[RFC,2/2] MIPS/GCC: IEEE Std 754 NaN interlinking support

Implement the GCC interface for IEEE Std 754 NaN interlinking, following 
"MIPS ABI Extension for IEEE Std 754 Non-Compliant Interlinking" 
<https://dmz-portal.mips.com/wiki/MIPS_ABI_-_NaN_Interlinking> and
the recommendations set out there as follows:

* implement driver and compiler command-line options to control the 
  compliance mode:

  -mieee=strict        -- to select the strict compliance mode enforcing 
                          compliance checks down to the static link and
                          therefore suitable for use cases where IEEE 
                          Std 754 compliance is critical,

  -mieee=relaxed-exec  -- to select the relaxed compliance mode for 
                          executable links only, and the strict 
                          compliance mode for other execution modes,
                          effectively producing objects and shared 
                          libraries still usable with software which is 
                          strictly compliant with IEEE Std 754,

  -mieee=relaxed       -- to select the relaxed compliance mode,

  -mieee=force-relaxed -- to select the relaxed compliance mode, 
                          additionally suppressing warnings from 
                          suspicious static links.

* implement corresponding configuration options to control the default 
  compliance mode:

  --with-ieee=strict       -- to select the strict compliance mode as 
                              the default,

  --with-ieee=relaxed-exec -- to select the relaxed compliance mode for 
                              executable links only as the default.

* implement compiler command-line options to control the NaN compliance 
  mode:

  -mrelaxed-nan=none  -- to output code suitable for use cases where 
                         IEEE Std 754 compliance is critical and 
                         therefore including `.ieee strict; .ieee warn' 
                         in assembly produced,

  -mrelaxed-nan=exec  -- to output code suitable for use cases where 
                         IEEE Std 754 compliance is not required, 
                         however still usable with software which is 
                         strictly compliant with IEEE Std 754, and 
                         therefore including `.ieee strict; .ieee 
                         nowarn' in assembly produced,

  -mrelaxed-nan=all   -- to output code suitable for use cases where 
                         IEEE Std 754 compliance is never required and 
                         therefore including `.ieee relaxed; .ieee 
                         nowarn' in assembly produced,

  -mrelaxed-nan=force -- same as `-mrelaxed-nan=all' to simplify 
                         handling in specs.

There is one issue with this change in that a `-mrelaxed-nan=' option 
supplied on a GCC command line does not override the default set with a 
`--with-ieee=' option.  This is because the driver stuffs the initial 
`-mieee=' option produced with OPTION_DEFAULT_SPECS *after* any 
`-mrelaxed-nan=' option provided by the user.  I take it as a driver bug
as user-supplied options are supposed to be placed last on the command 
line so as to allow any earlier or implied options to be overridden.
Therefore I am not going to address this problem with this change, the
user is advised to use `-mieee=' options wherever possible anyway.

	gcc/
	* flags.h (set_fast_math_flags): New prototype.
	* opts.c (set_fast_math_flags): Export.
	* common/config/mips/mips-common.c (mips_handle_option): Handle 
	`-mieee=' options.
	* config/mips/mips-opts.h (mips_relaxed_nan_setting): New enum.
	(mips_ieee_setting): Likewise.
	* config/mips/mips.h (OPTION_DEFAULT_SPECS): Add `ieee'.
	(ASM_SPEC): Handle `-mieee=' and `-mrelaxed-nan=' options.
	(LINK_SPEC): Likewise.
	[!HAVE_AS_IEEE] (HAVE_AS_IEEE): New macro.
	* config/mips/mips.c (mips_file_start): Handle `-mrelaxed-nan=' 
	options.
	* config/mips/mips.opt (mrelaxed-nan=, mieee=): New options.
	* doc/install.texi (Configuration): Add `--with-ieee=' options.
	* doc/invoke.texi (Option Summary) <MIPS Options>: Add 
	`-mrelaxed-nan=' and `-mieee=' options.
	(MIPS Options): Likewise.
	* config.gcc <mips*-*-*>: Handle `--with-ieee=' options.
	(all_defaults): Add `ieee'.
	* configure.ac <mips*-*-*>: Check for GAS `-mieee=' support.
	* config.in: Regenerate.
	* configure: Regenerate.

	gcc/testsuite/
	* gcc.target/mips/ieee-force-relaxed.c: New test.
	* gcc.target/mips/ieee-relaxed-exec.c: New test.
	* gcc.target/mips/ieee-relaxed.c: New test.
	* gcc.target/mips/ieee-strict.c: New test.
	* gcc.target/mips/relaxed-nan-all.c: New test.
	* gcc.target/mips/relaxed-nan-exec.c: New test.
	* gcc.target/mips/relaxed-nan-force.c: New test.
	* gcc.target/mips/relaxed-nan-none.c: New test.
---
gcc-mips-nan-interlink.diff

[RFC,2/2] MIPS/GCC: IEEE Std 754 NaN interlinking support

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