[llvm-commits] Fold() function on TargetFolder class - removal of redundant constantexpr

[Nick Lewycky]

Jin Gu Kang wrote:
> Hi Duncan
>
> I send patches about NoFolder.

The idea behind llvm::NoFolder is that you create instructions instead 
of constants; that's what prevents the folding. When IRBuilder<true, 
NoFolder>.CreateAdd(i32 1, i32 1) is called it should create an add 
instruction (instead of a constant) with i32 1 and i32 1 as arguments, 
and no folding will occur.

Adding a DoFold argument to every Constant constructor is not an 
acceptable solution to commit to LLVM.

Nick

> NoFolder's functions return pointer of Constant because llvm-gcc's codes which
> call TheFolder's functions wait for pointer of Constant. and some of get() functions
> in ConstantExpr have a additional argument "DoFold" which decide to enable or
> disable fold. This implementation may be inefficient because there are additional
> arguments and 'if' statements. Please comment about this.
>
> In this patch, bitcast is folded unconditionally due to a little of problems.
> For example, EmitSWITCH_EXPR() on llvm-gcc-4.2 changes case's low
> and high values to ConstantInt which is folded to integer constant.and
> The part of codes does not accept unfolded bitcast ConstantExpr.
> (Because ConvertINTEGER_CST() uses Type::getInt64Ty(Context) to make
> ConstantInt, bitcast is generated on types with other bitwidths)
>
> I completed compilation of llvm-gcc-4.2 on x86-32bit and ARM using this patch
> and tested test-suite. There are not compilation and execution errors on x86-32bit.
>
> Patch files are as following:
> 1. llvm patch
> 2. llvm-gcc test patch - insertion of define "LLVM_TEST_NOFOLDER"
>
> Please review these patches.
>
> Thanks,
> Jin-Gu Kang
> ________________________________________
> From: Duncan Sands [baldrick at free.fr]
> Sent: Wednesday, February 23, 2011 5:57 PM
> To: Jin Gu Kang
> Cc: Chris Lattner; llvm-commits at cs.uiuc.edu
> Subject: Re: [llvm-commits] Fold() function on TargetFolder class - removal of redundant constantexpr
>
> Hi Jin Gu Kang,
>
>> while processing "kang.temp.e = 1" in TreeToLLVM::EmitLV_COMPONENT_REF() function on llvm-gcc-4.2,
>> StructAddrLV.Ptr which is returned from EmitLV() is bitcast ConstantExpr because type of kang's initializer is different
>> from type of struct test and this StructAddrLV.Ptr is used as an argument by CreateStructGEP().
>> Later, this bitcast ConstantExpr is disappeared by SymbolicallyEvaluateGEP() like above ll code for getelementptr.
>>
>> so I suggested to keep this bitcast ConstantExpr in order to represent meaning of conversion.
>
> it is true that disabling all constant folding may be helpful for beginners who
> want to read the bitcode and understand how it relates to the original C.  This
> is not so easy to achieve however because a minimal amount of constant folding
> always occurs.  For example you can change llvm-gcc from using TargetFolder to
> ConstantFolder.  This will result in less constant folding because it will no
> longer use information about the target when constant folding.  However the fold
> you mentioned (bitcast turned into deeper GEP) will occur anyway because doing
> it does not require knowing the target.  There is also a NoFolder class which
> does no folding whatsoever, by creating instructions rather than constants, but
> I hear that it doesn't work anymore (it used to, but perhaps bitrotted because
> no one uses it).  If you want to disable all constant folding in llvm-gcc then
> I suggest you change all instances of TargetFolder to NoFolder in
> llvm-internal.h and llvm-backend.cpp.  This line
>     TheFolder = new TargetFolder(TheTarget->getTargetData());
> probably needs to become something like this:
>     TheFolder = new NoFolder(getGlobalContext());
> However as I mentioned it probably will require some work on IRBuilder and
> NoFolder to get it working properly.  If you succeed please send in a patch.
>
> Ciao, Duncan.
>
>
>
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