Hi Marc, On Thu, Oct 19, 2023 at 9:17 AM Marc Glisse <marc.glisse@xxxxxxxx> wrote: > > On Thu, 19 Oct 2023, Mathieu Malaterre via Gcc-help wrote: > > > After reading this SO post (*), I became curious as to what my gcc > > would do with the following code. It turns out that I cannot make any > > sense of the output: > > > > % gcc -fopt-info-missed -std=c11 -O3 -c generic.c > > generic.c:4:21: missed: couldn't vectorize loop > > generic.c:2:5: missed: not vectorized: relevant phi not supported: > > found_14 = PHI <found_5(7), 0(15)> > > If you try again with a snapshot of gcc-14, it does vectorize, although > the result doesn't seem as nice as what clang produces. > (in general I would also suggest adding -march=native or some recent arch) > > > Doing a quick search did not reveal anything meaningful. If my > > understanding is correct this is a basic information level (not meant > > for GCC developers): > > > > * https://gcc.gnu.org/onlinedocs/gcc/Developer-Options.html#index-fopt-info > > > > So my question is: what should the following sentence indicates > > > > ... > > missed: not vectorized: relevant phi not supported: found_14 = PHI > > <found_5(7), -1(15)> > > ... > > Some information is hard to translate to a user-understandable language > that refers to source code, this deep in the optimization, although in > this case the message is not very informative even for a gcc dev. > -fdump-tree-vect-details generates a file with more information, but that > can be hard to understand if you are not used to it. > > In this loop, some variables are 16 bits (haystack, needle), while the > reduction variable is 32 bits, and gcc has a hard time vectorizing mixed > sizes (and it doesn't realize that 'found' could be narrowed). If you > declare 'found' as 'short' instead, something different happens. Thanks very much for your answer, very helpful ! I was able to vectorize a different piece of C code, just using your trick. The type being reduced must be of the same type as the vector elements. I still do not understand what `PHI`, `found_14` or `<found_5(7), -1(15)>` refer to exactly, but I'll continue studying the *.vect output. > > % cat generic.c > > #include <stdint.h> > > int hasmatch(uint16_t needle, const uint16_t haystack[4]) { > > int found = 0; > > for (int i = 0; i < 4; ++i) { > > if (needle == haystack[i]) { > > found = 1; > > } > > } > > return found; > > } > > > > (*) https://stackoverflow.com/questions/74803190/fastest-way-to-find-16bit-match-in-a-4-element-short-array > > > > Thanks ! > > -- > Marc Glisse -- Mathieu
