[llvm-bugs] [Bug 49099] New: GVNHoist should drop debug location according to the debug info guide

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49099

            Bug ID: 49099
           Summary: GVNHoist should drop debug location according to the
                    debug info guide
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: yuanboli233 at gmail.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 24497
  --> https://bugs.llvm.org/attachment.cgi?id=24497&action=edit
abc_debug.ll

Overview, the GVNHoist pass preserves the debug location, however, according to
the LLVM debug info update guide, the debug location should be dropped.

Steps to reproduce:

$ clang++ --version
clang version 13.0.0 (https://github.com/llvm/llvm-project.git
0b5d8d668c210f263a93821fd51c564313c96099)
Target: x86_64-unknown-linux-gnu
Thread model: posix

The complete IR file abc_debug.ll is attached.

$ cat abc_debug.ll
...... (irrelevant content omitted)
entry:
  call void @llvm.dbg.value(metadata i32 0, metadata !9, metadata
!DIExpression()), !dbg !11
  switch i32 %c1, label %exit1 [
    i32 0, label %sw0
    i32 1, label %sw1
  ], !dbg !11

sw0:                                              ; preds = %entry
  store i32 1, i32* @G, align 4, !dbg !12
  br label %exit, !dbg !13

sw1:                                              ; preds = %entry
  store i32 1, i32* @G, align 4, !dbg !14
  br label %exit, !dbg !15

exit1:                                            ; preds = %entry
  store i32 1, i32* @G, align 4, !dbg !16
  ret void, !dbg !17
......


The gvn hoist will hoist the identical store instructions to the predecessor
basic block.

$ opt -gvn-hoist abc_debug.ll > result.bc
$ llvm-dis  result.bc 
$ cat result.ll
......(omit irrelevant content)
entry:
  call void @llvm.dbg.value(metadata i32 0, metadata !9, metadata
!DIExpression()), !dbg !11
  store i32 1, i32* @G, align 4, !dbg !12
  switch i32 %c1, label %exit1 [
    i32 0, label %sw0
    i32 1, label %sw1
  ], !dbg !11

sw0:                                              ; preds = %entry
  br label %exit, !dbg !13

sw1:                                              ; preds = %entry
  br label %exit, !dbg !14

exit1:                                            ; preds = %entry
  ret void, !dbg !15
......


However, according to the LLVM debug info update guide:
https://llvm.org/docs/HowToUpdateDebugInfo.html, "Hoisting identical
instructions which appear in several successor blocks into a predecessor block.
In this case there is no single merged instruction. The rule for dropping
locations applies". It seems that the debug location should be dropped instead
of preserving the "!dbg !12"

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