[llvm-commits] [llvm] r138722 - in /llvm/trunk: lib/Analysis/ConstantFolding.cpp lib/Transforms/InstCombine/InstCombineCasts.cpp test/Transforms/InstCombine/cast.ll

[Rotem, Nadav]

Hi Chris, 

I am not sure I understand the following rule in isEliminableCastPair:

02115   // If either of the casts are a bitcast from scalar to vector, disallow the
02116   // merging.
02117   if ((firstOp == Instruction::BitCast &&
02118        isa<VectorType>(SrcTy) != isa<VectorType>(MidTy)) ||
02119       (secondOp == Instruction::BitCast &&
02120        isa<VectorType>(MidTy) != isa<VectorType>(DstTy)))
02121     return 0; // Disallowed

I think that it is okay to simplify the bitcast A->B->A regardless of the type. Am I missing something ?


Thanks, 
Nadav



-----Original Message-----
From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Chris Lattner
Sent: Sunday, August 28, 2011 19:36
To: Duncan Sands
Cc: llvm-commits at cs.uiuc.edu
Subject: Re: [llvm-commits] [llvm] r138722 - in /llvm/trunk: lib/Analysis/ConstantFolding.cpp lib/Transforms/InstCombine/InstCombineCasts.cpp test/Transforms/InstCombine/cast.ll


On Aug 28, 2011, at 5:07 AM, Duncan Sands wrote:

> Hi Nadav,
> 
>> Bitcasts are transitive. Bitcast-Bitcast-X becomes Bitcast-X.
> 
> sadly this is not true because you are only allowed to bitcast an mmx type
> to and from a vector type, but you can bitcast vector to and from integers
> etc.

See also CastInst::isEliminableCastPair.

-Chris
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