[PATCH] Improve prepare_shrink_wrap to sink more instructions

[Jeff Law]

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
>> REG_P || CONSTANT_P
>    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",
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).

I suspect you still need the callback to verify the # of registers is 
just 1 so that the later tests work.  However, please don't use 
for_each_rtx, we're moving away from that to a more efficient walker 
FOR_EACH_SUBRTX.

Jeff