On Wed, 2009-11-11 at 14:18 +0000, Maciej W. Rozycki wrote: > On Wed, 11 Nov 2009, Wu Zhangjin wrote: > > > > No, register jumps cannot be predicted -- this is where the performance > > > goes on any serious processor -- the two extra instructions are nothing > > > compared to that. OTOH frame pointer calculations are pure arithmetic, so > > > you only lose time incurred by the instructions themselves. > > > > Yes, I only mean the -mlong-calls and the original -mno-long-calls with > > -pg. > > > > The orignal one looks like this: > > > > move ra, at > > jal _mcount > > > > The new one with -mlong-calls looks like this: > > > > lui v1, HI_16BIT_OF_MCOUNT > > addiu v1, v1, LOW_16BIT_OF_MCOUNT > > move ra, at > > jalr v1 > > > > both of them have a "jump" instruciton, so, only two lui, addiu added > > for -mlong-calls ;) > > > > what about the difference between that "jal _mcount" and "jalr v1"? > > As I say, the latter cannot be predicted and will incur a stall for any > decent pipeline. With the former the target address of the jump can be > calculated early and the instruction fetch unit can start feeding > instructions from there into the pipeline even before the jump has reached > the execution stage. > Get it, thanks! Regards, Wu Zhangjin
