[all-commits] [llvm/llvm-project] 9649c2: [InstCombine] Drop debug loc in TryToSinkInstructi...

[Vedant Kumar via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 9649c2095f07a392bc2b2a93b5bd6c4c9bf5ba34
      https://github.com/llvm/llvm-project/commit/9649c2095f07a392bc2b2a93b5bd6c4c9bf5ba34
  Author: Vedant Kumar <vsk at apple.com>
  Date:   2020-06-26 (Fri, 26 Jun 2020)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
    A llvm/test/Transforms/InstCombine/sink_to_unreachable_dbg.ll

  Log Message:
  -----------
  [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. 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