On May 15, 2013, at 1:27 AM, Richard Biener <richard.guent...@gmail.com> wrote:
> My question is, if you end up with
>
> (truncate:PSI (reg:SI 27))
>
> and then constant propagate 0x7fffffff to reg:SI 27, what does simplify-rtx.c
> do here? Truncate to _what_ precision exactly?
In my world, I change PSI to be P28SI, and then answer is that there are 28
bits. All ports that create partial int modes full well know the exact
precision, and that can be added. There is a ton of support already in the
compiler for this, and the last little bit in the mode def language and to
strap it in is light weight and obvious.
> Recent introduction of PTImode to rs6000 makes me think that the
> PARTIAL_INT_MODE()s are a hack to simply get another name for
> TImode (in this case).
>
> Thus to the middle-end it shouldn't be
>
> (truncate:PSI (reg:SI 27))
>
> but
>
> (set (reg:PSI 28 (reg:SI 27)))
>
> or maybe
>
> (subreg:PSI (reg:SI 27))
I see all forms as valid, but not the same.
(set (reg:P28SI (reg:SI 29))
(truncate:P28SI (reg:SI 27)))
is natural and reasonable, which combines two of the forms above. Using
truncate is fine.
> config/avr/avr-modes.def:FRACTIONAL_INT_MODE (PSI, 24, 3);
I never got any joy from FRACTIONAL_INT_MODE.
> So ... time to remove PARTIAL_INT_MODE ()s? Btw, I wonder why
No.
