https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101993
--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> --- On Fri, 20 Aug 2021, wwwhhhyyy333 at gmail dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101993 > > --- Comment #2 from Hongyu Wang <wwwhhhyyy333 at gmail dot com> --- > (In reply to Richard Biener from comment #1) > > We can vectorize this with masked moves when using AVX2. clang seems to > > simply remove the test completely - C seems to guarantee that a + i is a > > valid pointer > > if any of a + i is accessed and thus a + i is never NULL. > > > > But then - just don't write such stupid checks? What real-world code was > > this testcase created from? > > It came from 538.imagick_r > > --- > #define GetPixelIndex(indexes) \ > ((indexes == (const unsigned short *) NULL) ? 0 : (*(indexes))) > > for (v=0; v < (ssize_t) kernel->height; v++) { > for (u=0; u < (ssize_t) kernel->width; u++, k--) { > if ( IsNaN(*k) ) continue; > result.red += (*k)*k_pixels[u].red; > result.green += (*k)*k_pixels[u].green; > result.blue += (*k)*k_pixels[u].blue; > result.opacity += (*k)*k_pixels[u].opacity; > if ( image->colorspace == CMYKColorspace) > result.index += (*k)*GetPixelIndex(k_indexes+u); > } > k_pixels += virt_width; > k_indexes += virt_width; > } > --- > > I extracted it to a small test in https://godbolt.org/z/G5h6nWvb5 > which can be vectorized by clang but not gcc due to such pattern. I see - thanks. Maybe the opportunity can be realized by invariant motion, for for (i = 0; i < n; ++i) if (ptr + i != 0) ... hoist any NULL check on a POINTER_PLUS_EXPR with invariant pointer part as a NULL check on the invariant pointer. Possibly doing by replacing all if (ptr + offset != 0) by if (ptr != 0) and then relying on unswitching of invariant conditions. That said, I'm sure this is viewed as too aggressive ... ?
