[llvm-commits] [llvm] r69631 - /llvm/trunk/lib/Transforms/Scalar/LoopUnroll.cpp

[Dale Johannesen]

On Apr 20, 2009, at 3:25 PMPDT, Evan Cheng wrote:
> On Apr 20, 2009, at 3:19 PM, Dale Johannesen wrote:
>> Author: johannes
>> Date: Mon Apr 20 17:19:33 2009
>> New Revision: 69631
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=69631&view=rev
>> Log:
>> Adjust loop size estimate for full unrolling;
>> GEP's don't usually become instructions.
>
> Well... That's not entirely true. Each GEP can also become a several
> instructions. Is there a test case to demonstrate the merit of this
> change?
>
> Evan

It causes several loops in openssl(aes) to become completely unrolled,  
recovering 1/3 to 1/4 of the difference between llvm-gcc and gcc.    
All this is used for is determining whether to completely unroll  
loops, so it won't matter often.  But I have no problem doing this  
some other way, e.g. increasing the threshold, if you prefer.  Why  
don't we see how it does overnight?

> How about changing it to what InlineCost.cpp does?
>
>       } else if (const GetElementPtrInst *GEPI =
>                  dyn_cast<GetElementPtrInst>(II)) {
>         // If a GEP has all constant indices, it will probably be
> folded with
>         // a load/store.
>         bool AllConstant = true;
>         for (unsigned i = 1, e = GEPI->getNumOperands(); i != e; ++i)
>           if (!isa<ConstantInt>(GEPI->getOperand(i))) {
>             AllConstant = false;
>             break;
>           }
>         if (AllConstant) continue;

The subscripts involved are not constant (they're based off the  
induction variable, as you would expect in a loop), so that won't work.

>> Modified:
>>   llvm/trunk/lib/Transforms/Scalar/LoopUnroll.cpp
>>
>> Modified: llvm/trunk/lib/Transforms/Scalar/LoopUnroll.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopUnroll.cpp?rev=69631&r1=69630&r2=69631&view=diff
>>
>> =
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>> =====================================================================
>> --- llvm/trunk/lib/Transforms/Scalar/LoopUnroll.cpp (original)
>> +++ llvm/trunk/lib/Transforms/Scalar/LoopUnroll.cpp Mon Apr 20
>> 17:19:33 2009
>> @@ -89,6 +89,8 @@
>>        // Ignore instructions only used by the loop terminator.
>>      } else if (isa<DbgInfoIntrinsic>(I)) {
>>        // Ignore debug instructions
>> +      } else if (isa<GetElementPtrInst>(I) && I->hasOneUse()) {
>> +        // Ignore GEP as they generally are subsumed into a load or
>> store.
>>      } else if (isa<CallInst>(I)) {
>>        // Estimate size overhead introduced by call instructions
>> which
>>        // is higher than other instructions. Here 3 and 10 are magic
>>
>>
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