Re: [PATCH, i386]: Rewrite x87 sqrt patterns, take 2a (macroized)

[Uros Bizjak <ubizjak at gmail dot com>]

Roger Sayle wrote:

> Hi Uros,
>
> On Fri, 24 Nov 2006, Uros Bizjak wrote:
>  
>
> > 2006-11-24  Uros Bizjak  <ubizjak@gmail.com>
> >
> > 	config/i386/i386.md (UNSPEC_TRUNC_NOOP): New unspec definition.
> > 	(X87MODEF): New mode macro.
> > 	(ssemodefsuffix): New mode attribute.
> > 	(truncxf<mode>2_i387_noop_unspec): New insn pattern.
> > 	(sqrt_extend<mode>xf2_i387): New insn pattern.
> > 	(sqrt<mode>2): For non-SSE sqrt, emit sqrt_extend<mode>xf2_i387
> > 	insn and truncate result back to original mode using
> > 	UNSPEC_TRUNC_NOOP truncation.
> > 	(*sqrt<mode>2_sse): Implement using SSEMODEF mode macro and
> > 	ssemodefsuffix mode attribute.
> > 	(*sqrtsf2_mixed, *sqrtsf2_i387, *sqrtdf2_mixed, *sqrtdf2_i387)
> > 	(*sqrtextendsfdf2_i387, *sqrtextendsfxf2_i387)
> > 	(*sqrtextenddfxf2_i387): Remove insn patterns.
> >
> > 	(fmodsf3, fmoddf3, remaindersf3, remainderdf3): Use noop
> > 	truncation patterns.
> >
> > 	reg-stack.c (get_true_reg): Handle UNSPEC_TRUNC_NOOP.
> >    
>
> This patch looks like a step in the right direction.  Ok for mainline.
>
> However my brain isn't working at the moment, so I'm unsure whether and
> how the UNSPEC_TRUNC_NOOP gets optimized away with -ffast-math.  For
>  
This part of reg-stack.c change to get_true_reg() lowers 
UNSPEC_TRUNC_NOOP to plain move:

+      case UNSPEC:
+       if (XINT (*pat, 1) == UNSPEC_TRUNC_NOOP)
+         {
+           pat = & XVECEXP (*pat, 0, 0);
+           break;
+         }
+

When operands[0] == operands[1], the move itself is removed, and at this 
point the optimization you described below takes place.
(During the discussion, it was agreed that builtins can extend returned 
value to XFmode without truncating the value back to its original mode, 
as the user can use -ffloat store in this case or -fno-builtin-XX).

> example, i386.c's output_387_reg_move doesn't appear to do anything
> clever when operands[0] == operands[1].  However even without
> understanding exactly what's going on, I agree with the philosophy
> of making the XFmode operations more explicit and using macros to
> simplify the backends patterns, and I trust you enough that your
> implementation is sound.  I can figure out how things work once its
> in the tree.
>  
Roger, thanks for your review!
Uros.