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
