https://bugs.llvm.org/show_bug.cgi?id=33470
Bug ID: 33470
Summary: EarlyCSE/computeKnownBits regression - assume of xor
i1 true, %val is being removed
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
Assignee: unassignedb...@nondot.org
Reporter: arie...@mail.tau.ac.il
CC: llvm-bugs@lists.llvm.org
It seems that LLVM trunk's early-cse is spuriously removing calls to
`llvm.assume(xor %val, 1)`. This can prevent further optimizations downstream.
This seems to occur because computeKnownBits sees that because of the assume,
the condition must be true, and therefore `%val` must be false. This is sound
but of course kills the assume. There's an `isEphemeralValueOf` check which is
supposed to prevent this situation, but it is not working.
## Meta
Verified locally on r304178. Verified not to occur on Rust's branch of 4.0.
## STR
--
; RUN: opt -S -early-cse < %s | FileCheck %s
declare i1 @food()
declare void @llvm.assume(i1)
define void @hungry() {
; CHECK-LABEL: @hungry
entry-block:
%0 = call i1 @food()
; CHECK: call void @llvm.assume
%1 = xor i1 %0, true
call void @llvm.assume(i1 %1)
ret void
}
--
## Expected Results
The code should remain as-is - there is nothing to simplify.
## Actual Results
The code is optimized into this assume-less version:
--
; ModuleID = '<stdin>'
source_filename = "<stdin>"
declare i1 @food()
; Function Attrs: nounwind
declare void @llvm.assume(i1) #0
define void @hungry() {
entry-block:
%0 = call i1 @food()
ret void
}
---
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