[PATCH] Improve prepare_shrink_wrap to sink more instructions

Wang Jiong wong.kwongyuan.tools@gmail.com
Thu Sep 11 21:03:00 GMT 2014

2014-09-11 21:39 GMT+01:00 Jeff Law <law@redhat.com>:
> On 09/08/14 07:57, Jiong Wang wrote:
>>> Conceptually OK.  Some questions/concerns about the implementation.
>> Hi Jeff,
>>    thanks very much for your review.
>>> It seems to me that what you're trying to describe on the RHS is
>>    yes.
>>    and actually I am trying to detect all rtx which contains any number
>>    of RTX_CONST_OBJs and no more than one REG.
>>> 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.
>>    I am using the for_each_rtx because I want to scan "src" so that
>> any sub-operands are checked,  the number of REG and non-constant
>> objects are record in "reg_found" and "nonconst_found".  the embedded
>> register found also record in the "reg" field of the structure
>> "rtx_search_arg",

Hi Jeff,

 > Constants in this context are going to satisfy CONSTANT_P, you
don't need to manually verify them.  It will include simple
constants and constants which are built up out of multiple
instructions (symbolic constants in particular).

===>  thanks, will investigate this tomorrow.

> I suspect you still need the callback to verify the # of registers is just 1 so that the later tests work.

===>  the reg_found will record the # of registers, and there will be
a check to make sure no register or just 1 register in the src.
===>  so if "reg_found" is bigger than 1, then src will be assigned to
NULL_RTX, that we just stop the move.

          +      if (arg.nonconst_found
          +          || arg.reg_found > 1)
          +    src = NULL_RTX;

===> am I missing something here?

>However, please don't use for_each_rtx, we're moving away from that to a more efficient walker FOR_EACH_SUBRTX.

===> OK, will fix.  Thanks very much for review.


> Jeff

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