On Tue, Apr 06, 2010 at 09:58:23AM -0700, Ian Lance Taylor wrote: > In the code the register is always accessed via a subreg, so the > lower-subregs pass thinks that it is OK to decompose the register. > Once it is decomposed, nothing is expected to put it back together. > > To fix this, you should probably look at simple_move in > lower-subreg.c. You will want it to return NULL_RTX for a vector load > or store. Perhaps it should check costs, or perhaps it should never > decompose explicit vector modes.
Compiling anything that uses doubles on powerpc e500v2 produces awful code due in part to lower-subregs (the register allocator doesn't help, either, but that's a different story). Code that looks like: rY:DI = r<ARG>:DI rX:DI = rY:DI (subreg:DF rZ:DI 0) = rX:DI <ARG> is a hard register for argument passing; the code looks equally awful inside of a function, too. The above gets lowered to: 1: r<Y>:SI = r<ARG>:SI 2: r<Y+1>:SI = r<ARG+1>:SI 3: (subreg:SI rX:DI 0) = r<Y>:SI 4: (subreg:SI rX:DI 4) = r<Y+1>:SI 5: (subreg:DF rZ:DI 0) = rX:DI which usually results in two stores and a load against the stack, rather than a single-instruction dealing entirely in registers. I realize e500v2 is not exactly a mainstream target, but perhaps a target hook is appropriate here? I suppose checking costs might achieve the same thing. -Nathan
