[llvm-bugs] [Bug 32379] New: 64-bit bitfield and/or miscompiled on ARM

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Mar 22 10:35:18 PDT 2017


            Bug ID: 32379
           Summary: 64-bit bitfield and/or miscompiled on ARM
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: ARM
          Assignee: unassignedbugs at nondot.org
          Reporter: arielb1 at mail.tau.ac.il
                CC: llvm-bugs at lists.llvm.org

## Versions

This occurs on Debian's 3.9.1, Rust's fork of 3.9.1, and godbolt's 4.0 & trunk.

## STR

    $ cat trouble.c
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>

uint64_t test_function(uint64_t data, bool f1, bool f2) {
  if (f1) data &= ~2;
  if (f2) data |= 2;
  return data;

int main() {
  int data = (int)test_function(2, false, false);
  printf("%d\n", data);
  return 0;
    $ clang-3.9 test.c --target=armv7-unknown-linux-gnueabihf -O
    $ ./a.out

## Expected Result

Code should print 2

## Actual Result

Code prints 0, as the following assembly is emitted:
    test_function(unsigned long long, bool, bool):
            mov     r12, r0
            cmp     r2, #0
            bfc     r12, #1, #1
            moveq   r12, r0
            bfi     r12, r3, #1, #1
            mov     r0, r12
            bx      lr

The `bfi` instruction trashes the original value of r0.

This was originally reported as Rust bug

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