i386: Outline fast BF -> SF conversion and fix up sNaN handling in it [PR107628]

i386: Outline fast BF -> SF conversion and fix up sNaN handling in it [PR107628]

On Fri, Oct 21, 2022 at 10:23:14AM +0200, Uros Bizjak wrote:
> OK, but now we have two more copies of a function that effectively
> extends BF to SF. Can you please split this utility function out and
> use it here and in cbranchbf4/cstorebf4? I'm talking about this part:
>
> +      op = gen_lowpart (HImode, op1);
> +      if (CONST_INT_P (op))
> +       op = simplify_const_unary_operation (FLOAT_EXTEND, SFmode,
> +                                            op1, BFmode);
> +      else
> +       {
> +         rtx t1 = gen_reg_rtx (SImode);
> +         emit_insn (gen_zero_extendhisi2 (t1, op));
> +         emit_insn (gen_ashlsi3 (t1, t1, GEN_INT (16)));
> +         op = gen_lowpart (SFmode, t1);
> +       }
>
> Taking this a bit further, it looks like a generic function to extend
> BF to SF, when extendbfsf2 named function is not defined.
>
> The above could be a follow-up patch, the proposed patch is OK.

Sorry for the delay, only got to this now.
And I'm fixing the sNaN handling in it too.  If the argument is a BFmode sNaN
constant, we want in this case just a SFmode sNaN constant, but
simplify_const_unary_operation (FLOAT_EXTEND, ...)
in that case returns NULL (as normally conversions of a sNaN to some
other float type should raise an exception).  In this case we want
to bypass that, as we know the sNaN will be used immediately in the SFmode
comparison a few instructions later.  The patch fixes it by just
simplifying the lowpart to HImode and its zero extension to SImode, then
force into a pseudo and do the left shift and subreg to SFmode on the
pseudo.  CSE or combine can handle it later.

2022-11-19  Jakub Jelinek  <jakub@redhat.com>

	PR target/107628
	* config/i386/i386-protos.h (ix86_expand_fast_convert_bf_to_sf):
	Declare.
	* config/i386/i386-expand.cc (ix86_expand_fast_convert_bf_to_sf): New
	function.
	* config/i386/i386.md (cbranchbf4, cstorebf4): Use it.

	* gcc.target/i386/pr107628.c: New test.