Nikita Popov 0b83c5a78f [InstCombine] Combine neg of shl of sub (PR44529)
Fixes https://bugs.llvm.org/show_bug.cgi?id=44529. We already have
a combine to sink a negation through a left-shift, but it currently
only works if the shift operand is negatable without creating any
instructions. This patch introduces freelyNegateValue() as a more
powerful extension of dyn_castNegVal(), which allows negating a
value as long as this doesn't end up increasing instruction count.
Specifically, this patch adds support for negating A-B to B-A.

This mechanism could in the future be extended to handle general
negation chains that a) start at a proper 0-X negation and b) only
require one operand to be freely negatable. This would end up as a
weaker form of D68408 aimed at the most obviously profitable subset
that eliminates a negation entirely.

Differential Revision: https://reviews.llvm.org/D72978
2020-01-22 23:03:58 +01:00
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This directory contains test cases for the instcombine transformation.  The
dated tests are actual bug tests, whereas the named tests are used to test
for features that the this pass should be capable of performing.