https://llvm.org/bugs/show_bug.cgi?id=25585
Bug ID: 25585
Summary: Instcombine can't remove a PHI cycle when that cycle
should be replaced with a PHI
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: silviu.bara...@arm.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 15319
--> https://llvm.org/bugs/attachment.cgi?id=15319&action=edit
The reproducer
For the following IR, instcombine should be able to replace %p1 and %p2 with
%p0.
target triple = "aarch64-linux-gnueabi"
define i32 @f(i1 %c) {
entry:
br i1 %c, label %if.else, label %pre
if.else:
br label %pre
pre:
%p0 = phi i32 [ 0, %entry ], [ 1, %if.else ]
br label %loop
loop:
%p1 = phi i32 [ %p2, %loop ], [ %p0, %pre ]
%p2 = phi i32 [ %p0, %loop ] , [ %p1, %pre ]
br i1 %c, label %loop, label %exit
exit:
ret i32 %p2
}
We're already trying to do this, but the current phi cycle elimination
algorithm can't handle the case where the value that we should be replacing
other the PHIs with is another PHI (in this case %p).
When the value that we would replace the PHI cycle with is not a PHI, this
algorithm works as expected:
define i32 @g(i1 %c, i32 %val) {
entry:
br label %pre
pre:
br label %loop
loop:
%p1 = phi i32 [ %p2, %loop ], [ %val, %pre ]
%p2 = phi i32 [ %val, %loop ] , [ %p1, %pre ]
br i1 %c, label %loop, label %exit
exit:
ret i32 %p2
}
Reproduce with: opt -instcombine ind.ll -S -o -
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