Zack Rusin wrote: > On Friday 18 May 2007 05:07:08 am Keith Whitwell wrote: >> Sounds good. >> >> I've been thinking about LLVM and Mesa a little bit the last few days. >> If it can be made to work, it seems like a good way to go. There are a >> couple of practical issues that should be taken into account though: >> >> - Firstly, we need to continue supporting the existing drivers. In >> particular this means we need to generate programs they can understand >> (ie in the current Mesa IR), up until the point at which they all >> understand LLVM. > > Hmm, that's a good point. To be honest I haven't really thought about that. > >> - Secondly, I'm not yet convinced that LLVM in its current state really >> can support the wierdnesses of native GPU architectures. There are some >> assumptions that seem to be encoded in LLVM that don't apply on GPU's -- >> things like the availability of a general branch instruction, for instance. > > Yeah, I've been stressing over this a little bit as well. It's not so much > LLVM IR that stresses me as their optimization passes. LLVM IR is pretty > extensible and I thought about simply extending it with if/else constructs - > my main conceptual problem is that I was afraid that potentially arbitrary > optimization passes could destroy that structure.
It looks like conditional branches in the LLVM IR are somewhat structured. From http://llvm.org/docs/LangRef.html#terminators Test: %cond = icmp eq, i32 %a, %b br i1 %cond, label %IfEqual, label %IfUnequal IfEqual: ret i32 1 IfUnequal: ret i32 0 It might not be too hard to generate IF/ELSE/ENDIF GPU instructions from that. But if the optimizer changed the structure... hmmm. I was looking at LLVM's vector support a little more. There's basic vector add, sub, mul, divide and shuffling, but that's about it. GPU instruction sets have vector min/max, abs, lrp, dot product, frac and set-if-less/greater/equal, etc that are commonly used in shader code. I think it's important that the code generator emits those instructions, rather than scalar decompositions. It seems that either the LLVM IR would need some extensions for those things or the back-end code generator/instruction selector would have to look for patterns of scalar instructions and figure out where vector ops should be used. The later sounds unpleasant. -Brian ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ Mesa3d-dev mailing list Mesa3d-dev@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mesa3d-dev
