On 12/06/2013 10:36 PM, Alex Bennée wrote: >> While correct, surely better to work with tcg and select on opc:invert to >> generate andc/orc/eqv? > > Shouldn't the TCG optimiser/back-end just be smart enough to figure it > out? It seems clearer to express the tcg ops in terms of the front-end's > meaning?
No, the TCG optimizer is really quite stupid. It only does constant folding and dead code elimination. No peepholing or combination sorts of opts. >> Also, isn't MOV (register) canonical for ORR (rn=31 && shift_amount=0), and >> MVN >> (register) canonical for ORN (rn=31 && shift_amount=0), and both therefore >> also >> worth a special case? > > I suspect I'm being overly cheeky to expect the optimiser to detect and > optimise for that case as the ZR is a const ;-) It would. But since register-register move is a rather common operation, it will pay off to not require the optimizer to clean up that special case. Thus I only recommend special casing the official aliases, not any operation that could mathematically be considered an identity. r~
