On 08/18/2015 02:24 AM, Artyom Tarasenko wrote: > The unoptimized case is a sequence of multiple cmp and branch > operations (likely created by a "case" statement in the original > source code), especially where cmp is in a delay slot of a branch > instruction.
Interesting. > I wonder whether we always have to finish a TB on a conditional jump. > Maybe it would make sense to translate further if a destination of a > jump is not too far from dc->pc? The definition of "not too far" is > indeed tricky. We can only handle two chained exits from a TB. If we continue past a conditional branch, we may well encounter a second conditional branch, which would leave us with three different exits from the TB. Something that may be interesting to play with, however, is to change the TB with which the insn in a delay slot is connected. For instance, we currently spend some amount of effort computing and saving the branch condition, so that we can then execute the delay slot, and afterwards use the saved branch condition to perform the branch. Another way of doing this is to immediately branch, exiting the TB. But we set up PC+NPC for the next TB such that the delay slot is the first insn that is executed within the next TB. In that way, the compare in the delay slot that you mention *is* in the same TB as the branch that uses it, allowing the case to be optimized. This could wind up creating more TBs than the current solution, so it's not clear that it would be a win. One can mitigate that somewhat by noticing the case where the delay slot is a nop. I do think it's worth an experiment. r~
