[llvm-bugs] [Bug 36085] New: Debug fragment offsets are emitted in the wrong order for symbol locations

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36085

            Bug ID: 36085
           Summary: Debug fragment offsets are emitted in the wrong order
                    for symbol locations
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: david.stenberg at ericsson.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19747
  --> https://bugs.llvm.org/attachment.cgi?id=19747&action=edit
llc input

LLVM commit used: r323411.

When compiling foo.c:

  struct {
    long foo;
    void *bar;
    long baz;
    void *qux;
  } static var = { .bar = (void *)0x1234, .qux = (void *)0x5678 };

  void *ptr;

  int main() {
    if (var.foo == 0)
      var.bar = var.qux = ptr;
    return 0;
  }

using:

  clang -march=x86-64 -O0 -g2 -S -emit-llvm foo.c -o foo.ll
  opt -S -globalopt foo.ll -o foo.opt.ll
  llc -O0 foo.opt.ll -filetype=obj -o foo.obj 

the following debug info is emitted for the SRA'd struct variable, in which
the first and third fields have been optimized out:

   <1><40>: Abbrev Number: 4 (DW_TAG_variable)
      <41>   DW_AT_name        : (indirect string, offset: 0x10f): var
      <45>   DW_AT_type        : <0x66>
      <49>   DW_AT_decl_file   : 1
      <4a>   DW_AT_decl_line   : 6
      <4b>   DW_AT_location    : 26 byte block: 3 0 0 0 0 0 0 0 0 93 8 93 8 3 8
0 0 0 0 0 0 0 93 8 93 8   (DW_OP_addr: 0; DW_OP_piece: 8; DW_OP_piece: 8;
DW_OP_addr: 8; DW_OP_piece: 8; DW_OP_piece: 8)

As seen, the DW_OP_addr operations for the second and fourth field are emitted
before the pieces used as offsets for the optimized-out first and third
variable, leading the operations to be associated with wrong parts of the
variable.

This can be further seen by printing the variable in gdb:

echo "print /x var" | gdb a.out  | grep '\$1'  
(gdb) $1 = {foo = 0x1234, bar = <optimized out>, baz = 0x5678, qux = <optimized
out>}

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