https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77276
Bug ID: 77276
Summary: strlen expanded inline with -Os, emits larger code
than with -O2
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
On x86_64 (but not on powerpc64le, for example), a call to __builtin_strlen
with a non-constant argument is expanded inline at -Os (as well as -Og and
-O1), but not at -O2 even though the size of the code emitted at -O2 is less
than that at -Os.
$ (set -x && cat xyz.c && for o in s 2; do /build/gcc-trunk-svn/gcc/xgcc -B
/build/gcc-trunk-svn/gcc -O$o -Wall -Wextra -c xyz.c && objdump -d xyz.o; done)
+ cat xyz.c
static const char* volatile s = "123";
unsigned f (void)
{
return __builtin_strlen (s);
}
+ for o in s 2
+ /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -Os -Wall -Wextra
-c xyz.c
+ objdump -d xyz.o
xyz.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <f>:
0: 48 8b 3d 00 00 00 00 mov 0x0(%rip),%rdi # 7 <f+0x7>
7: 31 c0 xor %eax,%eax
9: 48 83 c9 ff or $0xffffffffffffffff,%rcx
d: f2 ae repnz scas %es:(%rdi),%al
f: 48 89 c8 mov %rcx,%rax
12: 48 f7 d0 not %rax
15: 48 ff c8 dec %rax
18: c3 retq
+ for o in s 2
+ /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -O2 -Wall -Wextra
-c xyz.c
+ objdump -d xyz.o
xyz.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <f>:
0: 48 83 ec 08 sub $0x8,%rsp
4: 48 8b 3d 00 00 00 00 mov 0x0(%rip),%rdi # b <f+0xb>
b: e8 00 00 00 00 callq 10 <f+0x10>
10: 48 83 c4 08 add $0x8,%rsp
14: c3 retq
