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

[guojiufu <guojiufu@linux.ibm.com>]

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.

BR,
Jeff (Jiufu)
> 
> It looks pretty good at this point.
> 
> jeff