On 05/26/2018 04:09 AM, Richard Sandiford wrote: > Steve Ellcey <sell...@cavium.com> writes: >> On Wed, 2018-05-16 at 22:11 +0100, Richard Sandiford wrote: >>> >>> TARGET_HARD_REGNO_CALL_PART_CLOBBERED is the only current way >>> of saying that an rtl instruction preserves the low part of a >>> register but clobbers the high part. We would need something like >>> Alan H's CLOBBER_HIGH patches to do it using explicit clobbers. >>> >>> Another approach would be to piggy-back on the -fipa-ra >>> infrastructure >>> and record that vector PCS functions only clobber Q0-Q7. If -fipa-ra >>> knows that a function doesn't clobber Q8-Q15 then that should >>> override >>> TARGET_HARD_REGNO_CALL_PART_CLOBBERED. (I'm not sure whether it does >>> in practice, but it should :-) And if it doesn't that's a bug that's >>> worth fixing for its own sake.) >>> >>> Thanks, >>> Richard >> >> Alan, >> >> I have been looking at your CLOBBER_HIGH patches to see if they >> might be helpful in implementing the ARM SIMD Vector ABI in GCC. >> I have also been looking at the -fipa-ra flag and how it works. >> >> I was wondering if you considered using the ipa-ra infrastructure >> for the SVE work that you are currently trying to support with >> the CLOBBER_HIGH macro? >> >> My current thought for the ABI work is to mark all the floating >> point / vector registers as caller saved (the lower half of V8-V15 >> are currently callee saved) and remove >> TARGET_HARD_REGNO_CALL_PART_CLOBBERED. >> This should work but would be inefficient. >> >> The next step would be to split get_call_reg_set_usage up into >> two functions so that I don't have to pass in a default set of >> registers. One function would return call_used_reg_set by >> default (but could return a smaller set if it had actual used >> register information) and the other would return regs_invalidated >> by_call by default (but could also return a smaller set). >> >> Next I would add a 'largest mode used' array to call_cgraph_rtl_info >> structure in addition to the current function_used_regs register >> set. >> >> Then I could turn the get_call_reg_set_usage replacement functions >> into target specific functions and with the information in the >> call_cgraph_rtl_info structure and any simd attribute information on >> a function I could modify what registers are really being used/invalidated >> without being saved. >> >> If the called function only uses the bottom half of a register it would not >> be marked as used/invalidated. If it uses the entire register and the >> function is not marked as simd, then the register would marked as >> used/invalidated. If the function was marked as simd the register would not >> be marked because a simd function would save both the upper and lower halves >> of a callee saved register (whereas a non simd function would only save the >> lower half). >> >> Does this sound like something that could be used in place of your >> CLOBBER_HIGH patch? > > One of the advantages of CLOBBER_HIGH is that it can be attached to > arbitrary instructions, not just calls. The motivating example was > tlsdesc_small_<mode>, which isn't treated as a call but as a normal > instruction. (And I don't think we want to change that, since it's much > easier for rtl optimisers to deal with normal instructions compared to > calls. In general a call is part of a longer sequence of instructions > that includes setting up arguments, etc.) Yea. I don't think we want to change tlsdesc*. Representing them as normal insns rather than calls seems reasonable to me.
Now that we're in stage1 I do want to revisit the CLOBBER_HIGH stuff. When we left things I think we were trying to decide between CLOBBER_HIGH and clobbering the appropriate subreg. The problem with the latter is the dataflow we compute is inaccurate (overly pessimistic) so that'd have to be fixed. Jeff
