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[CIR] Make use of !invariant.group metadata for const allocas
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This patch updates the LLVM lowering part of load and stores to const allocas
and makes use of the !invariant.group metadata in the result LLVM IR.

The HoistAlloca pass is also updated. The const flag on a hoisted alloca is
removed for now since their uses are not always invariants. Will update in later
patches to teach their invariants.
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@Lancern
Lancern committed Dec 4, 2024
1 parent 7fb608d commit 1a7b1ce
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Showing 4 changed files with 102 additions and 28 deletions.
12 changes: 11 additions & 1 deletion clang/lib/CIR/Dialect/Transforms/HoistAllocas.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -48,8 +48,18 @@ static void process(cir::FuncOp func) {

mlir::Operation *insertPoint = &*entryBlock.begin();

for (auto alloca : allocas)
for (auto alloca : allocas) {
alloca->moveBefore(insertPoint);
if (alloca.getConstant()) {
// Hoisted alloca may come from the body of a loop, in which case the
// stack slot is re-used by multiple objects alive in different iterations
// of the loop. In theory, each of these objects are still constant within
// their lifetimes, but currently we're not emitting metadata to further
// describe this. So for now let's behave conservatively and remove the
// const flag on nested allocas when hoisting them.
alloca.setConstant(false);
}
}
}

void HoistAllocasPass::runOnOperation() {
Expand Down
35 changes: 27 additions & 8 deletions clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -959,8 +959,7 @@ mlir::LogicalResult CIRToLLVMVTTAddrPointOpLowering::matchAndRewrite(

if (op.getSymAddr()) {
if (op.getOffset() == 0) {
rewriter.replaceAllUsesWith(op, llvmAddr);
rewriter.eraseOp(op);
rewriter.replaceOp(op, {llvmAddr});
return mlir::success();
}

Expand Down Expand Up @@ -1490,11 +1489,21 @@ mlir::LogicalResult CIRToLLVMLoadOpLowering::matchAndRewrite(
alignment = *alignOpt;
}

// TODO: nontemporal, invariant, syncscope.
auto invariant = false;
// Under -O1 or higher optimization levels, add the invariant metadata if the
// load operation loads from a constant object.
if (lowerMod &&
lowerMod->getContext().getCodeGenOpts().OptimizationLevel > 0) {
auto addrAllocaOp =
mlir::dyn_cast_if_present<cir::AllocaOp>(op.getAddr().getDefiningOp());
invariant = addrAllocaOp && addrAllocaOp.getConstant();
}

// TODO: nontemporal, syncscope.
rewriter.replaceOpWithNewOp<mlir::LLVM::LoadOp>(
op, llvmTy, adaptor.getAddr(), /* alignment */ alignment,
op.getIsVolatile(), /* nontemporal */ false,
/* invariant */ false, /* invariantGroup */ false, ordering);
/* invariant */ false, /* invariantGroup */ invariant, ordering);
return mlir::LogicalResult::success();
}

Expand All @@ -1515,10 +1524,20 @@ mlir::LogicalResult CIRToLLVMStoreOpLowering::matchAndRewrite(
alignment = *alignOpt;
}

auto invariant = false;
// Under -O1 or higher optimization levels, add the invariant metadata if the
// store operation stores to a constant object.
if (lowerMod &&
lowerMod->getContext().getCodeGenOpts().OptimizationLevel > 0) {
auto addrAllocaOp =
mlir::dyn_cast_if_present<cir::AllocaOp>(op.getAddr().getDefiningOp());
invariant = addrAllocaOp && addrAllocaOp.getConstant();
}

// TODO: nontemporal, syncscope.
rewriter.replaceOpWithNewOp<mlir::LLVM::StoreOp>(
op, adaptor.getValue(), adaptor.getAddr(), alignment, op.getIsVolatile(),
/* nontemporal */ false, /* invariantGroup */ false, ordering);
/* nontemporal */ false, /* invariantGroup */ invariant, ordering);
return mlir::LogicalResult::success();
}

Expand Down Expand Up @@ -3887,7 +3906,9 @@ void populateCIRToLLVMConversionPatterns(
CIRToLLVMConstantOpLowering,
CIRToLLVMDerivedDataMemberOpLowering,
CIRToLLVMGetRuntimeMemberOpLowering,
CIRToLLVMGlobalOpLowering
CIRToLLVMGlobalOpLowering,
CIRToLLVMLoadOpLowering,
CIRToLLVMStoreOpLowering
// clang-format on
>(converter, patterns.getContext(), lowerModule);
patterns.add<
Expand Down Expand Up @@ -3938,7 +3959,6 @@ void populateCIRToLLVMConversionPatterns(
CIRToLLVMIsConstantOpLowering,
CIRToLLVMIsFPClassOpLowering,
CIRToLLVMLLVMIntrinsicCallOpLowering,
CIRToLLVMLoadOpLowering,
CIRToLLVMMemChrOpLowering,
CIRToLLVMMemCpyInlineOpLowering,
CIRToLLVMMemCpyOpLowering,
Expand All @@ -3958,7 +3978,6 @@ void populateCIRToLLVMConversionPatterns(
CIRToLLVMShiftOpLowering,
CIRToLLVMSignBitOpLowering,
CIRToLLVMStackSaveOpLowering,
CIRToLLVMStoreOpLowering,
CIRToLLVMSwitchFlatOpLowering,
CIRToLLVMThrowOpLowering,
CIRToLLVMTrapOpLowering,
Expand Down
14 changes: 12 additions & 2 deletions clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.h
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Original file line number Diff line number Diff line change
Expand Up @@ -301,8 +301,13 @@ class CIRToLLVMAllocaOpLowering
};

class CIRToLLVMLoadOpLowering : public mlir::OpConversionPattern<cir::LoadOp> {
cir::LowerModule *lowerMod;

public:
using mlir::OpConversionPattern<cir::LoadOp>::OpConversionPattern;
CIRToLLVMLoadOpLowering(const mlir::TypeConverter &typeConverter,
mlir::MLIRContext *context,
cir::LowerModule *lowerModule)
: OpConversionPattern(typeConverter, context), lowerMod(lowerModule) {}

mlir::LogicalResult
matchAndRewrite(cir::LoadOp op, OpAdaptor,
Expand All @@ -311,8 +316,13 @@ class CIRToLLVMLoadOpLowering : public mlir::OpConversionPattern<cir::LoadOp> {

class CIRToLLVMStoreOpLowering
: public mlir::OpConversionPattern<cir::StoreOp> {
cir::LowerModule *lowerMod;

public:
using mlir::OpConversionPattern<cir::StoreOp>::OpConversionPattern;
CIRToLLVMStoreOpLowering(const mlir::TypeConverter &typeConverter,
mlir::MLIRContext *context,
cir::LowerModule *lowerModule)
: OpConversionPattern(typeConverter, context), lowerMod(lowerModule) {}

mlir::LogicalResult
matchAndRewrite(cir::StoreOp op, OpAdaptor,
Expand Down
69 changes: 52 additions & 17 deletions clang/test/CIR/CodeGen/const-alloca.cpp
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Original file line number Diff line number Diff line change
@@ -1,5 +1,7 @@
// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
// RUN: FileCheck --input-file=%t.cir %s
// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O1 -fclangir -emit-cir %s -o %t.cir
// RUN: FileCheck --input-file=%t.cir --check-prefix=CIR %s
// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O1 -fclangir -emit-llvm %s -o %t.ll
// RUN: FileCheck --input-file=%t.ll --check-prefix=LLVM %s

int produce_int();
void blackbox(const int &);
Expand All @@ -8,33 +10,33 @@ void local_const_int() {
const int x = produce_int();
}

// CHECK-LABEL: @_Z15local_const_intv
// CHECK: %{{.+}} = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init, const]
// CHECK: }
// CIR-LABEL: @_Z15local_const_intv
// CIR: %{{.+}} = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init, const]
// CIR: }

void param_const_int(const int x) {}

// CHECK-LABEL: @_Z15param_const_inti
// CHECK: %{{.+}} = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init, const]
// CHECK: }
// CIR-LABEL: @_Z15param_const_inti
// CIR: %{{.+}} = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init, const]
// CIR: }

void local_constexpr_int() {
constexpr int x = 42;
blackbox(x);
}

// CHECK-LABEL: @_Z19local_constexpr_intv
// CHECK: %{{.+}} = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init, const]
// CHECK: }
// CIR-LABEL: @_Z19local_constexpr_intv
// CIR: %{{.+}} = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init, const]
// CIR: }

void local_reference() {
int x = 0;
int &r = x;
}

// CHECK-LABEL: @_Z15local_referencev
// CHECK: %{{.+}} = cir.alloca !cir.ptr<!s32i>, !cir.ptr<!cir.ptr<!s32i>>, ["r", init, const]
// CHECK: }
// CIR-LABEL: @_Z15local_referencev
// CIR: %{{.+}} = cir.alloca !cir.ptr<!s32i>, !cir.ptr<!cir.ptr<!s32i>>, ["r", init, const]
// CIR: }

struct Foo {
int a;
Expand All @@ -47,6 +49,39 @@ void local_const_struct() {
const Foo x = produce_foo();
}

// CHECK-LABEL: @_Z18local_const_structv
// CHECK: %{{.+}} = cir.alloca !ty_Foo, !cir.ptr<!ty_Foo>, ["x", init, const]
// CHECK: }
// CIR-LABEL: @_Z18local_const_structv
// CIR: %{{.+}} = cir.alloca !ty_Foo, !cir.ptr<!ty_Foo>, ["x", init, const]
// CIR: }

[[clang::optnone]]
int local_const_load_store() {
const int x = produce_int();
int y = x;
return y;
}

// CIR-LABEL: @_Z22local_const_load_storev
// CIR: %{{.+}} = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init, const] {alignment = 4 : i64}
// CIR: }

// LLVM-LABEL: @_Z22local_const_load_storev
// LLVM: %[[#INIT:]] = call i32 @_Z11produce_intv()
// LLVM-NEXT: store i32 %[[#INIT]], ptr %[[#SLOT:]], align 4, !invariant.group !{{.+}}
// LLVM-NEXT: %{{.+}} = load i32, ptr %[[#SLOT]], align 4, !invariant.group !{{.+}}
// LLVM: }

int local_const_optimize() {
const int x = produce_int();
blackbox(x);
blackbox(x);
return x;
}

// LLVM-LABEL: @_Z20local_const_optimizev()
// LLVM-NEXT: %[[#slot:]] = alloca i32, align 4
// LLVM-NEXT: %[[#init:]] = tail call i32 @_Z11produce_intv()
// LLVM-NEXT: store i32 %[[#init]], ptr %[[#slot]], align 4, !invariant.group !{{.+}}
// LLVM-NEXT: call void @_Z8blackboxRKi(ptr nonnull %[[#slot]])
// LLVM-NEXT: call void @_Z8blackboxRKi(ptr nonnull %[[#slot]])
// LLVM-NEXT: ret i32 %[[#init]]
// LLVM-NEXT: }

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