> At least on x86 it should also be a good idea to know which way > the branch is going to go, because it doesn't have explicit branch > hints, you really want to be able to optimize the cold branch > predictor case if converting from cmov to conditional branches.
x86 as of Pentium 4 does have branch hint instruction prefixes, but their use is somewhat discouraged: from http://softwarecommunity.intel.com/articles/eng/3431.htm: " The Pentium® 4 Processor introduced new instructions for adding static hints to branches. It is not recommended that a programmer use these instructions, as they add slightly to the size of the code and are static hints only. It is best to use a conditional branch in the manner that the static predictor expects, rather than adding these branch hints. In the event that a branch hint is necessary, the following instruction prefixes can be added before a branch instruction to change the way the static predictor behaves: * 0x3E statically predict a branch as taken * 0x2E statically predict a branch as not taken " see also section 2.1.1 Instruction Prefixes in http://download.intel.com/design/processor/manuals/253666.pdf: " Branch hint prefixes (2EH, 3EH) allow a program to give a hint to the processor about the most likely code path for a branch. Use these prefixes only with conditional branch instructions (Jcc). Other use of branch hint prefixes and/or other undefined opcodes with Intel 64 or IA-32 instructions is reserved; such use may cause unpre- dictable behavior. " ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs
