[llvm-bugs] [Bug 36689] New: Failure to zextload due to unused bits

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36689

            Bug ID: 36689
           Summary: Failure to zextload due to unused bits
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: llvm-bugs at lists.llvm.org, spatel+llvm at rotateright.com

We correctly convert to a zextload for a basic load + mask, but this fails if
we set any extra bits, which I assume is due to SimplifyDemandedBits getting in
the way:

clang -g0 -O3 -march=btver2 

int masklo(int *p) {
    return (*p & 65535);
}

int masklo_set(int *p) {
    return (*p & 65535) | 255;
}

define i32 @_Z6maskloPi(i32*) {
  %2 = load i32, i32* %0
  %3 = and i32 %2, 65535
  ret i32 %3
}

define i32 @_Z10masklo_setPi(i32*) {
  %2 = load i32, i32* %0
  %3 = and i32 %2, 65280
  %4 = or i32 %3, 255
  ret i32 %4
}

_Z6maskloPi:
  movzwl (%rdi), %eax
  retq

_Z10masklo_setPi:
  movl $65280, %eax # imm = 0xFF00
  andl (%rdi), %eax
  orl $255, %eax
  retq

Ideally this should have been:

_Z10masklo_setPi:
  movzwl (%rdi), %eax
  orl $255, %eax
  retq

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