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[InstCombine] fold min/max tree with common operand (PR35717)

Authored by spatel on Jan 8 2018, 7:05 AM.

Description

[InstCombine] fold min/max tree with common operand (PR35717)

There is precedence for factorization transforms in instcombine for FP ops with fast-math.
We also have similar logic in foldSPFofSPF().

It would take more work to add this to reassociate because that's specialized for binops,
and min/max are not binops (or even single instructions). Also, I don't have evidence that
larger min/max trees than this exist in real code, but if we find that's true, we might
want to reorganize where/how we do this optimization.

In the motivating example from https://bugs.llvm.org/show_bug.cgi?id=35717 , we have:

int test(int xc, int xm, int xy) {

int xk;
if (xc < xm)
  xk = xc < xy ? xc : xy;
else
  xk = xm < xy ? xm : xy;
return xk;

}

This patch solves that problem because we recognize more min/max patterns after rL321672

https://rise4fun.com/Alive/Qjne
https://rise4fun.com/Alive/3yg

Differential Revision: https://reviews.llvm.org/D41603

llvm-svn: 321998

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