https://bugs.llvm.org/show_bug.cgi?id=44861
Bug ID: 44861
Summary: InstCombine incorrectly rewrites indices of gep
inbounds
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: juneyoung....@sf.snu.ac.kr
CC: llvm-bugs@lists.llvm.org
```
$ cat input.ll
declare void @g(i8*)
define void @foo(i8* %ptr, i1 %cc1, i1 %cc2, i1 %cc3, i32 %arg) {
entry:
br i1 %cc1, label %do.end, label %if.then
if.then:
br i1 %cc2, label %do.body, label %do.end
do.body:
%phi = phi i32 [ %arg, %if.then ]
%phi.ext = zext i32 %phi to i64
%ptr2 = getelementptr inbounds i8, i8* %ptr, i64 %phi.ext
%ptr3 = getelementptr inbounds i8, i8* %ptr2, i64 -1
call void @g(i8* %ptr3)
br i1 %cc3, label %do.end, label %if.then
do.end:
ret void
}
$ opt -instcombine -S -o - input.ll
declare void @g(i8*)
define void @foo(i8* %ptr, i1 %cc1, i1 %cc2, i1 %cc3, i32 %arg) {
br i1 %cc1, label %do.end, label %if.then
if.then: ; preds = %do.body, %entry
br i1 %cc2, label %do.body, label %do.end
do.body: ; preds = %if.then
%phi.ext = zext i32 %arg to i64
%ptr2 = getelementptr inbounds i8, i8* %ptr, i64 -1
%ptr3 = getelementptr inbounds i8, i8* %ptr2, i64 %phi.ext
call void @g(i8* nonnull %ptr3)
br i1 %cc3, label %do.end, label %if.then
do.end: ; preds = %do.body, %if.then,
%entry
ret void
}
```
If %arg is 1, ptr3 in the source is `gep inb (gep inb ptr, 1), -1` whereas ptr3
in the target is `gep inb (gep inb ptr, -1), 1`.
This is incorrect because ptr3 in the tgt may be poison whereas ptr3 in the
source isn't.
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