[llvm-bugs] [Bug 50552] New: Early loop unrolling causes missed SLP vectorization

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50552

            Bug ID: 50552
           Summary: Early loop unrolling causes missed SLP vectorization
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

#include <stdint.h>

#include <cstddef>

#define STRIPE_WIDTH 32
#define STRIPE_MASK (STRIPE_WIDTH - 1)

static int16_t dither_line[2 * STRIPE_WIDTH] = {
#if STRIPE_WIDTH > 8
    8,  40, 8,  40, 8,  40, 8,  40, 8,  40, 8,  40, 8,  40, 8,  40,
    56, 24, 56, 24, 56, 24, 56, 24, 56, 24, 56, 24, 56, 24, 56, 24,
#else
    8, 40, 8, 40, 8, 40, 8, 40, 56, 24, 56, 24, 56, 24, 56, 24,
#endif
};

void ass_stripe_pack_c(uint8_t *dst, ptrdiff_t dst_stride, const int16_t *src,
                       uintptr_t width, uintptr_t height) {
  uint8_t *ptr = dst;
  for (uintptr_t y = 0; y < height; ++y) {
    const int16_t *dither = dither_line + (y & 1) * STRIPE_WIDTH;
    for (int k = 0; k < STRIPE_WIDTH; ++k)
      ptr[k] = (uint16_t)(src[k] - (src[k] >> 8) + dither[k]) >> 6;

    ptr += dst_stride;
    src += STRIPE_WIDTH;
  }
  dst += STRIPE_WIDTH;
}


-O3 -fno-unroll-loops: Vectorized by Loop vectorizer
-O3: No vectorization by SLP

Current codegen:
https://godbolt.org/z/ocxbjx9ss

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