[llvm-bugs] [Bug 44880] New: Debug invariance in GlobalOpt

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44880

            Bug ID: 44880
           Summary: Debug invariance in GlobalOpt
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: DebugInfo
          Assignee: unassignedbugs at nondot.org
          Reporter: david.stenberg at ericsson.com
                CC: jdevlieghere at apple.com, keith.walker at arm.com,
                    llvm-bugs at lists.llvm.org,
                    paul_robinson at playstation.sony.com

Created attachment 23121
  --> https://bugs.llvm.org/attachment.cgi?id=23121&action=edit
IR reproducer.

After the introduction of "[InstCombine][DebugInfo] Fold constants wrapped in
metadata", some debug invariance started appearing.

Please note that that commit was reverted in
982944525c7706c4dee00042d5b7cf2f0d87804f due to this issue.

C reproducer:

    int a, b, c;
    static int arr[4];

    void foo() {
      const int *e[] = {&arr[2]};
      const int **f = &e[0];
      c = f != foo;
    }

    int main() {
      for (b = 0; b < 4; b++)
        a = a ^ arr[b];
      return 0;
    }

Commands:

    $ clang -O3 -g globalopt.c -S -o - -mllvm -disable-debug-info-print -o
with-debug.s
    $ clang -O3 globalopt.c -S -o - -mllvm -disable-debug-info-print -o
without-debug.s
    $ diff -u without-debug.s with-debug.s
    [...]
    +   movl    arr(%rip), %eax
    +   xorl    a(%rip), %eax
    +   xorl    arr+4(%rip), %eax
    +   xorl    arr+8(%rip), %eax
    +   xorl    arr+12(%rip), %eax
    +   movl    %eax, a(%rip)
        movl    $4, b(%rip)
        xorl    %eax, %eax
        retq

That commit appears to have exposed a pre-existing debug invariance issue in
GlobalOpt.

This can also be reproduced by just running GlobalOpt on the attached IR
reproducer.

    $ opt -strip-debug globalopt.ll | opt -S -globalopt -o without-debug.ll
    $ opt globalopt.ll | opt -S -globalopt -strip-debug -o with-debug.ll
    $ diff without-debug.ll with-debug.ll
    [...]
    -  %xor = xor i32 0, %a.promoted
    -  %xor.1 = xor i32 0, %xor
    -  %xor.2 = xor i32 0, %xor.1
    -  %xor.3 = xor i32 0, %xor.2
    +  %0 = load i32, i32* getelementptr inbounds ([4 x i32], [4 x i32]* @arr,
i64 0, i64 0), align 16
    +  %xor = xor i32 %0, %a.promoted
    +  %1 = load i32, i32* getelementptr inbounds ([4 x i32], [4 x i32]* @arr,
i64 0, i64 1), align 4
    +  %xor.1 = xor i32 %1, %xor
    +  %2 = load i32, i32* getelementptr inbounds ([4 x i32], [4 x i32]* @arr,
i64 0, i64 2), align 8
    +  %xor.2 = xor i32 %2, %xor.1
    +  %3 = load i32, i32* getelementptr inbounds ([4 x i32], [4 x i32]* @arr,
i64 0, i64 3), align 4
    +  %xor.3 = xor i32 %3, %xor.2

The issue seems to be that the dbg.value intrinsic:

  call void @llvm.dbg.value(metadata i64 ptrtoint (i32* getelementptr inbounds
([4 x i32], [4 x i32]* @arr, i64 0, i64 2) to i64), metadata !24, metadata
!DIExpression()), !dbg !32

seems to lead to GlobalStatus::analyzeGlobal() returning true that @arr has its
address taken. This seems to have to do with the analysis bailing out due to
the ptrtoint use of the GEP:

      // If the result of the constantexpr isn't pointer type, then we won't
      // know to expect it in various places.  Just reject early.
      if (!isa<PointerType>(CE->getType()))
        return true;

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