public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug middle-end/110052] New: useless local variable not optimized away
@ 2023-05-31  8:06 aldot at gcc dot gnu.org
  2023-05-31  8:31 ` [Bug middle-end/110052] " amonakov at gcc dot gnu.org
                   ` (4 more replies)
  0 siblings, 5 replies; 6+ messages in thread
From: aldot at gcc dot gnu.org @ 2023-05-31  8:06 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110052

            Bug ID: 110052
           Summary: useless local variable not optimized away
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aldot at gcc dot gnu.org
  Target Milestone: ---

Created attachment 55221
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55221&action=edit
reduced test, v1

A superfluous local variable is not optimized away.

Consider:
$ cat localvar.c ; echo EOF
#include <stddef.h>
// original flags: gcc-12 -std=gnu99 -malign-data=abi -finline-limit=0
-fno-builtin-strlen -fomit-frame-pointer -ffunction-sections -fdata-sections
-fno-guess-branch-probability -funsigned-char -falign-functions=1
-falign-jumps=1 -falign-labels=1 -falign-loops=1 -fno-unwind-tables
-fno-asynchronous-unwind-tables -fno-builtin-printf -Oz
extern void *realloc(void *ptr, size_t size);
static char * __attribute__((noipa))
myrealloc(void *ptr, int n, int *size)
{
  if (!ptr || n >= *size) {
    *size = n + (n >> 1) + 80;
    ptr = realloc (ptr, *size);
    if (ptr == NULL) /* mimic xrealloc */
      __builtin_abort();
  }
  return ptr;
}
typedef struct foo_s {
  int fd;
  char *buffer;
  int adv;
  int size;
  int pos;
  signed char boo;
} foo_t;

void bloat(foo_t *);
void bloat(foo_t *foo) {
  char *m = foo->buffer;
  /* This is the original code, the local variable is superfluous */
  int size = foo->size;
  if (!m)
    m = myrealloc(m, 256, &size);
  foo->size = size;
}

void ok(foo_t *);
void ok(foo_t *foo) {
  char *m = foo->buffer;
  if (!m)
    m = myrealloc(m, 256, &foo->size);
}
EOF


$ readelf -s localvar.o | grep "FUNC *GLOBAL"
    13: 0000000000000000    52 FUNC    GLOBAL DEFAULT    6 bloat
    14: 0000000000000000    24 FUNC    GLOBAL DEFAULT    8 ok
$ strip -R .comment localvar.o
$ objdump -D localvar.o

localvar.o:     file format elf64-x86-64


Disassembly of section .text.myrealloc:

0000000000000000 <.text.myrealloc>:
   0:   48 89 f8                mov    %rdi,%rax
   3:   48 85 ff                test   %rdi,%rdi
   6:   74 04                   je     0xc
   8:   39 32                   cmp    %esi,(%rdx)
   a:   7f 21                   jg     0x2d
   c:   51                      push   %rcx
   d:   89 f1                   mov    %esi,%ecx
   f:   48 97                   xchg   %rax,%rdi
  11:   d1 f9                   sar    %ecx
  13:   8d 74 0e 50             lea    0x50(%rsi,%rcx,1),%esi
  17:   89 32                   mov    %esi,(%rdx)
  19:   48 63 f6                movslq %esi,%rsi
  1c:   e8 00 00 00 00          callq  0x21
  21:   48 85 c0                test   %rax,%rax
  24:   75 05                   jne    0x2b
  26:   e8 00 00 00 00          callq  0x2b
  2b:   5a                      pop    %rdx
  2c:   c3                      retq   
  2d:   c3                      retq   

Disassembly of section .text.bloat:

0000000000000000 <.text.bloat>:
   0:   53                      push   %rbx
   1:   48 89 fb                mov    %rdi,%rbx
   4:   48 83 ec 10             sub    $0x10,%rsp
   8:   8b 47 14                mov    0x14(%rdi),%eax
   b:   48 83 7f 08 00          cmpq   $0x0,0x8(%rdi)
  10:   89 44 24 0c             mov    %eax,0xc(%rsp)
  14:   75 11                   jne    0x27
  16:   48 8d 54 24 0c          lea    0xc(%rsp),%rdx
  1b:   be 00 01 00 00          mov    $0x100,%esi
  20:   31 ff                   xor    %edi,%edi
  22:   e8 00 00 00 00          callq  0x27
  27:   8b 44 24 0c             mov    0xc(%rsp),%eax
  2b:   89 43 14                mov    %eax,0x14(%rbx)
  2e:   48 83 c4 10             add    $0x10,%rsp
  32:   5b                      pop    %rbx
  33:   c3                      retq   

Disassembly of section .text.ok:

0000000000000000 <.text.ok>:
   0:   48 83 7f 08 00          cmpq   $0x0,0x8(%rdi)
   5:   75 10                   jne    0x17
   7:   48 8d 57 14             lea    0x14(%rdi),%rdx
   b:   be 00 01 00 00          mov    $0x100,%esi
  10:   31 ff                   xor    %edi,%edi
  12:   e9 00 00 00 00          jmpq   0x17
  17:   c3                      retq   

object size with each myrealloc and either bloat or ok:
$ size localvar-*.o
   text    data     bss     dec     hex filename
     70       0       0      70      46 localvar-ok.o
     98       0       0      98      62 localvar-bloat.o

^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-06-01 12:48 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-05-31  8:06 [Bug middle-end/110052] New: useless local variable not optimized away aldot at gcc dot gnu.org
2023-05-31  8:31 ` [Bug middle-end/110052] " amonakov at gcc dot gnu.org
2023-05-31  8:51 ` rguenth at gcc dot gnu.org
2023-05-31 18:12 ` aldot at gcc dot gnu.org
2023-05-31 18:24 ` aldot at gcc dot gnu.org
2023-06-01 12:48 ` amonakov at gcc dot gnu.org

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).