[llvm-commits] [PATCH] Add the transformation ~(~X >>s Y) --> (X >>s Y)

Chris Lattner clattner at apple.com
Tue Jan 19 10:17:24 PST 2010

On Jan 19, 2010, at 8:36 AM, Edmund Grimley Evans wrote:

> Add this transformation in InstCombineAndOrXor.cpp:
> ~(~X >>s Y) --> (X >>s Y)
> It seems unlikely it could do any harm and it does have at least one
> use: together with other standard transformations it enables the
> following way of expressing an arithmetic shift right in portable C to
> be simplified into a single ASR operation:
> int asr(int a, int b)
> {
>  return a < 0 ? -(-a - 1 >> b) - 1 : a >> b;
> }

Looks great, I tweaked the testcases a bit and committed it as r93884.

The tweaks are to add the minimal testcase to an existing .ll file  
(xor2.ll) and make the "whole idiom recognizer" testcase into a new  
idioms.ll file, which only tests the instcombine part of the xform.

Thanks for the patch!


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