https://bugs.llvm.org/show_bug.cgi?id=41443
Bug ID: 41443
Summary: Canonicalize (-X srem Y) to -(X srem Y)
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: chenzheng1...@hotmail.com
CC: llvm-bugs@lists.llvm.org
This is from code review comment of https://reviews.llvm.org/D60395
In that patch, canonicalization is done for '-X s/ Y', so it can save one div
for -(X/Y) and (-X/Y).
There should be same opportunity for srem.
----------------------------------------
Optimization: -X srem Y -> -(X srem Y)
Precondition: true
%o0 = sub nsw i8 0, %x
%r = srem i8 %o0, %y
=>
%n0 = srem i8 %x, %y
%r = sub nsw i8 0, %n0
Done: 1
Optimization is correct
(X srem -Y) is not equal to -(X srem Y).
----------------------------------------
Optimization: X srem -Y -> -(X srem Y)
Precondition: true
%t0 = sub nsw i8 0, %y
%r = sdiv i8 %x, %t0
=>
%n0 = sdiv i8 %x, %y
%r = sub i8 0, %n0
ERROR: Target introduces undefined behavior for i8 %r
Example:
i8 %x = poison
i8 %y = 0xff (255, -1)
i8 %t0 = 0x01 (1)
source: poison
target: undefined
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