[LLVMdev] Testing LLVM on OS X

[Patrick Flanagan]

Sorry for the delayed response, we're having finals and stuff here so 
I've been pretty distracted studying for those. I will grab the latest 
from CVS tonight and rerun that and see what the difference is.

Patrick

On May 9, 2004, at 4:53 PM, Chris Lattner wrote:

> On Tue, 4 May 2004, Chris Lattner wrote:
>
>> On Tue, 4 May 2004, Chris Lattner wrote:
>>> I suspect that a large reason that LLVM does worst than a native C
>>> compiler with the CBE+GCC is that LLVM generates very low-level C 
>>> code,
>>> and I'm not convinced that GCC is doing a very good job (ie, without
>>> syntactic loops).
>>
>> Yup, this is EXACTLY what is going on.
>>
>> I took this very simple C function:
>>
>> int Array[1000];
>> void test(int X) {
>>   int i;
>>   for (i = 0; i < 1000; ++i)
>>     Array[i] += X;
>> }
>>
>> Compile with -O3 on OS/X gave me this:
>>
>> _test:
>>         mflr r5
>>         bcl 20,31,"L00000000001$pb"
>> "L00000000001$pb":
>>         mflr r2
>>         mtlr r5
>>         addis r4,r2,ha16(L_Array$non_lazy_ptr-"L00000000001$pb")
>>         li r2,0
>>         lwz r9,lo16(L_Array$non_lazy_ptr-"L00000000001$pb")(r4)
>>         li r4,1000
>>         mtctr r4
>> L9:
>>         lwzx r7,r2,r9          ; load
>>         add r6,r7,r3           ; add
>>         stwx r6,r2,r9          ; store
>>         addi r2,r2,4           ; Increment pointer
>>         bdnz L9                ; Decrement count register, branch 
>> while not zero
>>         blr
>>
>> This is nice code, good GCC.  :)
>
> Okay, I changed the C backend to emit syntactic loops around the real
> loops, and it seems to make a big difference.  LLVM now generates this
> code (note that the actual loop is not actually responsible for control
> flow, it's unreachable):
>
> void test(int l7_X) {
>   unsigned l8_indvar;
>   unsigned l8_indvar__PHI_TEMPORARY;
>   int *l14_tmp_2E_4;
>   int l7_tmp_2E_7;
>   unsigned l8_indvar_2E_next;
>
>   l8_indvar__PHI_TEMPORARY = 0u;   /* for PHI node */
>   goto l13_no_exit;
>
>   do {     /* Syntactic loop 'no_exit' to make GCC happy */
> l13_no_exit:
>   l8_indvar = l8_indvar__PHI_TEMPORARY;
>   l14_tmp_2E_4 = &Array[l8_indvar];
>   l7_tmp_2E_7 = *l14_tmp_2E_4;
>   *l14_tmp_2E_4 = (l7_tmp_2E_7 + l7_X);
>   l8_indvar_2E_next = l8_indvar + 1u;
>   l8_indvar__PHI_TEMPORARY = l8_indvar_2E_next;   /* for PHI node */
>   if ((l8_indvar_2E_next == 1000u)) {
>     goto l13_return;
>   } else {
>     goto l13_no_exit;
>   }
>
>   } while (1); /* end of syntactic loop 'no_exit' */
> l13_return:
>   return;
> }
>
> Instead of:
>
>> void test(int l7_X) {
>>   unsigned l8_indvar;
>>   unsigned l8_indvar__PHI_TEMPORARY;
>>   int *l14_tmp_2E_5;
>>   int l7_tmp_2E_9;
>>   unsigned l8_indvar_2E_next;
>>
>>   l8_indvar__PHI_TEMPORARY = 0u;   /* for PHI node */
>>
>> l13_no_exit:
>>   l8_indvar = l8_indvar__PHI_TEMPORARY;
>>   l14_tmp_2E_5 = &Array[l8_indvar];
>>   l7_tmp_2E_9 = *l14_tmp_2E_5;
>>   *l14_tmp_2E_5 = (l7_tmp_2E_9 + l7_X);
>>   l8_indvar_2E_next = l8_indvar + 1u;
>>   if (!(l8_indvar_2E_next == 1000u)) {
>>     l8_indvar__PHI_TEMPORARY = l8_indvar_2E_next;   /* for PHI node */
>>     goto l13_no_exit;
>>   }
>>   return;
>> }
>
> The new CBE generated code causes GCC to compile the function into:
>
> _test:
>         mflr r5
>         bcl 20,31,"L00000000001$pb"
> "L00000000001$pb":
>         mflr r4
>         mtlr r5
>         addis r2,r4,ha16(_Array-"L00000000001$pb")
>         li r4,1000
>         mtctr r4
>         la r9,lo16(_Array-"L00000000001$pb")(r2)
>         li r2,0
> L10:
> L2:
>         lwzx r7,r2,r9
>         add r6,r7,r3
>         stwx r6,r2,r9
>         addi r2,r2,4
>         bdnz L10
> L7:
>         blr
>
> ... which is exactly what we want.  Patrick, I would appreciate it if 
> you
> could rerun your tests on PPC and let me know if this helps.  :)
>
> -Chris
>
> -- 
> http://llvm.cs.uiuc.edu/
> http://www.nondot.org/~sabre/Projects/
>
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