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[InstCombine] fold zext of masked bit set/clear

Authored by spatel on May 29 2021, 5:52 AM.

Description

[InstCombine] fold zext of masked bit set/clear

This does not solve PR17101, but it is one of the
underlying diffs noted here:
https://bugs.llvm.org/show_bug.cgi?id=17101#c8

We could ease the one-use checks for the 'clear'
(no 'not' op) half of the transform, but I do not
know if that asymmetry would make things better
or worse.

Proofs:
https://rise4fun.com/Alive/uVB

Name: masked bit set
%sh1 = shl i32 1, %y
%and = and i32 %sh1, %x
%cmp = icmp ne i32 %and, 0
%r = zext i1 %cmp to i32

>

%s = lshr i32 %x, %y
%r = and i32 %s, 1

Name: masked bit clear
%sh1 = shl i32 1, %y
%and = and i32 %sh1, %x
%cmp = icmp eq i32 %and, 0
%r = zext i1 %cmp to i32

>

%xn = xor i32 %x, -1
%s = lshr i32 %xn, %y
%r = and i32 %s, 1

Note: this is a re-post of a patch that I committed at:
rGa041c4ec6f7a

The commit was reverted because it exposed another bug:
rGb212eb7159b40

But that has since been corrected with:
rG8a156d1c2795189 ( D101191 )

Differential Revision: https://reviews.llvm.org/D72396

Details

Committed
spatelMay 29 2021, 5:52 AM
Differential Revision
D72396: [InstCombine] fold zext of masked bit set/clear
Parents
rG52f297003601: [InstCombine] reduce code duplication; NFC
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