| Issue |
159419
|
| Summary |
[clang] pointer subtraction should use ptrtoaddr rather than ptrtoint
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
bababuck
|
Not sure this is an issue or not, but seems incorrect from what I was able to gather.
>From the C-specification,
```
Pointer subtraction works by subtracting p1’s numeric value from p3’s, and dividing by target object size. The two pointer arguments should point into the same array.
```
My understanding is that pointer subtraction should return the number of elements between the two pointers. From this, it follows that the provenance of the pointer should not be considered, and thus a ` ptrtoaddr` instruction should be used.
However, `clang` currently lowers pointer subtraction using `ptrtoint`.
```
#include <stdint.h>
#include <stddef.h>
ptrdiff_t foot(int8_t *a, int8_t *b) {
return a - b;
}
```
```
./bin/clang -O2 -S -emit-llvm -o ptr.ll ptr.c
```
```
; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
define dso_local noundef i64 @foot(ptr noundef %a, ptr noundef %b) local_unnamed_addr #0 {
entry:
%sub.ptr.lhs.cast = ptrtoint ptr %a to i64
%sub.ptr.rhs.cast = ptrtoint ptr %b to i64
%sub.ptr.sub = sub i64 %sub.ptr.lhs.cast, %sub.ptr.rhs.cast
ret i64 %sub.ptr.sub
}
```
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