[llvm-commits] [llvm] r61424 - in /llvm/trunk/lib/Target/X86: X86Instr64bit.td X86InstrInfo.td

[Evan Cheng]

On Dec 24, 2008, at 10:01 PM, Eli Friedman wrote:

> On Wed, Dec 24, 2008 at 9:32 PM, Chris Lattner <clattner at apple.com>  
> wrote:
>> On Dec 24, 2008, at 9:21 PM, Eli Friedman wrote:
>>>>> Also, even ignoring that, performance is hugely different: on a  
>>>>> Core
>>>>> 2, "bt %ebx, %eax" is one uop, but "bt %ebx, (%esp)" is 10 uops.
>>>>> The
>>>>> difference isn't quite as severe on other processors, but the reg-
>>>>> reg
>>>>> form is still significantly faster even if a load from memory is
>>>>> necessary.
>>>>
>>>> Are you sure you aren't thinking of btc/bts?  bt doesn't modify any
>>>> operands.
>>>
>>> Oh, oops, s/modifies/tests/.  The rest is correct.
>>
>> Do you have a benchmark to show this?  If it shows that it is slower
>> in practice, I think it would make sense to have a "has slow bt from
>> memory" subtarget flag that would be a nice predicate for the memory
>> form of the instructions.
>

Yes, the load folding form of bt does seem slow. According to Agner's  
information, it's slow in all Intel Pentium M and Core and AMD  
implementations.

Evan

> I'm going by timings from http://www.agner.org/optimize/.
>
> If you want a benchmark, try the following; it's a completely silly
> benchmark, but it shows the issue at hand.
>
> #include <stdlib.h>
>
> int main() {
>  int testlen = 1000000000;
>  int* a = malloc(testlen/8);
>  unsigned i;
> #if 1
>  for (i = 0; i < testlen; i++) {
>    asm volatile ("btl %0, (%1)" : : "r"(i), "r"(a));
>  }
> #else
>  for (i = 0; i < testlen; i++) {
>    asm volatile ("mov %0, %%eax;"
>                  "shrl $5, %%eax;"
>                  "mov (%1,%%eax,4), %%eax;"
>                  "btl %0, %%eax" : : "r"(i), "r"(a) : "eax");
>  }
> #endif
> }
>
> The two branches do approximately the same thing; the second version
> is almost twice as fast as the first on my computer (a Core Duo).
>
> -Eli
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits