https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51017
--- Comment #10 from Alexander Peslyak <solar-gcc at openwall dot com> --- I decided to take a look at the generated code. Compared to 4.6.2, GCC 4.9.2 started generating lots of xorps, orps, andps, andnps where it previously generated pxor, por, pand, pandn. Changing those with: sed -i 's/xorps/pxor/g; s/orps/por/g; s/andps/pand/g; s/andnps/pandn/g' made no difference for performance on this machine (still 4.9.2's poor performance). The next suspect were the varieties of MOV instructions. In 4.9.2's generated code, there were 1319 movaps, 721 movups. In 4.6.2's, there were 1258 movaps, 465 movups. Simply changing all movups to movaps in 4.9.2's original code with sed (thus, with no other changes except for this one), resulting in a total of 2040 movaps, brought the performance to levels similar to GCC 4.4 and 4.5's (and is better than 4.6's, but worse than 4.3's). So movups appear to be the main culprit. The same hack for 4.6.2's code brought its performance almost to 4.3's level (still 5% worse, though), and significantly above 4.9.2's (so there's still some other, smaller regression with 4.9.2). Here are my new results: 4.1.0o - 2960K c/s, 28182 bytes, 1758 movaps, 0 movups 4.3.6o - 2956K c/s, 28229 bytes, 1755 movaps, 0 movups 4.4.6o - 2694K c/s, 29316 bytes, 1709 movaps, 7 movups 4.4.6h - 2714K c/s, 29316 bytes, 1716 movaps, 0 movups 4.5.3o - 2709K c/s, 29203 bytes, 1669 movaps, 0 movups 4.6.2o - 2121K c/s, 29624 bytes, 1258 movaps, 465 movups 4.6.2h - 2817K c/s, 29624 bytes, 1723 movaps, 0 movups 4.9.2o - 1852K c/s, 28256 bytes, 1319 movaps, 721 movups 4.9.2h - 2688K c/s, 28256 bytes, 2040 movaps, 0 movups "o" means original, "h" means hacked generated assembly code (all movups changed to movaps). (BTW, there were no movdqa/movdqu in any of these code versions.) Now I am wondering to what extent this is a GCC issue and to what extent it might be my source code's, if GCC is somehow unsure it can assume alignment. What are the conditions when GCC should in fact use movups? Is it intentional that newer versions of GCC are being more careful at this, resulting in worse performance?
