On Mon, Feb 04, 2002 at 09:46:45AM +0000, Dominic Sweetman wrote: > > cgd@broadcom.com (cgd@broadcom.com) writes: > > > Branch-likely instructions probably _do_ buy you something (at > > least, slightly less code size) on some CPUs, probably even some > > CPUs which are still being produced. > > Here's how branch likely is used to improve performance in a simple > MIPS CPU, and why it has no effect on code size. > > You start off with this: > > loopstart: insn 1 > insn 2 > .... > insn last > branch to loopstart > nop > > In small loops, the last instruction in the loop might well calculate > the branch condition, so it can't be moved into the delay slot of the > loop-closing branch. That puts a no-op into every loop iteration. > With branch-likely, you can transform the loop to > > loopstart: insn 1 > loop2: insn 2 > .... > insn last > branch-likely loop2 > insn 1 (copy) > > The nop is replaced by a duplicate instruction from the top > of the loop. Good for performance, no effect on code size. > > Builders of clever modern CPUs full of branch prediction hardware, > multiple instruction issue and instruction re-ordering find the > coupling of the branch-likely to the following instruction makes their > CPUs more complicated. That's why MIPS have warned that the > instructions will be removed from some future version of the MIPS > instruction set. I can change glibc not to use branch-likely without using nop. But it may require one or two instructions outside of the loop. Should I do it given what we know now? H.J.
