Re: [PATCH V5] Use reg mode to move sub blocks for parameters and returns

[Jeff Law <jeffreyalaw@gmail.com>]

On 5/3/23 23:49, guojiufu wrote:
> Hi,
> 
> On 2023-05-01 03:00, Jeff Law wrote:
>> On 3/16/23 21:39, Jiufu Guo wrote:
>>> Hi,
>>>
>>> When assigning a parameter to a variable, or assigning a variable to
>>> return value with struct type, and the parameter/return is passed
>>> through registers.
>>> For this kind of case, it would be better to use the nature mode of
>>> the registers to move the content for the assignment.
>>>
>>> As the example code (like code in PR65421):
>>>
>>> typedef struct SA {double a[3];} A;
>>> A ret_arg_pt (A *a) {return *a;} // on ppc64le, expect only 3 lfd(s)
>>> A ret_arg (A a) {return a;} // just empty fun body
>>> void st_arg (A a, A *p) {*p = a;} //only 3 stfd(s)
>>>
>>> Comparing with previous version:
>>> https://gcc.gnu.org/pipermail/gcc-patches/2023-January/609394.html
>>> This version refine code to eliminated reductant code in  the sub
>>> routine "move_sub_blocks".
>>>
>>> Bootstrap and regtest pass on ppc64{,le}.
>>> Is this ok for trunk?
>>>
> ...
> 
>>> diff --git a/gcc/expr.cc b/gcc/expr.cc
>>> index 15be1c8db99..97a7be9542e 100644
>>> --- a/gcc/expr.cc
>>> +++ b/gcc/expr.cc
>>> @@ -5559,6 +5559,41 @@ mem_ref_refers_to_non_mem_p (tree ref)
>>>     return non_mem_decl_p (base);
>>>   }
>>>   +/* Sub routine of expand_assignment, invoked when assigning from a
>>> +   parameter or assigning to a return val on struct type which may
>>> +   be passed through registers.  The mode of register is used to
>>> +   move the content for the assignment.
>>> +
>>> +   This routine generates code for expression FROM which is BLKmode,
>>> +   and move the generated content to TO_RTX by su-blocks in 
>>> SUB_MODE.  */
>>> +
>>> +static void
>>> +move_sub_blocks (rtx to_rtx, tree from, machine_mode sub_mode)
>>> +{
>>> +  gcc_assert (MEM_P (to_rtx));
>>> +
>>> +  HOST_WIDE_INT size = MEM_SIZE (to_rtx).to_constant ();
>> Consider the case of a BLKmode return value.  Isn't TO_RTX in this
>> case a BLKmode object?
> 
> Thanks for this question!
> 
> Yes, the mode of TO_RTX is BLKmode.
> As we know, when the function returns via registers, the mode of
> the `return-rtx` could also be BLKmode.  This patch is going to
> improve these kinds of cases.
> 
> For example:
> ```
> typedef struct FLOATS
> {
>    double a[3];
> } FLOATS;
> FLOATS ret_arg_pt (FLOATS *a){return *a;}
> ```
> 
> D.3952 = *a_2(D); //this patch enhance this assignment
> return D.3952;
> 
> The mode is BLKmode for the rtx of `D.3952` is BLKmode, and the
> rtx for "DECL_RESULT(current_function_decl)".  And the DECL_RESULT
> represents the return registers.
I didn't think MEM_SIZE  worked for BLKmode.  BUt looking at its 
definition, it's pulling the size out of the attributes rather than from 
the mode.  SO I guess there's a reasonable chance it's going to work :-)

OK for the trunk.

jeff