[llvm] [InstCombine] Add support for GEPs in `simplifyNonNullOperand` (PR #128365)

[Alexander Kornienko via llvm-commits]

alexfh wrote:

> Google is seeing some miscompiles that track back to this PR. I'm working on a sharable test case, but just wanted to put that out there in case someone else sees something similar.

I got an automatically reduced version of the code @Sterling-Augustine is talking about here. I'm not quite sure the interesting bits weren't distorted during reduction, but maybe this will help figuring out where the problem is: https://gcc.godbolt.org/z/YYcvG69Tf

```
$ cat reduced.ll
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%"S1" = type { %"S2", ptr, ptr, ptr, ptr, ptr }
%"S2" = type { ptr, ptr, ptr, ptr, ptr, %"S3", i32, i8, i8, ptr, ptr, ptr }
%"S3" = type { i32, i8, i8, %union.anon.55 }
%union.anon.55 = type { ptr }

@_f1 = external global %"S1"

define void @_g1() {
  %1 = call { ptr, i8 } @_h4()
  ret void
}

define { ptr, i8 } @_h4() {
  %1 = load volatile ptr, ptr null, align 8
  %2 = call ptr @_h3(ptr %1)
  %3 = call i1 @_h1(ptr %2)
  ret { ptr, i8 } zeroinitializer
}

define ptr @_h3(ptr %0) {
  %2 = call ptr @_f3()
  %3 = load volatile ptr, ptr null, align 8
  %4 = call i1 @_f4(ptr %2, ptr %3)
  %. = select i1 %4, ptr null, ptr %0
  ret ptr %.
}

define i1 @_h1(ptr %0) {
  %2 = getelementptr inbounds i8, ptr %0, i64 16
  %3 = call ptr @_h2(ptr %2)
  ret i1 false
}

define i1 @_f4(ptr %0, ptr %1) {
  %3 = call i1 @_f5(ptr %0, ptr %1)
  %4 = xor i1 %3, true
  ret i1 %4
}

define ptr @_f3() {
  %1 = call ptr @_f2()
  ret ptr %1
}

define i1 @_f5(ptr %0, ptr %1) {
  %3 = icmp eq ptr %0, %1
  ret i1 %3
}

define ptr @_f2() {
  ret ptr @_f1
}

declare ptr @_h2(ptr)

$ diff -u <(./clang-good -O3 -g0 -emit-llvm -S -o - reduced.ll) <(./clang-bad -O3 -g0 -emit-llvm -S -o - reduced.ll)
--- /dev/fd/63  2025-02-27 15:04:35.706601363 +0100
+++ /dev/fd/62  2025-02-27 15:04:35.710601401 +0100
@@ -13,9 +13,7 @@
 define void @_g1() local_unnamed_addr {
   %1 = load volatile ptr, ptr null, align 4294967296
   %2 = load volatile ptr, ptr null, align 4294967296
-  %.not.i.i = icmp eq ptr %2, @_f1
-  %..i.i = select i1 %.not.i.i, ptr %1, ptr null
-  %3 = getelementptr inbounds nuw i8, ptr %..i.i, i64 16
+  %3 = getelementptr inbounds nuw i8, ptr %1, i64 16
   %4 = tail call ptr @_h2(ptr nonnull %3)
   ret void
 }
@@ -23,9 +21,7 @@
 define { ptr, i8 } @_h4() local_unnamed_addr {
   %1 = load volatile ptr, ptr null, align 4294967296
   %2 = load volatile ptr, ptr null, align 4294967296
-  %.not.i = icmp eq ptr %2, @_f1
-  %..i = select i1 %.not.i, ptr %1, ptr null
-  %3 = getelementptr inbounds nuw i8, ptr %..i, i64 16
+  %3 = getelementptr inbounds nuw i8, ptr %1, i64 16
   %4 = tail call ptr @_h2(ptr nonnull %3)
   ret { ptr, i8 } zeroinitializer
 }
```

https://github.com/llvm/llvm-project/pull/128365