On Sat, Jan 23, 2010 at 6:33 PM, Steve White <swh...@aip.de> wrote: > Hi, Andrew! > > Thanks for the suggestion, but it didn't make any difference for me. > Neither the speed nor the assembler was significantly altered. > > Which version of gcc did you use? Mine is 4.4.1. > > I threw everything at it: > gcc -std=c99 -Wall -pedantic -O3 -ffast-math -mmmx -msse -msse2 > -mfpmath=sse -mtune=pentium-m -o dt dt.c -lm -lc > I should say, I have tried a lot of other combinations. > I have never got gcc to perform well with this test. You will also > see that I thought of alignment, and tried to correct for that. > > Nevermind icc for the moment, with whatever trick it may be doing. > Why is the SSE2 division so slow, compared to multiplication? > > Change one character in the division test to make a multiplication test. > It is an order of magnitude difference in speed.
It's because multiplication latency is like 4 cycles while division is about 20, also one mutliplication can be issued per cycle while only every 17th instruction can be a division (AMD Fam10 values). GCC performs loop interchange with -ftree-loop-linear but the pass is scheduled in an unfortunate place so no further optimization happens. Richard. > Try it yourself! > > Thanks! > > On 23.01.10, Andrew Pinski wrote: >> On Sat, Jan 23, 2010 at 8:47 AM, Steve White <swh...@aip.de> wrote: >> > gcc has this (gcc -std=c99 -O3 -msse2 -mfpmath=sse -lm -S dt.c) >> > icc has this (icc -Wall -w2 -fast -c dt.c) >> >> icc's -fast is equivalent to gcc's -ffast-math option which you did >> not supply so you comparing apples to oranges. >> >> Note supplying -ffast-math will have gcc to pull out the division out >> of the loop which should speed up your program with some loss of >> precision. >> >> Thanks, >> Andrew Pinski >> > > -- > | - - - - - - - - - - - - - - - - - - - - - - - - - > | Steve White +49(331)7499-202 > | e-Science / AstroGrid-D Zi. 35 Bg. 20 > | - - - - - - - - - - - - - - - - - - - - - - - - - > | Astrophysikalisches Institut Potsdam (AIP) > | An der Sternwarte 16, D-14482 Potsdam > | > | Vorstand: Prof. Dr. Matthias Steinmetz, Peter A. Stolz > | > | Stiftung privaten Rechts, Stiftungsverzeichnis Brandenburg: III/7-71-026 > | - - - - - - - - - - - - - - - - - - - - - - - - - >
