On 20/07/16 16:02, Jiong Wang wrote: > On 20/07/16 15:18, Richard Earnshaw (lists) wrote: >> On 20/07/16 14:03, Jiong Wang wrote: >>> Those stack adjustment sequences inside aarch64_expand_prologue/epilogue >>> are doing exactly what's aarch64_add_constant offered, except they also >>> need to be aware of dwarf generation. >>> >>> This patch teach existed aarch64_add_constant about dwarf generation and >>> currently SP register is supported. Whenever SP is updated, there >>> should be CFA update, we then mark these instructions as frame related, >>> and if the update is too complex for gcc to guess the adjustment, we >>> attach explicit annotation. >>> >>> Both dwarf frame info size and pro/epilogue scheduling are improved >>> after >>> this patch as aarch64_add_constant has better utilization of scratch >>> register. >>> >>> OK for trunk? >>> >>> gcc/ >>> 2016-07-20 Jiong Wang <jiong.w...@arm.com> >>> >>> * config/aarch64/aarch64.c (aarch64_add_constant): Mark >>> instruction as frame related when it is. Generate CFA >>> annotation when it's necessary. >>> (aarch64_expand_prologue): Use aarch64_add_constant. >>> (aarch64_expand_epilogue): Likewise. >>> >> Are you sure using aarch64_add_constant is unconditionally safe? Stack >> adjustments need to be done very carefully to ensure that we never >> transiently deallocate part of the stack. > > Richard, > > Thanks for the review, yes, I believe using aarch64_add_constant is > unconditionally > safe here. Because we have generated a stack tie to clobber the whole > memory thus > prevent any instruction which access stack be scheduled after that. > > The access to deallocated stack issue was there and fixed by > > https://gcc.gnu.org/ml/gcc-patches/2014-09/msg02292.html. > > aarch64_add_constant itself is generating the same instruction > sequences as the > original code, except for a few cases, it will prefer > > move scratch_reg, #imm > add sp, sp, scratch_reg > > than: > add sp, sp, #imm_part1 > add sp, sp, #imm_part2 > > > >
But can you guarantee we will never get and add and a sub in a single adjustment? If not, then we need to ensure the two instructions appear in the right order. R.
