On Mon, 23 May 2011, Aurelien Jarno wrote:
> > Thanks for this patch series. Your approach to solve this issue is > really different than mine. Instead I added more state to the dead/live > states, and use them to mark some input deads even for global, and mark > some output arguments to be synced. This informations are then used > directly in the tcg_reg_alloc_* functions to make better usage of the > available registers. On the other hand my patch series only tries to > really lower the number of spills and doesn't try to make better spill > choices. > > I guess it would be a good idea that I continue with this approach (I > basically just have to fix a few cases were some regs are wrongly copied > back to memory), so that we can more easily compare the two approaches. > Your last patch is anyway interesting, having some statistics is always > something interesting. > > In any case I really think we need a better register allocator before we > can do any serious optimization passes like constant or copy propagation, > otherwise we end up with a lot of register in use for no real reason. > When I started working on this patch series I first wanted to write a better register allocator, something linear scan based. But TBs currently have quite specific and very simple structure. They have globals which are alive everywhere and temps, packed in a count of nests. Each nest is a result of translation of one guest instruction. Live ranges of temps in one nest always intersect, while live ranges of temps from different nests never intersect. As a result more sophisticated algorithm being applied to this test case works very similar to a simple greedy algorithm we have right now. Gathered statistics shows some interesting things too. I've run matrix multiplication benchmark (guest - ARM, host - x86, linux-user mode, with my patches applied) and here are the results: spill count 3916 real spills 32 spills at bb end 1023 spills at call: globals 2755 iarg passing 0 call cloobers 106 Real spills are spills generated by register allocator when it runs out of registers. They are less than 1% of all spills. Other tests show similar behavior. I think any further improvements to register allocator without leveling conventions about saving globals at calls and BB ends somehow is useless. Currently we are looking if we can pass some globals on registers through basic block boundaries (inside one TB of course).