[PATCH] Change illegitimate constant into memref of constant pool in change_zero_ext.

Segher Boessenkool segher@kernel.crashing.org
Tue Aug 24 21:12:38 GMT 2021


On Tue, Aug 24, 2021 at 04:55:30PM +0800, liuhongt wrote:
>   This patch extend change_zero_ext to change illegitimate constant
> into constant pool, this will enable simplification of below:

It should be in a separate function.  recog_for_combine will call both.
But not both for the same RTL!  This is important.  Originally, combine
tried only one thing for every combination of input instructions it got.
Every extra attempt causes quite a bit more garbage (not to mention it
takes time as well, recog isn't super cheap), so we should try to not
make recog_for_combine exponential in the variants it tries..

And of course the function name should always be descriptive of what a
function does :-)

>  change_zero_ext (rtx pat)
> @@ -11417,6 +11418,23 @@ change_zero_ext (rtx pat)
>      {
>        rtx x = **iter;
>        scalar_int_mode mode, inner_mode;
> +      machine_mode const_mode = GET_MODE (x);
> +
> +      /* Change illegitimate constant into memref of constant pool.  */
> +      if (CONSTANT_P (x)
> +	  && !const_vec_duplicate_p (x)

This is x86-specific?  It makes no sense in generic code, anyway.  Or if
it does it needs a big fat comment :-)

> +	  && const_mode != BLKmode
> +	  && GET_CODE (x) != HIGH
> +	  && GET_MODE_SIZE (const_mode).is_constant ()
> +	  && !targetm.legitimate_constant_p (const_mode, x)
> +	  && !targetm.cannot_force_const_mem (const_mode, x))

You should only test that it did not recog, and then force a constant
to memory.  You do not want to do this for every constant (rotate by 42
won't likely match better if you force 42 to memory) so some
sophistication will help here, but please do not make it target-

> +	{
> +	  x = force_const_mem (GET_MODE (x), x);

That mode is const_mode.

Ideally you will have some example where it benefits some other target,
too.  Running recog twice for a big fraction of all combine attempts,
for no benefit at all, is not a good idea.  The zext* thing is there
because combine *itself* creates a lot of extra zext*, whether those
exist for the target or not.  So this isn't obvious precedent (and that
wouldn't mean it is a good idea anyway ;-) )


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