On Tue, Feb 04, 2003 at 03:02:29PM +0100, Jan Hudec wrote: > On Mon, Feb 03, 2003 at 04:36:01PM +0100, Erik Mouw wrote: > > On Mon, Feb 03, 2003 at 04:46:28PM +0200, Gilad Benjamini wrote: > > > What's the exact syntax ? > > > > __asm__ ( "asm code" : "input variables" : "output variables" : "clobber" ); > > While assembly can't be optimized, this syntax allows gcc to optimize > the code around the assembly. It allows gcc to minimize number of stores > and loads needed around the assembly bit. Most important it allows gcc to decide which registers to assign to the instructions. On register-starved architectures like the x86 there are not that many choices, but on RISC architectures (which usually have at least 8 general purpose registers) this can make a difference. > > BIG FAT WARNING: Try to avoid assembly! It makes your code non-portable > > and hard to maintain! > > And a notice: you don't need gcc to optimize assembly, because you only > write code in assembly if you already optimized it better than gcc can > ever do. It also helps to rewrite the C code so the optimiser can do a better job. Properly rewritten code can sometimes yield four times faster code. For a (documented) example with properly rewritten C code, have a look at the CRC32 implementation I did for the blob ARM bootloader: http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/blob/blob/src/lib/crc32.c?rev=1.3&content-type=text/vnd.viewcvs-markup Erik -- J.A.K. (Erik) Mouw Email: J.A.K.Mouw@its.tudelft.nl mouw@nl.linux.org WWW: http://www-ict.its.tudelft.nl/~erik/
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