On 27 October 2010 21:21, Jeff Law <l...@redhat.com> wrote: > On 10/27/10 12:15, Frederic Riss wrote: >> On 26 October 2010 16:22, Jeff Law<l...@redhat.com> wrote: >> >> The thing is the cprop pass doesn't look at insn costs while doing its >> job AFAICS. I'm interested to see how insn splitting can help with >> this if you don't care to explain. > > Certainly the SSA propagators don't use costing information; CSE on the > other hand does using costing info, but not always in the way you might > think (addresses in memory references for example are often backwards from > what you might think)
Care to extend on that use of the costing info? I followed the code path in gcse.c and e.g. do_local_cprop doesn't seem to care about costs. BTW, I'm on the 4.5 branch if that matters. > This is a common problem. For constants, its generally preferable to first > load them into registers and allow CSE to try and commonize the large > constants. Combine then will propagate single use constants into their use, > leaving the multi-use constants commonized. That's the situation I managed to get by preventing cprop to propagate large constants. Once propagated, no CSE pass will extract and commonize them again. > Register pressure isn't as much of a problem as you might think because > constants are relatively easy to rematerialize when there is excess register > pressure. That's what I thought, but I have yet to see GCC split the liveness of the commonized constants. When should that be done? Many thanks, Fred
