[Bug target/34653] unnecessary REX prefix
dean at arctic dot org
gcc-bugzilla@gcc.gnu.org
Thu Jan 3 20:10:00 GMT 2008
------- Comment #1 from dean at arctic dot org 2008-01-03 19:27 -------
oops i should have used an "unsigned long" for the tag rather than unsigned
long long, not that it matters much.
here's an expanded example showing another unnecessary REX:
extern unsigned long table[];
unsigned long foo(unsigned char *p) {
unsigned long tag = *p;
return table[tag >> 4] + table[tag & 0xf];
}
which generates:
0: 0f b6 17 movzbl (%rdi),%edx
3: 48 89 d0 mov %rdx,%rax
6: 48 c1 ea 04 shr $0x4,%rdx
a: 83 e0 0f and $0xf,%eax
d: 48 8b 04 c5 00 00 00 mov 0x0(,%rax,8),%rax
14: 00
11: R_X86_64_32S table
15: 48 03 04 d5 00 00 00 add 0x0(,%rdx,8),%rax
1c: 00
19: R_X86_64_32S table
1d: c3 retq
and in this case the "mov %rdx,%rax" could be "mov %edx,%eax" because of the
dominating movzbl.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34653
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