Re: [PATCH v3] EXPR: Emit an truncate if 31+ bits polluted for SImode

[Jeff Law <jeffreyalaw@gmail.com>]

On 12/24/23 05:24, Roger Sayle wrote:
> 
>>>> What's exceedingly weird is T_N_T_M_P (DImode, SImode) isn't
>>>> actually a truncation!  The output precision is first, the input
>>>> precision is second.  The docs explicitly state the output precision
>>>> should be smaller than the input precision (which makes sense for truncation).
>>>>
>>>> That's where I'd start with trying to untangle this mess.
>>>
>>> Thanks (both) for correcting my misunderstanding.
>>> At the very least might I suggest that we introduce a new
>>> TRULY_NOOP_EXTENSION_MODES_P target hook that MIPS can use for this
>>> purpose?  It'd help reduce confusion, and keep the
>>> documentation/function naming correct.
>>>
>>
>> Yes. It is good for me.
>> T_N_T_M_P is a really confusion naming.
> 
> Ignore my suggestion for a new target hook.  GCC already has one.
> You shouldn't be using TRULY_NOOP_TRUNCATION_MODES_P
> with incorrectly ordered arguments. The correct target hook is
> TARGET_MODE_REP_EXTENDED, which the MIPS backend correctly
> defines via mips_mode_rep_extended.
> 
> It's MIPS definition of (and interpretation of) mips_truly_noop_truncation
> that's suspect.
> 
> My latest theory is that these sign extensions should be:
> (set (reg:DI) (sign_extend:DI (truncate:SI (reg:DI))))
> and not
> (set (reg:DI) (sign_extend:DI (subreg:SI (reg:DI))))
In isolation these are the same.  I think the fact that the MIPS backend 
wipes out the sign extension turning the result into a NOP is what makes 
them different.

Of course that's kind of the point behind the TRULY_NOOP_TRUNCATION 
macro.  That's what allows the MIPS target to wipe out the sign extension.

ISTM this might be worth noting in the docs for TRULY_NOOP_TRUNCATION.

Jeff