[llvm-bugs] [Bug 38092] New: Missed elimination of moves

via llvm-bugs llvm-bugs at lists.llvm.org
Sun Jul 8 06:44:55 PDT 2018


            Bug ID: 38092
           Summary: Missed elimination of moves
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org


struct replace_info {
      int n;
      char *text;

static int compare(const void *a, const void *b) {
      struct replace_info *x = (struct replace_info *) a;
      struct replace_info *y = (struct replace_info *) b;
      return x->n - y->n;

int main(void) {
      struct replace_info info[3] = {
          {5, "Aaaa"},
          {7, "Bbbb"},
          {3, "Cccc"}

      qsort(info, 3, sizeof(struct replace_info), compare);

Clang trunk (-O3):
  sub rsp, 56
  xorps xmm0, xmm0                    // *****
  movaps xmmword ptr [rsp], xmm0       // *****
  movaps xmmword ptr [rsp + 16], xmm0   // *****
  movaps xmmword ptr [rsp + 32], xmm0    // *****
  mov dword ptr [rsp], 5
  mov qword ptr [rsp + 8], offset .L.str
  mov dword ptr [rsp + 16], 7
  mov qword ptr [rsp + 24], offset .L.str.1
  mov dword ptr [rsp + 32], 3
  mov qword ptr [rsp + 40], offset .L.str.2
  mov rdi, rsp
  mov esi, 3
  mov edx, 16
  mov ecx, offset compare
  call qsort
  xor eax, eax
  add rsp, 56

I think lines marked with "*****" are useless and GCC confirms it (does not
generate them).

  sub rsp, 56
  mov ecx, OFFSET FLAT:compare
  mov edx, 16
  mov esi, 3
  mov rdi, rsp
  mov DWORD PTR [rsp], 5
  mov QWORD PTR [rsp+8], OFFSET FLAT:.LC0
  mov DWORD PTR [rsp+16], 7
  mov QWORD PTR [rsp+24], OFFSET FLAT:.LC1
  mov DWORD PTR [rsp+32], 3
  mov QWORD PTR [rsp+40], OFFSET FLAT:.LC2
  call qsort
  xor eax, eax
  add rsp, 56

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