[llvm-dev] Implementing a proposed InstCombine optimization

Carlos Liam via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 7 06:17:45 PDT 2016


I'm interested in implementing an InstCombine optimization that I discovered and verified with Alive-NJ (with the help of the authors of Alive-NJ). The optimization is described in Alive-NJ format as follows:

Name: xor->fsub
Pre: isSignBit(C)
%x = bitcast %A
%y = xor %x, C
%z = bitcast %y
%z = fsub -0.0, %A

Effectively the optimization targets code that casts a float to an int with the same width, XORs the sign bit, and casts back to float, and replaces it with a subtraction from -0.0.

I am not very familiar with C++ or the LLVM codebase so I would greatly appreciate some help in writing a patch adding this optimization.

Thanks in advance.

 - CL

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