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

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

OK for trunk?

2016-07-20  Jiong Wang  <>

             * 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.


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

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

  add sp, sp, #imm_part1
  add sp, sp, #imm_part2

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