[all-commits] [llvm/llvm-project] 77ffce: [Instruction] Set metadata uses to undef on deletion

[Vedant Kumar via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 77ffce6954470018de1748d33f9ac6aa66121e9f
      https://github.com/llvm/llvm-project/commit/77ffce6954470018de1748d33f9ac6aa66121e9f
  Author: Vedant Kumar <vsk at apple.com>
  Date:   2020-05-21 (Thu, 21 May 2020)

  Changed paths:
    M llvm/lib/IR/Instruction.cpp
    M llvm/test/DebugInfo/NVPTX/debug-addr-class.ll
    M llvm/test/DebugInfo/NVPTX/debug-info.ll
    M llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-di.ll
    M llvm/test/Transforms/SafeStack/X86/debug-loc2.ll
    M llvm/unittests/IR/DebugInfoTest.cpp

  Log Message:
  -----------
  [Instruction] Set metadata uses to undef on deletion

Summary:
Replace any extant metadata uses of a dying instruction with undef to
preserve debug info accuracy. Some alternatives include:

- Treat Instruction like any other Value, and point its extant metadata
  uses to an empty ValueAsMetadata node. This makes extant dbg.value uses
  trivially dead (i.e. fair game for deletion in many passes), leading to
  stale dbg.values being in effect for too long.

- Call salvageDebugInfoOrMarkUndef. Not needed to make instruction removal
  correct. OTOH results in wasted work in some common cases (e.g. when all
  instructions in a BasicBlock are deleted).

This came up while discussing some basic cases in
https://reviews.llvm.org/D80052.

Reviewers: jmorse, TWeaver, aprantl, dexonsmith, jdoerfert

Subscribers: jholewinski, qcolombet, hiraditya, jfb, sstefan1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80264