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#290838 - 26/11/2006 22:43 Boring C question
andym carpal tunnel Registered: 17/01/2002 Posts: 3996 Loc: Manchester UK	My google-fu is weak..... I've got two arrays of floats, I need to add them together. Currently I'm doing it like so: Code: for (j = 0 ; j < numFrames ; j++) tempVal[j] += buffer[j]; It works, but is there a quicker way? _________________________ Cheers, Andy M
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#290839 - 26/11/2006 22:50 Re: Boring C question [Re: andym]
mlord carpal tunnel Registered: 29/08/2000 Posts: 14496 Loc: Canada	Quote: for (j = 0 ; j < numFrames ; j++) tempVal[j] += buffer[j]; Unroll the loop (some compilers may partially do this already for you), and reverse the loop index to count down to zero (saves 1-2 instructions, usually). So, something like:Code: // Assumes numFrames is multiple of 4 for ( j = numFrames - 4; j != 0; j -= 4) { tempVal[j] = buffer[j]; tempVal[j+1] = buffer[j+1]; tempVal[j+2] = buffer[j+2]; tempVal[j+3] = buffer[j+3]; } Switching to pointers rather than table indexes might save another couple of instructions per iteration. But for real speed, one could use MMX/SSE(2) etc.. instructions. Any experts here? Cheers
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#290840 - 26/11/2006 23:20 Re: Boring C question [Re: mlord]
andym carpal tunnel Registered: 17/01/2002 Posts: 3996 Loc: Manchester UK	I now remember reading about this ages ago. I think the article it was in was about early computer graphics or something. I did also consider using pointers too. Will give them both a try. Thanks Mark! _________________________ Cheers, Andy M
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#290841 - 27/11/2006 07:36 Re: Boring C question [Re: mlord]
peter carpal tunnel Registered: 13/07/2000 Posts: 4180 Loc: Cambridge, England	Quote: But for real speed, one could use MMX/SSE(2) etc.. instructions. Any experts here? I'm no expert, but I do recall that modern GCC will use SSE instructions automatically in this sort of situation, especially if you've unrolled the loop in that way. It can't do that, though, unless it can prove that the two arrays don't overlap (consider "float buffer[101]; tempVal=&buffer[1]"). If one or more of buffer and tempVal is a local array variable, it can work out that they don't overlap; if this code is in a function where tempVal and buffer are both passed-in as float pointers, mark them "restrict" if you're sure they'll never overlap. Peter
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#290842 - 27/11/2006 12:14 Re: Boring C question [Re: peter]
mlord carpal tunnel Registered: 29/08/2000 Posts: 14496 Loc: Canada	Ahh.. good information there. Also, gcc might not use MMX/SSE(2) instructions unless you explicitly tell it that it's okay to do so. On my Pentium-M notebook, I generally provide these flags: "-march=pentium-m -mfpmath=sse -msse2 -malign-double"
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#290843 - 28/11/2006 06:42 Re: Boring C question [Re: mlord]
julf veteran Registered: 01/10/2001 Posts: 1307 Loc: Amsterdam, The Netherlands	Quote: On my Pentium-M notebook, I generally provide these flags: "-march=pentium-m -mfpmath=sse -msse2 -malign-double" By the way, have you come across a tool that would test your CPU and find out it's capabilities (and tell what optimisations make sense)?
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#290844 - 28/11/2006 12:43 Re: Boring C question [Re: julf]
mlord carpal tunnel Registered: 29/08/2000 Posts: 14496 Loc: Canada	Quote: Quote: On my Pentium-M notebook, I generally provide these flags: "-march=pentium-m -mfpmath=sse -msse2 -malign-double" By the way, have you come across a tool that would test your CPU and find out it's capabilities (and tell what optimisations make sense)? Under Linux: cat /proc/cpuinfo Cheers
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#290845 - 28/11/2006 14:01 Re: Boring C question [Re: mlord]
andy carpal tunnel Registered: 10/06/1999 Posts: 5916 Loc: Wivenhoe, Essex, UK	Quote: Under Linux: cat /proc/cpuinfo Under Windows: http://www.cpuid.com/cpuz.php _________________________ Remind me to change my signature to something more interesting someday
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#290846 - 28/11/2006 18:21 Re: Boring C question [Re: mlord]
julf veteran Registered: 01/10/2001 Posts: 1307 Loc: Amsterdam, The Netherlands	Quote: Under Linux: cat /proc/cpuinfo Duhh! Indeed, the first sign of Alzheimer's is... Hold on... It will come to me...
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