Re: [AArch64] Use scvtf fbits option where appropriate

[Richard Sandiford <richard.sandiford@arm.com>]

Wilco Dijkstra <Wilco.Dijkstra@arm.com> writes:
>  > +/* If X is a positive CONST_DOUBLE with a value that is the reciprocal of a
>  > +   power of 2 (i.e 1/2^n) return the number of float bits. e.g. for x==(1/2^n)
>  > +   return n. Otherwise return -1.  */
>  > +int
>  > +aarch64_fpconst_pow2_recip (rtx x)
>  > +{
>  > +  REAL_VALUE_TYPE r0;
>  > +
>  > +  if (!CONST_DOUBLE_P (x))
>  > +    return -1;
>  
>> CONST_DOUBLE can be used for things other than floating point.  You
>> should really check that the mode on the double in is in class MODE_FLOAT.
>  
> Several other functions (eg aarch64_fpconst_pow_of_2) do the same since
> this function is only called with HF/SF/DF mode. We could add an assert for
> SCALAR_FLOAT_MODE_P (but then aarch64_fpconst_pow_of_2 should do
> the same).

IMO we should leave it as-is.  aarch64.h has:

#define TARGET_SUPPORTS_WIDE_INT 1

which makes it invalid to use CONST_DOUBLE for anything other than
floating-point constants.  The handling of CONST_DOUBLEs with integer
modes is effectively compiled out in key places so it would be very hard
to create one accidentally.  And even if somehow we did, it would fail
noisily in other ways.

So I think it would be redundant to assert that CONST_DOUBLE has a float
mode here, much like we (rightly) don't assert that CONST_VECTORs have
vector modes.

Thanks,
Richard