[llvm-commits] PATCH: review request: X86: Use StackRegister instead of FrameRegister in getFrameIndexReference if stack needs realignment

[Alexey Samsonov]

Hi, llvm-commits!

It looks like debug info generated for variables is incorrect in case the
stack needs realignment. Consider:
$ cat test.cc
#include <stdio.h>
void run() {
  int x __attribute__((aligned(32))); // enforce alignment
  printf("%p\n", &x);
}
$ clang++ -g -c test.cc
$ objdump -d test.o
<...>
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 81 e4 e0 ff ff ff    and    $0xffffffffffffffe0,%rsp   <-----
stack gets aligned here
   b:   48 83 ec 40             sub    $0x40,%rsp
   f:   48 8d 3c 25 00 00 00    lea    0x0,%rdi
  16:   00
  17:   48 8d 74 24 20          lea    0x20(%rsp),%rsi
  1c:   b0 00                   mov    $0x0,%al
  1e:   e8 00 00 00 00          callq  23 <_Z3runv+0x23>
<...>
$ readelf -wi test.o
<...>
 <3><55>: Abbrev Number: 4 (DW_TAG_variable)
    <56>   DW_AT_name        : (indirect string, offset: 0xa4): x
    <5a>   DW_AT_decl_file   : 1
    <5b>   DW_AT_decl_line   : 3
    <5c>   DW_AT_type        : <0x65>
    <60>   DW_AT_location    : 2 byte block: 91 20      (DW_OP_fbreg: 32)

That is, dwarf debug info says that "x" is located at offset 0x20 from
frame register (%rbp), which is wrong:
1) the offset from %rbp should clearly be negative
2) we can't calculate it that easy, as stack is aligned (%rsp is changed)
after old rsp value is assigned to rbp.
3) 0x20 is actually the offset from *%rsp*, not %rbp

I may be wrong, but X86FrameLowering::getFrameIndexOffset in case the stack
needs realignment (
http://llvm.org/docs/doxygen/html/X86FrameLowering_8cpp_source.html#l01144)
seems to be calculating the offset from %rsp. If this is true, we should
enforce the usage of rsp when calculating variable locations, even if we
have frame pointers. This can be fixed by applying the patch.

Codereview link: http://codereview.appspot.com/6127054/

-- 
Alexey Samsonov, MSK
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