[PATCH] Improve prepare_shrink_wrap to sink more instructions

Jeff Law law@redhat.com
Fri Sep 5 19:48:00 GMT 2014


On 09/04/14 08:15, Jiong Wang wrote:
> currently, the instruction sink in "prepare_shrink_wrap" is a bit
> conservative
> that some further optimization opportunities have been missed.
>
> given the prologue use register A by:
>
>    (store A, [sp + offset])
>
> then given the entry_basic_block contains a simply register copy like:
>
>    (move A, B)
>
> current "prepare_shrink_wrap" will sink the move instruction as deep as
> it can,
> then the entry_basic_block could be marked as "don't need prologue".
>
> while if we replace "(move A, B)" into either one of
>
>    * "(move B, CONST_K)",
>    * "(move B, (plus A, CONST_K))"
>
> we still could do the same sink optimization, but *current gcc do not*.
>
> pattern like (move B, CONST_K) are very normal for some RISC targets.
>
> for example on AArch64, we could have the following pair:
>
>    adrp    x22, global_data_a
>    add     x0, x22, :lo12:global_data_a
>
> if "adrp" be scheduled into the entry_basic_block then the write of x22 may
> prevent shrink-wrap happen.
>
> when judge whether one instruction is sink-able,
> move_insn_for_shrink_wrap only
> accept simply reg copy that both dest and src are REG_P, while the second
> operand of adrp is actually a SYMBOL_REF, thus it's reject by the
> optimization.
>
> this patch relax the restriction on src to accept any one of the following:
>
>    + REG
>    + CONST_OBJ, like SYMBOL_REF
>    + combination of single REG and any other CONST_OBJs.
>      (reg def/use calculation will not affected by CONST_OBJs)
>
> RISC backend may benefit more from this relax, although there still
> be minor improvements on x86. for example, there are 17 more functions
> shrink-wrapped during x86-64 bootstrap, like sort_bucket in ira-color.c.
>
> test done
> =========
>    no regression on aarch64-none-elf bare-metal.
>    no regression on x86-64 check-gcc.
>    both aarch64 and x86-64 bootstrap OK.
>
> ok for install?
>
> 2014-09-04 Jiong Wang<jiong.wang@arm.com>
>
> gcc/
>    * shrink-wrap.c (rtx_search_arg): New structure type.
>    (rtx_search_arg_p): New typedef.
>    (count_reg_const): New callback function.
>    (move_insn_for_shrink_wrap): Relax the restriction on src operand.
>
Conceptually OK.  Some questions/concerns about the implementation.

It seems to me that what you're trying to describe on the RHS is
REG_P || CONSTANT_P

Note that CONSTANT_P will catch things like (high (symbol_ref)) and 
(lo_sum (reg) (symbol_ref)) which are often used to build addresses on 
some targets.

With that in mind, rather than using a for_each_rtx callback, test
if (REG_P (src) || CONSTANT_P (src))

Note that SRC, when it is a CONSTANT_P,  may have a variety of forms 
with embedded registers.  You'll need to verify that all of those 
registers are not assigned after the insn with the CONSTANT_P source 
operand.  Right now you only perform that check when SRC is a REG_P.

Jeff



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