[llvm-bugs] [Bug 50118] New: llvm incorrectly compiles bitshift on x86

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50118

            Bug ID: 50118
           Summary: llvm incorrectly compiles bitshift on x86
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: corydoras at ridiculousfish.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, pengfei.wang at intel.com,
                    spatel+llvm at rotateright.com

clang miscompiles a bit shift at -O1 when targeting x86-64 or i386.

To reproduce, on an x86 machine:

```
#include <stdio.h>

__attribute__((noinline))
unsigned func(unsigned v0, unsigned v1, unsigned shift) {
  v1 <<= shift;
  return v1 | (v0 >> ((32u - shift) & 31u));
}

int main(void) {
  unsigned r = func(42, 0, 0);
  printf("%u\n", r);
  return 0;
}
```

Compile with `clang -O1 test.c && ./a.out`

This program should output 42, and it does so at -O0. However with -O1 it
outputs 0.

Reproes with clang 11.1 on Linux, clang 12.0 on x86 Mac. Does not repro with
clang 12 on ARM Mac. This does not repro on ARM so I'll direct to the x86
backend.

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