SIMD & reduction on signed types emits sign-conversion diagnostic

[llvmbot]

Bugzilla Link	45959
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor

Extended Description

With -Wsign-conversion, an & reduction on an integer value will emit a diagnostic. Here is a simple test case:

  int foo(int* v);

  int foo(int* v) {
    int r = ~0;

    #pragma omp simd reduction(&:r)
    for (int i = 0 ; i < 8 ; i++) {
      r &= v[i];
    }

    return r;
  }

When compiling:

  $ clang -Weverything -fopenmp-simd -c -o simd.o simd.c
  simd.c:6:32: warning: implicit conversion changes signedness: 'unsigned int' to 'int' [-Wsign-conversion]
    #pragma omp simd reduction(&:r)
                               ^
  1 warning generated.

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#50798

[mr-c]

I can confirm this still happens with clang-17

$ clang-17 -Weverything -fopenmp-simd -c -o simd.o llvm-45304.c 
llvm-45304.c:6:32: warning: implicit conversion changes signedness: 'unsigned int' to 'int' [-Wsign-conversion]
    6 |   #pragma omp simd reduction(&:r)
      |                                ^
llvm-45304.c:8:10: warning: unsafe buffer access [-Wunsafe-buffer-usage]
    8 |     r &= v[i];
      |          ^
2 warnings generated.
$ clang-17 --version
Debian clang version 17.0.4 (1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin