[llvm-bugs] [Bug 39908] New: [InstCombine] GEP is wrongly optimized for 32-bit targets

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39908

            Bug ID: 39908
           Summary: [InstCombine] GEP is wrongly optimized for 32-bit
                    targets
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: mati865 at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Forwarding https://github.com/rust-lang/rust/issues/52026

Minimal example by neivv and rkruppe:
```
target datalayout = "p:32:32"

%S = type { [2 x i32] }

define i1 @foo([0 x %S]* %p, i32 %n) {
  %start.cast = bitcast [0 x %S]* %p to %S*
  %end = getelementptr inbounds [0 x %S], [0 x %S]* %p, i32 0, i32 %n, i32 0,
i32 0
  %end.cast = bitcast i32* %end to %S*
  %last = getelementptr inbounds %S, %S* %end.cast, i32 -1
  %cmp = icmp eq %S* %last, %start.cast
  ret i1 %cmp
}
```
GodBolt link: https://godbolt.org/z/C84Axg

For 32 bit targets it turns to:
define i1 @foo([0 x %S]* %p, i32 %n) {
  %cmp = icmp eq i32 %n, -536870911
  ret i1 %cmp
}

While 64 bit targets are optimized correctly:
define i1 @foo([0 x %S]* %p, i32 %n) {
  %cmp = icmp eq i32 %n, 1
  ret i1 %cmp
}

Explanation of the issue from
https://github.com/rust-lang/rust/issues/52026#issuecomment-403628144:
In this issue, the code is comparing roughly (S *)slice_start == (S
*)slice_start + slice_len - 1.
This function is extracting the constant byte offset, -8, and dividing it by 8,
sizeof(S). However, before that, compiling to a 32-bit target, the values are
masked by PtrSizeMask 0xFFFFFFFF, which discards the sign bit of int64 and
gives +4294967288, which divided by 8 is the same 536870911 which ends up being
in the result.

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