[LLVMdev] [PATCH] Minor fix to StackColoring to avoid needlessly clearing mem operands.

[Nadav Rotem]

The patch LGTM.  The StackColoring patch is still too conservative and I am consulting with Jakob and Andy about possible solutions.

On May 13, 2013, at 10:33 AM, Akira Hatanaka <ahatanak at gmail.com> wrote:

> This is the email I sent last week.
> 
> ---------- Forwarded message ----------
> From: Akira Hatanaka <ahatanak at gmail.com>
> Date: Wed, May 8, 2013 at 7:04 PM
> Subject: [PATCH] Minor fix to StackColoring to avoid needlessly clearing mem operands.
> To: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
> 
> 
> The following code snippet taken from StackColoring::remapInstructions clears a mem operand if it can't guarantee whether the memoperand's underlying value aliases with the merged allocas:
> 
> // Update the MachineMemOperand to use the new alloca.
>  522       for (MachineInstr::mmo_iterator MM = I->memoperands_begin(),
> ....
>          // Climb up and find the original alloca.
>  532         V = GetUnderlyingObject(V);
>  533         // If we did not find one, or if the one that we found is not in our
>  534         // map, then move on.
>  535         if (!V || !isa<AllocaInst>(V)) {
>  536           // Clear mem operand since we don't know for sure that it doesn't
>  537           // alias a merged alloca.
>  538           MMO->setValue(0);
>  539           continue;
>  540         }
> 
> The attached patch makes the code above less conservative. It avoids clearing a mem operand if its underlying value is a PseudoSourceValue and PseudoSourceValue::isConstant returns true. This enables MachineLICM to hoist loads from GOT out of a loop (see test case in stackcoloring-test.patch).
> 
> Please review.
> 
> Question: Why does it need to clear a mem operand if the underlying object is not an AllocaInst? I am not sure if I understand the reason for this.
> 
> 
> <stackcoloring.patch><stackcoloring-test.patch>_______________________________________________
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