On Mon, May 02, 2011 at 05:01:24PM +0100, Peter Maydell wrote: > On 20 April 2011 15:52, Peter Maydell <peter.mayd...@linaro.org> wrote: > > Move VLD/VST multiple into helper functions, as some cases can > > generate more TCG ops than the maximum per-instruction limit > > and certainly more than the recommended 20. > > I've had a review comment in private email that this patch > slows down ffmpeg by a little over 4% (in a linux-user execution > run where about 5% of instructions were an affected vld/vst). So > perhaps the right approach is to increase the max-ops-per-insn limit > and update the guidance in tcg/README instead? >
Does this patch fixes a real issue (ie most probably a crash), or it is just to make the arm target compliant with the README? Two remarks there: - The guidance in tcg/README is probably true for complex helpers in the sense of doing a lot of logic/arithmetic operations for which the TCG instruction set if not really rich and for which the compiler can do a lot better. When the instruction is emulated mostly by load/store ops, there isn't much possible optimizations left. - Your patch calls the slow version of the _mmu functions, which doesn't use the QEMU TLB. It is therefore normal to expect a slow down. Unfortunately the fast version of _mmu functions are not really usable from an helper, or at least they need a lot of code to convert an mmu index to the right function. It is something I have on my TODO list for some months, haven't found time to look at it. -- Aurelien Jarno GPG: 1024D/F1BCDB73 aurel...@aurel32.net http://www.aurel32.net
