[llvm-bugs] [Bug 44861] New: InstCombine incorrectly rewrites indices of gep inbounds

[via llvm-bugs]

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: unassignedbugs at nondot.org
          Reporter: juneyoung.lee at sf.snu.ac.kr
                CC: llvm-bugs at 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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