Re: [PATCH v2] Store_bit_field_1: Use SUBREG instead of REG if possible

[YunQiang Su <wzssyqa@gmail.com>]

Eric Botcazou <botcazou@adacore.com> 于2023年7月19日周三 17:45写道：
>
> > I don't see that.  That's definitely not what GCC expects here,
> > the left-most word of the doubleword should be unchanged.
> >
> > Your testcase should be a dg-do-run and probably more like
> >
> > NOMIPS16 int __attribute__((noipa)) test (const unsigned char *buf)
> > {
> >   int val;
> >   ((unsigned char*)&val)[0] = *buf++;
> >   ((unsigned char*)&val)[1] = *buf++;
> >   ((unsigned char*)&val)[2] = *buf++;
> >   ((unsigned char*)&val)[3] = *buf++;
> >   return val;
> > }
> > int main()
> > {
> >   int val = 0x01020304;
> >   val = test (&val);
> >   if (val != 0x01020304)
> >     abort ();
> > }
> >
> > not sure if I got endianess correct.  Now, the question is what
> > WORD_REGISTER_OPERATIONS implies for a bitfield insert and what
> > the MIPS ABI says for returning SImode.
>

MIPS N64 ABI uses 2 GPR for integer return values.
If the return value is SImode, the first v0 register is used, and it
must be sign-extended,
aka the bits[64-31] are all same.

Yes, it is same for signed and unsigned int32.

https://irix7.com/techpubs/007-2816-004.pdf
Page 6:
32-bit integer (int) parameters are always sign-extended when passed
in registers,
whether of signed or unsigned type. [This issue does not arise in the
o32-bit ABI.]


> WORD_REGISTER_OPERATIONS must *not* be taken account for bit-fields, see e;g.
> word_register_operation_p:
>
> /* Return true if X is an operation that always operates on the full
>    registers for WORD_REGISTER_OPERATIONS architectures.  */
>
> inline bool
> word_register_operation_p (const_rtx x)
> {
>   switch (GET_CODE (x))
>     {
>     case CONST_INT:
>     case ROTATE:
>     case ROTATERT:
>     case SIGN_EXTRACT:
>     case ZERO_EXTRACT:
>       return false;
>
>     default:
>       return true;
>     }
> }
>
> --
> Eric Botcazou
>
>


-- 
YunQiang Su