[llvm-bugs] [Bug 40371] New: IndVarSimplify incorrectly replaces a pointer with another

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40371

            Bug ID: 40371
           Summary: IndVarSimplify incorrectly replaces a pointer with
                    another
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            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

Created attachment 21352
  --> https://bugs.llvm.org/attachment.cgi?id=21352&action=edit
test.ll

$ cat test.ll

; while (p != q) p++;
; return p;

define i8* @iterate_until_p_becomes_q(i8* %p, i8* %q) {
label:
  br label %while.cond

while.cond:
  %p.addr.0 = phi i8* [ %p, %label ], [ %incdec.ptr, %while.body ]
  %cmp = icmp ne i8* %p.addr.0, %q
  br i1 %cmp, label %while.body, label %while.end

while.body:
  %incdec.ptr = getelementptr inbounds i8, i8* %p.addr.0, i32 1
  br label %while.cond

while.end:
  ret i8* %p.addr.0
}


After `opt test.ll -indvars -S -o -`, the returned value %p.addr.0 is replaced
with %q:


define i8* @iterate_until_p_becomes_q(i8* %p, i8* %q) {
label:
  br label %while.cond

while.cond:                                       ; preds = %while.body, %label
  %p.addr.0 = phi i8* [ %p, %label ], [ %incdec.ptr, %while.body ]
  %cmp = icmp ne i8* %p.addr.0, %q
  br i1 %cmp, label %while.body, label %while.end

while.body:                                       ; preds = %while.cond
  %incdec.ptr = getelementptr inbounds i8, i8* %p.addr.0, i32 1
  br label %while.cond

while.end:                                        ; preds = %while.cond
  ret i8* %q
}

This is incorrect because %p and %q may have different provenances. This causes
miscompilation in this case:

char p[4]; // 0x100
char q[4]; // 0x104
char *ptr = iterate_until_p_becomes_q(&p[0], &q[0]); // ptr = &p[4]
ptr--;
*ptr = 'a'; // After the optimization, this raises UB because ptr becomes
q[-1].


Interesting thing is that if %p and %q are defined as non-i8-type (e.g. i16*),
LLVM compiles correctly, by introducing uglygep (https://godbolt.org/z/Rk1dWt
).

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