) => {
                let size = Size::from_bits(bits);
                SpirvConst::Scalar(if signed {
                    size.sign_extend(val) as u128
                } else {
                    size.truncate(val)
                })
            }
            _ => val,
        };

        let val_with_type = WithType { ty, val };
        if let Some(entry) = self.const_to_id.borrow().get(&val_with_type) {
            // FIXME(eddyb) deduplicate this `if`-`else` and its other copies.
            let kind = if entry.legal.is_ok() {
                SpirvValueKind::Def(entry.val)
            } else {
                SpirvValueKind::IllegalConst(entry.val)
            };
            return SpirvValue { kind, ty };
        }
        let val = val_with_type.val;

        // FIXME(eddyb) make this an extension method on `rspirv::dr::Builder`?
        let const_op = |builder: &mut Builder, op, lhs, maybe_rhs: Option<_>| {
            // HACK(eddyb) remove after `OpSpecConstantOp` support gets added to SPIR-T.
            let spirt_has_const_op = false;

            if !spirt_has_const_op {
                let zombie = builder.undef(ty, None);
                cx.zombie_with_span(
                    zombie,
                    DUMMY_SP,
                    &format!("unsupported constant of type `{}`", cx.debug_type(ty)),
                );
                return zombie;
            }

            let id = builder.id();
            builder
                .module_mut()
                .types_global_values
                .push(Instruction::new(
                    Op::SpecConstantOp,
                    Some(ty),
                    Some(id),
                    [
                        Operand::LiteralSpecConstantOpInteger(op),
                        Operand::IdRef(lhs),
                    ]
                    .into_iter()
                    .chain(maybe_rhs.map(Operand::IdRef))
                    .collect(),
                ));
            id
        };

        let mut builder = self.global_builder();
        let id = match val {
            SpirvConst::Scalar(v) => match scalar_ty.unwrap() {
                SpirvType::Integer(..=32, _) | SpirvType::Float(..=32) => {
                    builder.constant_bit32(ty, v as u32)
                }
                SpirvType::Integer(64, _) | SpirvType::Float(64) => {
                    builder.constant_bit64(ty, v as u64)
                }
                SpirvType::Integer(128, false) => {
                    // HACK(eddyb) avoid borrow conflicts.
                    drop(builder);

                    let const_64_u32_id = cx.const_u32(64).def_cx(cx);
                    let [lo_id, hi_id] =
                        [v as u64, (v >> 64) as u64].map(|half| cx.const_u64(half).def_cx(cx));

                    builder = self.global_builder();
                    let mut const_op =
                        |op, lhs, maybe_rhs| const_op(&mut builder, op, lhs, maybe_rhs);
                    let [lo_u128_id, hi_shifted_u128_id] =
                        [(lo_id, None), (hi_id, Some(const_64_u32_id))].map(
                            |(half_u64_id, shift)| {
                                let mut half_u128_id = const_op(Op::UConvert, half_u64_id, None);
                                if let Some(shift_amount_id) = shift {
                                    half_u128_id = const_op(
                                        Op::ShiftLeftLogical,
                                        half_u128_id,
                                        Some(shift_amount_id),
                                    );
                                }
                                half_u128_id
                            },
                        );
                    const_op(Op::BitwiseOr, lo_u128_id, Some(hi_shifted_u128_id))
                }
                SpirvType::Integer(128, true) | SpirvType::Float(128) => {
                    // HACK(eddyb) avoid borrow conflicts.
                    drop(builder);

                    let v_u128_id = cx.const_u128(v).def_cx(cx);

                    builder = self.global_builder();
                    const_op(&mut builder, Op::Bitcast, v_u128_id, None)
                }
                SpirvType::Bool => match v {
                    0 => builder.constant_false(ty),
                    1 => builder.constant_true(ty),
                    _ => cx
                        .tcx
                        .dcx()
                        .fatal(format!("invalid constant value for bool: {v}")),
                },
                other => cx.tcx.dcx().fatal(format!(
                    "SpirvConst::Scalar does not support type {}",
                    other.debug(ty, cx)
                )),
            },

            SpirvConst::Null => builder.constant_null(ty),
            SpirvConst::Undef
            | SpirvConst::ZombieUndefForFnAddr
            | SpirvConst::ConstDataFromAlloc(_) => builder.undef(ty, None),

            SpirvConst::Composite(v) => builder.constant_composite(ty, v.iter().copied()),

            SpirvConst::PtrTo { pointee } => {
                builder.variable(ty, None, StorageClass::Private, Some(pointee))
            }
        };
        #[allow(clippy::match_same_arms)]
        let legal = match val {
            SpirvConst::Scalar(_) => Ok(()),

            SpirvConst::Null => {
                // FIXME(eddyb) check that the type supports `OpConstantNull`.
                Ok(())
            }
            SpirvConst::Undef => {
                // FIXME(eddyb) check that the type supports `OpUndef`.
                Ok(())
            }

            SpirvConst::ZombieUndefForFnAddr => {
                // This can be considered legal as it's already marked as zombie.
                // FIXME(eddyb) is it possible for the original zombie to lack a
                // span, and should we go through `IllegalConst` in order to be
                // able to attach a proper usesite span?
                Ok(())
            }

            SpirvConst::Composite(v) => v
                .iter()
                .map(|field| {
                    let field_entry = &self.id_to_const.borrow()[field];
                    field_entry.legal.and(
                        // `field` is itself some legal `SpirvConst`, but can we have
                        // it as part of an `OpConstantComposite`?
                        match field_entry.val {
                            SpirvConst::PtrTo { .. } => Err(IllegalConst::Shallow(
                                LeafIllegalConst::CompositeContainsPtrTo,
                            )),
                            _ => Ok(()),
                        },
                    )
                })
                .reduce(|a, b| {
                    match (a, b) {
                        (Ok(()), Ok(())) => Ok(()),
                        (Err(illegal), Ok(())) | (Ok(()), Err(illegal)) => Err(illegal),

                        // Combining two causes of an illegal `SpirvConst` has to
                        // take into account which is "worse", i.e. which imposes
                        // more restrictions on how the resulting value can be used.
                        // `Indirect` is worse than `Shallow` because it cannot be
                        // materialized at runtime in the same way `Shallow` can be.
                        (Err(illegal @ IllegalConst::Indirect(_)), Err(_))
                        | (Err(_), Err(illegal @ IllegalConst::Indirect(_)))
                        | (
                            Err(illegal @ IllegalConst::Shallow(_)),
                            Err(IllegalConst::Shallow(_)),
                        ) => Err(illegal),
                    }
                })
                .unwrap_or(Ok(())),

            SpirvConst::PtrTo { pointee } => match self.id_to_const.borrow()[&pointee].legal {
                Ok(()) => Ok(()),

                // `Shallow` becomes `Indirect` when placed behind a pointer.
                Err(IllegalConst::Shallow(cause) | IllegalConst::Indirect(cause)) => {
                    Err(IllegalConst::Indirect(cause))
                }
            },

            SpirvConst::ConstDataFromAlloc(_) => Err(IllegalConst::Shallow(
                LeafIllegalConst::UntypedConstDataFromAlloc,
            )),
        };
        let val = val.tcx_arena_alloc_slices(cx);
        assert_matches!(
            self.const_to_id
                .borrow_mut()
                .insert(WithType { ty, val }, WithConstLegality { val: id, legal }),
            None
        );
        assert_matches!(
            self.id_to_const
                .borrow_mut()
                .insert(id, WithConstLegality { val, legal }),
            None
        );
        // FIXME(eddyb) deduplicate this `if`-`else` and its other copies.
        let kind = if legal.is_ok() {
            SpirvValueKind::Def(id)
        } else {
            SpirvValueKind::IllegalConst(id)
        };
        SpirvValue { kind, ty }
    }

    pub fn lookup_const_by_id(&self, id: Word) -> Option<SpirvConst<'tcx, 'tcx>> {
        Some(self.id_to_const.borrow().get(&id)?.val)
    }

    pub fn lookup_const(&self, def: SpirvValue) -> Option<SpirvConst<'tcx, 'tcx>> {
        match def.kind {
            SpirvValueKind::Def(id) | SpirvValueKind::IllegalConst(id) => {
                self.lookup_const_by_id(id)
            }
            _ => None,
        }
    }

    pub fn lookup_const_scalar(&self, def: SpirvValue) -> Option<u128> {
        match self.lookup_const(def)? {
            SpirvConst::Scalar(v) => Some(v),
            _ => None,
        }
    }

    pub fn file_line_col_range_for_debuginfo(
        &self,
        span: Span,
    ) -> (DebugFileSpirv<'tcx>, Range<(u32, u32)>) {
        // HACK(eddyb) this is similar to what `#[track_caller]` does, and it
        // allows us to point to the use site of a macro, instead of inside the
        // macro (but ideally we would record the entire macro backtrace).
        let span = span.ctxt().outer_expn().expansion_cause().unwrap_or(span);

        let (lo, hi) = (span.lo(), span.hi());

        let lo_loc = self.source_map.lookup_char_pos(lo);
        let lo_line_col = (lo_loc.line as u32, lo_loc.col_display as u32);

        // Only use `hi` if the span is actually a range within a file.
        let hi_line_col = if lo <= hi {
            let hi_loc = self.source_map.lookup_char_pos(hi);
            if lo_loc.file.start_pos == hi_loc.file.start_pos {
                (hi_loc.line as u32, hi_loc.col_display as u32)
            } else {
                lo_line_col
            }
        } else {
            lo_line_col
        };

        (self.def_debug_file(lo_loc.file), lo_line_col..hi_line_col)
    }

    fn def_debug_file(&self, sf: Arc<SourceFile>) -> DebugFileSpirv<'tcx> {
        *self
            .debug_file_cache
            .borrow_mut()
            .entry(DebugFileKey(sf))
            .or_insert_with_key(|DebugFileKey(sf)| {
                let mut builder = self.global_builder();

                // FIXME(eddyb) it would be nicer if we could just rely on
                // `RealFileName::to_string_lossy` returning `Cow<'_, str>`,
                // but sadly that `'_` is the lifetime of the temporary `Arc`,
                // not `'tcx`, so we have to arena-allocate to get `&'tcx str`.
                let file_name = match &sf.name {
                    FileName::Real(name) => {
                        name.to_string_lossy(FileNameDisplayPreference::Remapped)
                    }
                    _ => sf.name.prefer_remapped_unconditionally().to_string().into(),
                };
                let file_name = {
                    // FIXME(eddyb) it should be possible to arena-allocate a
                    // `&str` directly, but it would require upstream changes,
                    // and strings are handled by string interning in `rustc`.
                    fn arena_alloc_slice<'tcx, T: Copy>(
                        dropless_arena: &'tcx DroplessArena,
                        xs: &[T],
                    ) -> &'tcx [T] {
                        if xs.is_empty() {
                            &[]
                        } else {
                            dropless_arena.alloc_slice(xs)
                        }
                    }
                    str::from_utf8(arena_alloc_slice(self.dropless_arena, file_name.as_bytes()))
                        .unwrap()
                };
                let file_name_op_string_id = builder.string(file_name.to_owned());

                let file_contents = self
                    .source_map
                    .span_to_snippet(Span::with_root_ctxt(sf.start_pos, sf.end_position()))
                    .ok();

                // HACK(eddyb) this logic is duplicated from `spirt::spv::lift`.
                let op_source_and_continued_chunks = file_contents.as_ref().map(|contents| {
                    // The maximum word count is `2**16 - 1`, the first word is
                    // taken up by the opcode & word count, and one extra byte is
                    // taken up by the nil byte at the end of the LiteralString.
                    const MAX_OP_SOURCE_CONT_CONTENTS_LEN: usize = (0xffff - 1) * 4 - 1;

                    // `OpSource` has 3 more operands than `OpSourceContinued`,
                    // and each of them take up exactly one word.
                    const MAX_OP_SOURCE_CONTENTS_LEN: usize =
                        MAX_OP_SOURCE_CONT_CONTENTS_LEN - 3 * 4;

                    let (op_source_str, mut all_op_source_continued_str) =
                        contents.split_at(contents.len().min(MAX_OP_SOURCE_CONTENTS_LEN));

                    // FIXME(eddyb) `spirt::spv::lift` should use this.
                    let all_op_source_continued_str_chunks = iter::from_fn(move || {
                        let contents_rest = &mut all_op_source_continued_str;
                        if contents_rest.is_empty() {
                            return None;
                        }

                        // FIXME(eddyb) test with UTF-8! this `split_at` should
                        // actually take *less* than the full possible size, to
                        // avoid cutting a UTF-8 sequence.
                        let (cont_chunk, rest) = contents_rest
                            .split_at(contents_rest.len().min(MAX_OP_SOURCE_CONT_CONTENTS_LEN));
                        *contents_rest = rest;
                        Some(cont_chunk)
                    });
                    (op_source_str, all_op_source_continued_str_chunks)
                });

                if let Some((op_source_str, all_op_source_continued_str_chunks)) =
                    op_source_and_continued_chunks
                {
                    builder.source(
                        SourceLanguage::Unknown,
                        0,
                        Some(file_name_op_string_id),
                        Some(op_source_str),
                    );
                    for cont_chunk in all_op_source_continued_str_chunks {
                        builder.source_continued(cont_chunk);
                    }
                }

                DebugFileSpirv {
                    file_name,
                    file_name_op_string_id,
                }
            })
    }

    pub fn set_global_initializer(&self, global: Word, initializer: Word) {
        let mut builder = self.builder.borrow_mut();
        let module = builder.module_mut();
        let index = module
            .types_global_values
            .iter()
            .enumerate()
            .find_map(|(index, inst)| {
                if inst.result_id == Some(global) {
                    Some(index)
                } else {
                    None
                }
            })
            .expect("set_global_initializer global not found");
        // Remove and push it to the end, to keep spir-v definition order.
        let mut inst = module.types_global_values.remove(index);
        assert_eq!(inst.class.opcode, Op::Variable);
        assert_eq!(
            inst.operands.len(),
            1,
            "global already has initializer defined: {global}"
        );
        inst.operands.push(Operand::IdRef(initializer));
        module.types_global_values.push(inst);
    }
}