RE: [PATCH][GCC][AArch64] optimize float immediate moves (1 /4) - infrastructure.

[Tamar Christina <Tamar dot Christina at arm dot com>]

Hi James,

I have updated the patch and have responded to your question blow.

Ok for trunk?

Thanks,
Tamar

> >  static bool
> > @@ -5857,12 +5955,6 @@ aarch64_preferred_reload_class (rtx x,
> reg_class_t regclass)
> >        return NO_REGS;
> >      }
> >
> > -  /* If it's an integer immediate that MOVI can't handle, then
> > -     FP_REGS is not an option, so we return NO_REGS instead.  */
> > -  if (CONST_INT_P (x) && reg_class_subset_p (regclass, FP_REGS)
> > -      && !aarch64_simd_imm_scalar_p (x, GET_MODE (x)))
> > -    return NO_REGS;
> > -
> 
> I don't understand this relaxation could you explain what this achieves and
> why it is safe in this patch?

Because this should be left up to the pattern to decide what should happen and not reload.
Leaving the check here also means you'll do a reasonably expensive check twice for each constant
you can emit a move for.

Removing extra restriction on the constant classes leaves it up to aarch64_legitimate_constant_p to decide if
if the constant can be emitted as a move or should be forced to memory.
aarch64_legitimate_constant_p also calls aarch64_cannot_force_const_mem.

The documentation for TARGET_PREFERRED_RELOAD_CLASS also states: 

"One case where TARGET_PREFERRED_RELOAD_CLASS must not return rclass is if x is a legitimate constant which cannot be loaded into some register class. By returning NO_REGS you can force x into a memory location. For example, rs6000 can load immediate values into general-purpose registers, but does not have an instruction for loading an immediate value into a floating-point register, so TARGET_PREFERRED_RELOAD_CLASS returns NO_REGS when x is a floating-point constant. If the constant can't be loaded into any kind of register, code generation will be better if TARGET_LEGITIMATE_CONSTANT_P makes the constant illegitimate instead of using TARGET_PREFERRED_RELOAD_CLASS. "

So it seems that not only did the original constraint not add anything, we may also generate better code now.

Attachment: float-const-patch-1.patch
Description: float-const-patch-1.patch