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Use computed constants in lowering rather than lowering instructions (#3905)

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First steps towards using constant values in lowering.

For now, we reuse the regular instruction lowering to lower constants.
This mostly works, because we don't actually need an `llvm::Function` or
a current basic block when lowering a constant most of the time.
However, a special case is needed for lowering aggregate value constants
because they would otherwise create a stack alloca to store the
constant. Separate constant lowering code will be added in a future
change to clean this up.

When lowering a constant initializing expression, the result is a value
of the destination type, rather than code to initialize the destination,
so a separate copy step is required when finishing initialization from a
constant for a type that uses in-place initialization. Handling this
required extending `ReturnExpr` to track its destination location.

We currently often create non-constant `*_access` SemIR instructions
that are only used by constant `*_init` instructions. These cause
lowering to leave behind `getelementptr` instructions in the lowered IR
that are now unused. It should be possible to detect this case and avoid
producing these instructions, or to produce them lazily, but for now
we're just leaving them around for LLVM to clean up.
This commit is contained in:
Richard Smith
2024-04-23 16:23:24 +00:00
committed by GitHub
parent e0b8728263
commit ccf87f0a38
60 changed files with 357 additions and 162 deletions
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toolchain/lower/function_context.cpp
+36 -18
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@@ -46,17 +46,25 @@ auto FunctionContext::TryToReuseBlock(SemIR::InstBlockId block_id,
}
auto FunctionContext::LowerBlock(SemIR::InstBlockId block_id) -> void {
for (const auto& inst_id : sem_ir().inst_blocks().Get(block_id)) {
auto inst = sem_ir().insts().Get(inst_id);
CARBON_VLOG() << "Lowering " << inst_id << ": " << inst << "\n";
builder_.getInserter().SetCurrentInstId(inst_id);
switch (inst.kind()) {
for (auto inst_id : sem_ir().inst_blocks().Get(block_id)) {
// Skip over constants. `FileContext::GetGlobal` lowers them as needed.
if (sem_ir().constant_values().Get(inst_id).is_constant()) {
continue;
}
LowerInst(inst_id);
}
}
auto FunctionContext::LowerInst(SemIR::InstId inst_id) -> void {
auto inst = sem_ir().insts().Get(inst_id);
CARBON_VLOG() << "Lowering " << inst_id << ": " << inst << "\n";
builder_.getInserter().SetCurrentInstId(inst_id);
switch (inst.kind()) {
#define CARBON_SEM_IR_INST_KIND(Name) \
case SemIR::Name::Kind: \
Handle##Name(*this, inst_id, inst.As<SemIR::Name>()); \
break;
#include "toolchain/sem_ir/inst_kind.def"
}
}
builder_.getInserter().SetCurrentInstId(SemIR::InstId::Invalid);
}
@@ -90,7 +98,13 @@ auto FunctionContext::FinishInit(SemIR::TypeId type_id, SemIR::InstId dest_id,
SemIR::InstId source_id) -> void {
switch (SemIR::GetInitRepr(sem_ir(), type_id).kind) {
case SemIR::InitRepr::None:
break;
case SemIR::InitRepr::InPlace:
if (sem_ir().constant_values().Get(source_id).is_constant()) {
// When initializing from a constant, emission of the source doesn't
// initialize the destination. Copy the constant value instead.
CopyValue(type_id, source_id, dest_id);
}
break;
case SemIR::InitRepr::ByCopy:
CopyValue(type_id, source_id, dest_id);
@@ -108,24 +122,28 @@ auto FunctionContext::CopyValue(SemIR::TypeId type_id, SemIR::InstId source_id,
case SemIR::ValueRepr::Copy:
builder().CreateStore(GetValue(source_id), GetValue(dest_id));
break;
case SemIR::ValueRepr::Pointer: {
const auto& layout = llvm_module().getDataLayout();
auto* type = GetType(type_id);
// TODO: Compute known alignment of the source and destination, which may
// be greater than the alignment computed by LLVM.
auto align = layout.getABITypeAlign(type);
// TODO: Attach !tbaa.struct metadata indicating which portions of the
// type we actually need to copy and which are padding.
builder().CreateMemCpy(GetValue(dest_id), align, GetValue(source_id),
align, layout.getTypeAllocSize(type));
case SemIR::ValueRepr::Pointer:
CopyObject(type_id, source_id, dest_id);
break;
}
case SemIR::ValueRepr::Custom:
CARBON_FATAL() << "TODO: Add support for CopyValue with custom value rep";
}
}
auto FunctionContext::CopyObject(SemIR::TypeId type_id, SemIR::InstId source_id,
SemIR::InstId dest_id) -> void {
const auto& layout = llvm_module().getDataLayout();
auto* type = GetType(type_id);
// TODO: Compute known alignment of the source and destination, which may
// be greater than the alignment computed by LLVM.
auto align = layout.getABITypeAlign(type);
// TODO: Attach !tbaa.struct metadata indicating which portions of the
// type we actually need to copy and which are padding.
builder().CreateMemCpy(GetValue(dest_id), align, GetValue(source_id), align,
layout.getTypeAllocSize(type));
}
auto FunctionContext::Inserter::InsertHelper(
llvm::Instruction* inst, const llvm::Twine& name, llvm::BasicBlock* block,
llvm::BasicBlock::iterator insert_pt) const -> void {