[PATCH] MIPS: IEEE 754-2008 features support

[Richard Sandiford]

"Maciej W. Rozycki" <macro@codesourcery.com> writes:
>  The run-time selection options are -mnan=2008 and -mnan=legacy for the 
> 2008 NaN and the legacy NaN mode respectively.  The binary patterns of NaN 
> data produced by compiler builtins or otherwise is adjusted accordingly, 
> as is a `.nan' directive included now with the assembly source produced, 
> used by GAS to set the NAN2008 ELF file header flag appropriately.  These 
> options are also passed down to GAS by the GCC driver so that user 
> assembly sources have the said NAN2008 flag set correctly in the absence 
> of a `.nan' directive.  Finally the name of the dynamic linker is changed 
> accordingly if the driver is used to invoke the linker.

I was a bit surprised that a change to the NaN format requires a change
to the dynamic linker, but I suppose that's the consequence of forcing
every ELF object to fall on one side of the fence?

I assume eventually there'll be an -mmac={legacy|2008} option too,
although of course it's good that that's a separate patch.  But then
the question is: is there a plan for handling all 8 possible combinations
of -mnan, -mabs and -mmac?  How would the FCSR be set up?

Even if we do plan to have all three be separately configurable,
I imagine most people will want them all set the same way, so it would
probably make sense to have something like -mfp-model={legacy|2008}
for that case.  And then my inclination would be to start with that
option and only add the split-out versions if we have a specific
(rather than theoretical) use case for them.

The patch mostly looks good apart from that, but please use a single
enum for the 2008/legacy thing, both in mips.h and mips.opt.

> +;; On pre IEEE 754-2008 processors abs.fmt is an arithmetic instruction
> +;; and treats all NaN inputs as invalid; it does not clear their sign bit.
> +;; We therefore can't use abs.fmt unless in the IEEE 754-2008 mode or if
> +;; the signs of NaNs matter.

"don't matter".  Or maybe to avoid the double negative:

;; The "legacy" (as opposed to "2008") form of ABS.FMT is an arithmetic
;; instruction that treats all NaN inputs as invalid; it does not flip
;; their sign bit.  We therefore can't use that form if the signs of
;; NaNs matter.

> +These options control the treatment of the special not-a-number (NaN)
> +IEEE 754 floating-point data with the @code{abs.@i{fmt}} and
> +@code{neg.@i{fmt}} machine instructions.
> +
> +By default or when the @option{-mabs=legacy} is used the legacy
> +treatment is selected.  In this case these instructions are considered
> +arithmetic and avoided where correct operation is required and the
> +input operand might be a NaN.  A longer sequence of instructions that
> +manipulate the sign bit of floating-point datum manually is used
> +instead unless the @option{-ffinite-math-only} option has also been
> +specified.
> +
> +The @option{-mabs=2008} option selects the IEEE 754-2008 treatment.  In

I think it reads better without the "-2008" bit, since even the 1985
version suggested this behaviour in the appendix.

> +The @option{-mnan=legacy} option selects the legacy encoding.  In this
> +case quiet NaNs (qNaNs) are denoted by the first bit of their trailing
> +significand field being 0, whereas signalling NaNs (sNaNs) are denoted
> +by the first bit of their trailing significand field being 1.
> +
> +The @option{-mnan=2008} option selects the IEEE 754-2008 encoding.  In
> +this case qNaNs are denoted by the first bit of their trailing
> +significand field being 1, whereas sNaNs are denoted by the first bit of
> +their trailing significand field being 0.

Very minor, but I'd prefer this without the "trailing"s.

> +These options are only supported on targets using the ELF binary format.

Let's drop that bit.  We don't support non-ELF on MIPS.

> +/* { dg-do compile } */
> +/* { dg-options "-mhard-float -mabs=2008" } */

Please instead make "-mabs=" imply "-mhard-float" in mips.exp.
(It is an "imply"; you can still test for -msoft-float -mabs=...
if you want.)

> +# Return 1 if this is a MIPS target supporting -mnan=.
> +# Old versions of binutils may not support this option.
> +
> +proc check_effective_target_mips_nan { } {
> +    if { ![istarget mips*-*-*] } {
> +	return 0
> +    }
> +    return [check_no_compiler_messages mips_nan object {
> +	int dummy;
> +    } "-mnan=2008"]
> +}

The tests you added are dg-do compile tests, which stop after assembly
generation, so this guard shouldn't be needed.  FWIW, if you did have
dg-do assemble or dg-do run tests, the mips.exp approach would be to
downgrade them to dg-do compile rather than skip them altogether.
We can add that later if we need it though...

Thanks,
Richard