Sometimes we can see 2 bytes nop (xchg %ax,%ax) between leaveq and retq.
IMHO, better to remove xchg %ax,%ax
Examples from Kernel 2.6.29.1:
> gcc --version
gcc (SUSE Linux) 4.3.2 [gcc-4_3-branch revision 141291]
> objdump vmlinux
...
ffffffff804262e0 <set_blitting_type>:
ffffffff804262e0: 55 push %rbp
ffffffff804262e1: 0f b7 07 movzwl (%rdi),%eax
ffffffff804262e4: 4c 8b 86 d0 03 00 00 mov 0x3d0(%rsi),%r8
ffffffff804262eb: 48 c1 e0 07 shl $0x7,%rax
ffffffff804262ef: 48 89 e5 mov %rsp,%rbp
ffffffff804262f2: 48 05 40 1f 9c 80 add $0xffffffff809c1f40,%rax
ffffffff804262f8: 49 89 80 90 01 00 00 mov %rax,0x190(%r8)
ffffffff804262ff: 8b 46 04 mov 0x4(%rsi),%eax
ffffffff80426302: 89 c1 mov %eax,%ecx
ffffffff80426304: 81 e1 00 00 02 00 and $0x20000,%ecx
ffffffff8042630a: 75 2c jne ffffffff80426338
<set_blitting_type+0x58>
ffffffff8042630c: 48 8b 86 d0 03 00 00 mov 0x3d0(%rsi),%rax
ffffffff80426313: 4c 89 c7 mov %r8,%rdi
ffffffff80426316: 48 8b 90 90 01 00 00 mov 0x190(%rax),%rdx
ffffffff8042631d: 8b 52 1c mov 0x1c(%rdx),%edx
ffffffff80426320: 83 fa 03 cmp $0x3,%edx
ffffffff80426323: 0f 4e ca cmovle %edx,%ecx
ffffffff80426326: 89 88 b0 01 00 00 mov %ecx,0x1b0(%rax)
ffffffff8042632c: e8 2f 4d 00 00 callq ffffffff8042b060
<fbcon_set_bitops>
ffffffff80426331: c9 leaveq
ffffffff80426332: c3 retq
ffffffff80426333: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
ffffffff80426338: e8 23 61 00 00 callq ffffffff8042c460
<fbcon_set_tileops>
ffffffff8042633d: c9 leaveq
ffffffff8042633e: 66 90 xchg %ax,%ax
ffffffff80426340: c3 retq
...
...
ffffffff8042b060 <fbcon_set_bitops>:
ffffffff8042b060: 55 push %rbp
ffffffff8042b061: 48 c7 07 d0 ad 42 80 movq
$0xffffffff8042add0,(%rdi)
ffffffff8042b068: 8b 87 b0 01 00 00 mov 0x1b0(%rdi),%eax
ffffffff8042b06e: 48 89 e5 mov %rsp,%rbp
ffffffff8042b071: 48 c7 47 08 30 ae 42 movq
$0xffffffff8042ae30,0x8(%rdi)
ffffffff8042b078: 80
ffffffff8042b079: 48 c7 47 10 c0 b7 42 movq
$0xffffffff8042b7c0,0x10(%rdi)
ffffffff8042b080: 80
ffffffff8042b081: 48 c7 47 18 10 af 42 movq
$0xffffffff8042af10,0x18(%rdi)
ffffffff8042b088: 80
ffffffff8042b089: 48 c7 47 20 10 b1 42 movq
$0xffffffff8042b110,0x20(%rdi)
ffffffff8042b090: 80
ffffffff8042b091: 48 c7 47 28 c0 b0 42 movq
$0xffffffff8042b0c0,0x28(%rdi)
ffffffff8042b098: 80
ffffffff8042b099: 48 c7 47 30 00 00 00 movq $0x0,0x30(%rdi)
ffffffff8042b0a0: 00
ffffffff8042b0a1: 85 c0 test %eax,%eax
ffffffff8042b0a3: 75 0b jne ffffffff8042b0b0
<fbcon_set_bitops+0x50>
ffffffff8042b0a5: c9 leaveq
ffffffff8042b0a6: c3 retq
ffffffff8042b0a7: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
ffffffff8042b0ae: 00 00
ffffffff8042b0b0: e8 4b 15 00 00 callq ffffffff8042c600
<fbcon_set_rotate>
ffffffff8042b0b5: c9 leaveq
ffffffff8042b0b6: 66 90 xchg %ax,%ax
ffffffff8042b0b8: c3 retq
--
Summary: Nonoptimal code - leaveq; xchg %ax,%ax; retq
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: vvv at ru dot ru
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39942
