On Sat, Nov 01, 2008 at 08:33:03PM +0000, Maciej W. Rozycki wrote: > > There are two ways we could handle this: > > > > - Make -mfix-r10000 require -mbranch-likely. (It mustn't _imply_ > > -mbranch-likely. It should simply check that -mbranch-likely is > > already in effect.) > > > > - Make -mfix-r10000 insert nops when -mbranch-likely is not in effect. > > If I recall right, these is something special about the pipeline in this > context making the branch-likely instructions the only ones that work. > Which would make the option you proposed first the only viable. I am not > absolutely sure and I have no reference handy. Perhaps Ralf or someone at > linux-mips will know? There are two possible workarounds. The other which IRIX and the Linux kernel are using is based on the branch-likely instruction. The way it works is that R10000 family processors have a fairly cheesy branch prediction for branch likely (unlike all MIPS32 and MIPS64 processors I know of!) which predicts branch likely instructions as always taken. So if a SC instruction succeeds the loop closure branch of the usual LL/SC loop will be miss-predicted and the pipeline restarted. The alternative is to put enough NOPs (upto 28) after the loop closure brach to avoid a sequence of 4 problematic instructions being active in the pipeline at the same time. SSNOP won't cut it btw. SSNOP don't have any influence on the predecode and reordering buffers - even assuming the R10000 actually honors SSNOP. Implementing the special treatment of SSNOP (which is encoded as SLL $0, $0, 1) just doesn't make sense for an R10000 calibre processor. Is gcc capable of guaranteeing a certain minimum number of instructions between one LL and another LL instruction? Then this knowledge could be used to avoid the branch likely or cut down the padding NOPs. Ralf
