Re: [AArch64][3/3] Migrate aarch64_expand_prologue/epilogue to aarch64_add_constant

[Jiong Wang <jiong dot wang at foss dot arm dot com>]

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.wang@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