[PATCH] D80691: Proposed fix for PR46114

[Orlando Cazalet-Hyams via Phabricator via llvm-commits]

Orlando added a project: debug-info.
Orlando added a comment.

Hi @helloqirun,

Thanks for working on this!

Assuming your IR is from the code you provided in your reproducer PR46114 [1],
compiled with clang at 709c52b9553 <https://reviews.llvm.org/rG709c52b9553f4ba83ab901a873392f8a7f2c56a5> (18 May 2020) I see this:

  $ cat test.c
  int a = 1;
  short b;
  char c;
  int main() {
    int d = a, l_52 = 0;
    b = (l_52 = a) - c; //   qirun0
    if (d) {
      int e=1;
    }
  }
  
  $ clang test.c -O3 -g -mllvm -print-after-all
  ...
  *** IR Dump After SROA *** // BEFORE EARLY CSE
  ; Function Attrs: nounwind uwtable
  define dso_local i32 @main() #0 !dbg !17 {
  entry:
    %0 = load i32, i32* @a, align 4, !dbg !26, !tbaa !27
    call void @llvm.dbg.value(metadata i32 %0, metadata !21, metadata !DIExpression()), !dbg !31
    call void @llvm.dbg.value(metadata i32 0, metadata !22, metadata !DIExpression()), !dbg !31
    %1 = load i32, i32* @a, align 4, !dbg !32, !tbaa !27
    call void @llvm.dbg.value(metadata i32 %1, metadata !22, metadata !DIExpression()), !dbg !31
    %2 = load i8, i8* @c, align 1, !dbg !33, !tbaa !34
    %conv = sext i8 %2 to i32, !dbg !33
    ...
  
  *** IR Dump After Early CSE ***
  ; Function Attrs: nounwind uwtable
  define dso_local i32 @main() #0 !dbg !17 {
  entry:
    %0 = load i32, i32* @a, align 4, !dbg !26, !tbaa !27
    call void @llvm.dbg.value(metadata i32 %0, metadata !21, metadata !DIExpression()), !dbg !31
    call void @llvm.dbg.value(metadata i32 0, metadata !22, metadata !DIExpression()), !dbg !31
    call void @llvm.dbg.value(metadata i32 %0, metadata !22, metadata !DIExpression()), !dbg !31
    %1 = load i8, i8* @c, align 1, !dbg !32, !tbaa !33
    %conv = sext i8 %1 to i32, !dbg !32
    ...

During EarlyCSE we detect that `%0 = load i32, i32* @a` is the same as 
`%1 = load i32, i32* @a`, so the latter is removed. We rewrite the dbg.value
which uses %1 to use %0 instead, because %1 is being removed. IIUC this
behaviour in EarlyCSE looks correct, and makes me feel like the issue
comes from somewhere else?

[1] https://bugs.llvm.org/show_bug.cgi?id=46114


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D80691