[LLVMdev] llvm register reload/spilling around calls

[Roland Scheidegger]

Hi,

I was investigating some performance issues with llvm JIT-generated code
(x86_64), and looking at the assembly it indeed seemed quite suboptimal.
In particular, the code is basically implementing some kind of caching.
If there's a cache hit, the code just takes the value from the cache, if
not it will do whatever is necessary to update the cache - this is
expensive but happens only in about 1% of all cases and just calls a
function to do it.
So I saw that the code is doing lots of register spilling/reloading. Now
I understand that due to calling conventions, there's not really a way
to avoid this - I tried using coldcc but apparently the backend doesn't
implement it and hence this is ignored.
But what is really bad about this, is that the spilling/reloading ALWAYS
happens, regardless if the branch containing the call is taken or not.
Since the branch is almost never taken, that is obviously quite bad (but
even if the branch would be taken more often, which the compiler can't
know, I can't see why the reloading is always happening).
I'm not quite sure what performance impact this has, but it looks to me
like it definitely would make a difference, as the code not taking the
branch is quite simple.
I tried with both llvm 2.7 and 2.8, no difference.
So is there any optimization option I'm missing which could improve
this? Or is this simply the way things are (would that be considered a
bug?). If this is a known limitation, any ideas if it's possible to work
around that (by changing the affected jit code)?
I'm attaching the IR I've hack-extracted from the jit code (it might be
bogus but it compiles just fine). I think the assembly shows what I'm
talking about quite well (even has the comments about the
restore/spills). I used llc -O3 to compile.


Roland


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