From: anton@mips.complang.tuwien.ac.at
EricP writes:
>Anton Ertl wrote:
>> When I compile
>>
>> long foo(long a, long b)
>> {
>> if (a+b<0)
>> return a-b;
>> else
>> return a*b;
>> }
>>
>> with gcc-12.2.0 -O -c on AMD64, I get
>>
>> 0000000000000000 :
>> 0: 48 89 f8 mov %rdi,%rax
>> 3: 48 89 fa mov %rdi,%rdx
>> 6: 48 01 f2 add %rsi,%rdx
>> 9: 78 05 js 10
>> b: 48 0f af c6 imul %rsi,%rax
>> f: c3 ret
>> 10: 48 29 f0 sub %rsi,%rax
>> 13: c3 ret
...
>This could be 1 MOV shorter.
>It didn't need to MOV %rdi, %rdx as it already copied rdi to rax.
>Just ADD %rsi,%rdi and after that use the %rax copy.
Yes, I often see more register-register moves in gcc-generated code
than necessary.
>For that optimization { ADD CMP Bcc } => { ADD Bcc }
>to work those three instructions must be adjacent.
>In this case it wouldn't make a difference but in general
>I think they would want the freedom to move code about and not have
>the ADD bound to the Bcc too early so this would have to be about
>the very last optimization so it didn't interfere with code motion.
Yes, possible. When I look at what clang-14.0.6 -O -c produces, it's
this:
0000000000000000 :
0: 48 89 f9 mov %rdi,%rcx
3: 48 29 f1 sub %rsi,%rcx
6: 48 89 f0 mov %rsi,%rax
9: 48 0f af c7 imul %rdi,%rax
d: 48 01 fe add %rdi,%rsi
10: 48 0f 48 c1 cmovs %rcx,%rax
14: c3 ret
clang seems to prefer using cmov. The interesting thing here is that
it puts the add right in front of the cmovs, after the code for "a-b"
and "a*b". When I do
long foo(long a, long b)
{
if (a+b*111<0)
return a-b;
else
return a*b;
}
clang produces this code:
0000000000000000 :
0: 48 6b ce 6f imul $0x6f,%rsi,%rcx
4: 48 89 f8 mov %rdi,%rax
7: 48 29 f0 sub %rsi,%rax
a: 48 0f af f7 imul %rdi,%rsi
e: 48 01 f9 add %rdi,%rcx
11: 48 0f 49 c6 cmovns %rsi,%rax
15: c3 ret
I.e., rcx=b*111 is first, but a+rcx is late, right before the cmovns.
So it seems to have some mechanism for keeping the add and the
cmov(n)s as one unit.
- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
Mitch Alsup,
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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