Re: [PATCH, i386] Avoid FP constants in wrong register set

[Uros Bizjak <uros at kss-loka dot si>]

Richard Henderson wrote:

> On Tue, Jan 25, 2005 at 10:43:25AM +0100, Uros Bizjak wrote:
>  
>
> > @@ -3755,6 +3760,10 @@ standard_80387_constant_rtx (int idx)
> > int
> > standard_sse_constant_p (rtx x)
> > {
> > +  /* Do not put x87 constants into SSE register for !TARGET_SSE_MATH.  */
> > +  if (!TARGET_SSE_MATH && SSE_FLOAT_MODE_P (GET_MODE (x)))
> > +    return 0;
> >    
>
> I would expect this to start loading 0 from memory instead of
> using xorps when talking about xmmintrin.h stuff.
>
>  
Hm, aren't these always vector mode (V4SF etc.) zeros? SSE_FLOAT_MODE_P 
shouldn't trigger for this.

However, patched standard_{sse,x87}_constant_p functions now break 
because of this change to ix86_preferred_reload_class():

 /* All classes can load zeros.  */
 if (x == CONST0_RTX (GET_MODE (x)))
   return class;

In this case, reg allocator sometimes can't decide whether x87 or SSE 
register should be used to load zero. If standard_{sse,x87} constant_p 
functions are patched to reject wrong {DF,SF}mode registers, 
-mfpmath=sse breaks because allocator already choosed wrong preferred 
register set:

pattern.cpp:2537: error: insn does not satisfy its constraints:
(insn 367 187 280 8 (set (reg/v:DF 8 st [orig:76 cf ] [76])
       (const_double:DF 0 [0x0] 0.0 [0x0.0p+0])) 64 {*movdf_nointeger} 
(nil)
   (nil))

I suggest that for TARGET_SSE_MATH, SSE registers are preferred for 
{DF,SF}mode constants (including zero) and vice versa for -mfpmath=387. 
Otherwise, these situations can occur:

...
80fe5bc:    d9 ee                    fldz  
80fe5be:    dd 5c 24 40              fstpl  0x40(%esp,1)
80fe5c2:    f2 0f 10 5c 24 40        movsd  0x40(%esp,1),%xmm3
...

and

...
804c4d7:    d9 e8                    fld1  
804c4d9:    dd 5c 24 28              fstpl  0x28(%esp,1)
804c4dd:    f2 0f 10 64 24 28        movsd  0x28(%esp,1),%xmm4
...

If constants would be  _forced_ into XMM register, further calculations 
wouldn't load constant from memory, because it will be already in the 
XMM register.

Uros.