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? Steve Ellcey sell...@cavium.com
