[llvm] 9649c20 - [InstCombine] Drop debug loc in TryToSinkInstruction (reland)

[David Blaikie via llvm-commits]

On Fri, Jun 26, 2020 at 5:18 PM Vedant Kumar via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
>
>
> Author: Vedant Kumar
> Date: 2020-06-26T17:18:15-07:00
> New Revision: 9649c2095f07a392bc2b2a93b5bd6c4c9bf5ba34
>
> URL: https://github.com/llvm/llvm-project/commit/9649c2095f07a392bc2b2a93b5bd6c4c9bf5ba34
> DIFF: https://github.com/llvm/llvm-project/commit/9649c2095f07a392bc2b2a93b5bd6c4c9bf5ba34.diff
>
> LOG: [InstCombine] Drop debug loc in TryToSinkInstruction (reland)
>
> Summary:
> The advice in HowToUpdateDebugInfo.rst is to "... preserve the debug
> location of an instruction if the instruction either remains in its
> basic block, or if its basic block is folded into a predecessor that
> branches unconditionally".
>
> TryToSinkInstruction doesn't seem to satisfy the criteria as it's
> sinking an instruction to some successor block. Preserving the debug loc
> can make single-stepping appear to go backwards, or make a breakpoint
> hit on that location happen "too late" (since single-stepping from that
> breakpoint can cause the function to return unexpectedly).
>
> So, drop the debug location.
>
> This was reverted in ee3620643dfc because it removed source locations
> from inlinable calls, breaking a verifier rule. I've added an exception
> for calls because the alternative (setting a line 0 location) is not
> better.

What do you mean by "is not better"?

My understanding is that the other reason for not moving such
locations is profile accuracy. If the line is preserved when an
instruction (including a call) is sunk into a codepath that doesn't
necessarily pass through the original location, a sample based profile
could incorrectly conclude that a conditional was taken that would've
reached the original location of the instruction.

> I tested the updated patch by completing a stage2 RelWithDebInfo
> build.
>
> Reviewers: aprantl, davide
>
> Subscribers: hiraditya, llvm-commits
>
> Tags: #llvm
>
> Differential Revision: https://reviews.llvm.org/D82487
>
> Added:
>     llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll
>
> Modified:
>     llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
>
> Removed:
>
>
>
> ################################################################################
> diff  --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
> index 1f97f0c1ac99..6a406314f63c 100644
> --- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
> +++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
> @@ -3355,6 +3355,12 @@ static bool TryToSinkInstruction(Instruction *I, BasicBlock *DestBlock) {
>    I->moveBefore(&*InsertPos);
>    ++NumSunkInst;
>
> +  // Drop the debug loc of non-inlinable instructions. This prevents
> +  // single-stepping from going backwards. See HowToUpdateDebugInfo.rst for
> +  // the full rationale.
> +  if (!isa<CallBase>(I))
> +    I->setDebugLoc(DebugLoc());
> +
>    // Also sink all related debug uses from the source basic block. Otherwise we
>    // get debug use before the def. Attempt to salvage debug uses first, to
>    // maximise the range variables have location for. If we cannot salvage, then
>
> diff  --git a/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll b/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll
> new file mode 100644
> index 000000000000..e642276224b8
> --- /dev/null
> +++ b/llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll
> @@ -0,0 +1,46 @@
> +; RUN: opt -debugify -debugify-level=locations -instcombine -S < %s | FileCheck %s
> +
> +; CHECK-LABEL: @test1(
> +; CHECK: [[phi:%.*]] = phi i32
> +; CHECK-NEXT: [[add:%.*]] = add i32 {{.*}}, 1{{$}}
> +; CHECK-NEXT: add i32 [[phi]], [[add]], !dbg
> +define i32 @test1(i32 %0, i1 %1) {
> +  %3 = add i32 %0, 1
> +  br i1 %1, label %4, label %5
> +
> +4:                                                ; preds = %2
> +  br label %6
> +
> +5:                                                ; preds = %2
> +  br label %6
> +
> +6:                                                ; preds = %5, %4
> +  %7 = phi i32 [ 0, %4 ], [ 1, %5 ]
> +  %8 = add i32 %7, %3
> +  ret i32 %8
> +}
> +
> +; Function Attrs: nounwind readnone
> +declare i32 @external(i32) #0
> +
> +; CHECK-LABEL: @test2(
> +; CHECK: [[phi:%.*]] = phi i32
> +; CHECK-NEXT: [[add:%.*]] = call i32 @external(i32 {{.*}}), !dbg
> +; CHECK-NEXT: add i32 [[phi]], [[add]], !dbg
> +define i32 @test2(i32 %0, i1 %1) {
> +  %3 = call i32 @external(i32 %0)
> +  br i1 %1, label %4, label %5
> +
> +4:                                                ; preds = %2
> +  br label %6
> +
> +5:                                                ; preds = %2
> +  br label %6
> +
> +6:                                                ; preds = %5, %4
> +  %7 = phi i32 [ 0, %4 ], [ 1, %5 ]
> +  %8 = add i32 %7, %3
> +  ret i32 %8
> +}
> +
> +attributes #0 = { nounwind readnone }
>
>
>
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