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[KeyInstr][Clang] While stmt atom

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See test comment for possible future improvement.

This patch is part of a stack that teaches Clang to generate Key Instructions
metadata for C and C++.

The Key Instructions project is introduced, including a "quick summary" section
at the top which adds context for this PR, here:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

The feature is only functional in LLVM if LLVM is built with CMake flag
LLVM_EXPERIMENTAL_KEY_INSTRUCTIONs. Eventually that flag will be removed.

The Clang-side work is demoed here:
https://github.com/llvm/llvm-project/pull/130943
This commit is contained in:
Orlando Cazalet-Hyams 2025-04-03 18:49:32 +01:00
parent 2ee4d451f7
commit 38f3eec43e
2 changed files with 42 additions and 1 deletions
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9
clang/lib/CodeGen/CGStmt.cpp
View File

@ -1130,7 +1130,14 @@ void CodeGenFunction::EmitWhileStmt(const WhileStmt &S,
if (!Weights && CGM.getCodeGenOpts().OptimizationLevel)
BoolCondVal = emitCondLikelihoodViaExpectIntrinsic(
BoolCondVal, Stmt::getLikelihood(S.getBody()));
Builder.CreateCondBr(BoolCondVal, LoopBody, ExitBlock, Weights);
auto *I = Builder.CreateCondBr(BoolCondVal, LoopBody, ExitBlock, Weights);
// Key Instructions: Emit the condition and branch as separate atoms to
// match existing loop stepping behaviour. FIXME: We could have the branch
// as the backup location for the condition, which would probably be a
// better experience. Explore this later.
if (auto *I = dyn_cast<llvm::Instruction>(BoolCondVal))
addInstToNewSourceAtom(I, nullptr);
addInstToNewSourceAtom(I, nullptr);
if (ExitBlock != LoopExit.getBlock()) {
EmitBlock(ExitBlock);

34
clang/test/KeyInstructions/while.c Normal file
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@ -0,0 +1,34 @@
// RUN: %clang -gkey-instructions -x c++ -std=c++17 %s -gmlt -S -emit-llvm -o - \
// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
// RUN: %clang -gkey-instructions -x c %s -gmlt -S -emit-llvm -o - \
// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
// Perennial quesiton: should the `dec` be in its own source atom or not
// (currently it is).
// We've made the cmp and br separate source atoms for now, to match existing
// behaviour in this case:
// 1. while (
// 2. int i = --End
// 3. ) {
// 4. useValue(i);
// 5. }
// Without Key Instructions we go: 2, 1[, 4, 2, 1]+
// Without separating cmp and br with Key Instructions we'd get:
// 1[, 4, 1]+. If we made the cmp higher precedence than the
// br and had them in the same group, we could get:
// 2, [4, 2]+ which might be nicer. FIXME: do that later.
void a(int A) {
// CHECK: %dec = add nsw i32 %0, -1, !dbg [[G1R2:!.*]]
// CHECK: store i32 %dec, ptr %A.addr{{.*}}, !dbg [[G1R1:!.*]]
// CHECK: %tobool = icmp ne i32 %dec, 0, !dbg [[G2R1:!.*]]
// CHECK: br i1 %tobool, label %while.body, label %while.end, !dbg [[G3R1:!.*]]
while (--A) { };
}
// CHECK: [[G1R2]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 2)
// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
// CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
// CHECK: [[G3R1]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 1)