[llvm-bugs] [Bug 44321] New: InstCombine incorrectly folds 'gep(bitcast ptr), idx' into 'gep ptr, idx'

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44321

            Bug ID: 44321
           Summary: InstCombine incorrectly folds 'gep(bitcast ptr), idx'
                    into 'gep ptr, idx'
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Transformation Utilities
          Assignee: unassignedbugs at nondot.org
          Reporter: juneyoung.lee at sf.snu.ac.kr
                CC: llvm-bugs at lists.llvm.org

```
$ cat gep-vector.ll
define i32* @bitcast_vec_to_array_gep(<7 x i32>* %x, i64 %y, i64 %z) {
  %arr_ptr = bitcast <7 x i32>* %x to [7 x i32]*
  %gep = getelementptr [7 x i32], [7 x i32]* %arr_ptr, i64 %y, i64 %z
  ret i32* %gep
}
$ opt -instcombine gep-vector.ll -S -o -
define i32* @bitcast_vec_to_array_gep(<7 x i32>* %x, i64 %y, i64 %z) {
  %gep = getelementptr <7 x i32>, <7 x i32>* %x, i64 %y, i64 %z
  ret i32* %gep
}
```

This is incorrect because DataLayout::getTypeAllocSize(< 7 x i32 >) and
getTypeAllocSize([ 7 x i32 ]) may differ.

This can be double-checked by emitting assembly code before/after optimization.
https://godbolt.org/z/xB-p5u
Before optimization, rax = rdi + 28 * rsi + 4 * rdx
After instcombine, rax = rdi + 32 * rsi + 4 * rdx.

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