https://llvm.org/bugs/show_bug.cgi?id=28556
Bug ID: 28556
Summary: [InstCombine] SimplifyDemandedBits makes suboptimal
transform
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
define i5 @or_3_or_1(i3 %a) {
%or1 = or i3 %a, 3
%z = zext i3 %or1 to i5
%or2 = or i5 %z, 1
ret i5 %or2
}
The 2nd 'or' is covered by the 1st, but SimplifyDemandedBits calls
ShrinkDemandedConstant and causes this:
$ ./opt -instcombine -S sandwiches.ll
define i5 @or_3_or_1(i3 %a) {
%or1 = or i3 %a, 2 <--- shrink constant causing later 'or' to be needed
%z = zext i3 %or1 to i5
%or2 = or i5 %z, 1
ret i5 %or2
}
I don't know if this is working as intended or if this is a bug.
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