Re: RFA: Simplifying truncation and integer lowpart subregs

[Eric Botcazou <ebotcazou@adacore.com>]

> I think modelling it as a TRUNCATE operation is correct for
> !TRULY_NOOP_TRUNCATION (it's the bug that Andrew pointed out).
> And we shouldn't generate an actual TRUNCATE rtx for
> TRULY_NOOP_TRUNCATION (the thing about making
> simplify_gen_unary (TRUNCATE, ...) no worse than simplify_gen_subreg
> for those targets).  I suppose:
> 
>       /* We can't handle truncation to a partial integer mode here
>          because we don't know the real bitsize of the partial
>          integer mode.  */
>       if (GET_MODE_CLASS (mode) == MODE_PARTIAL_INT)
>         break;
> 
> might be a problem though; we should still allow a subreg to be
> generated.  Is that what you were thinking of, or something else?

I was thinking of the !TRULY_NOOP_TRUNCATION case, where the two operations 
aren't equivalent.  Generating TRUNCATE in simplify_subreg seems suspicious to 
me in this case but, if not doing it is the source of the bug, I guess I need 
to do some homework on this TRULY_NOOP_TRUNCATION stuff. :-)

Maybe add a blurb to the head comment of simplify_truncation, explaining that
it is valid to call the function both for TRUNCATEs and truncations to the 
lowpart, and why it is correct to generate new TRUNCATEs in the latter case.

-- 
Eric Botcazou