[llvm-dev] Reasoning about results of min and max with a constant

Carlos Liam via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 29 19:08:23 PDT 2016

Hi all,

Say we have this IR:

%1 = icmp slt i16 %x, 0
%.x = select i1 %1, i16 0, i16 %x

This is the canonical form of what is effectively max(x, 0).

From what I can tell LLVM has no facilities to determine from this code that %.x >= 0, so (for example) an SExt on %.x will not be converted to a ZExt.

I'm interested in seeing what sorts of changes would be needed to recognize this pattern in existing facilities (computeKnownBits, etc.) in order to more broadly apply optimizations that use those facilities.

Any insight would be appreciated. Thanks!

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