Re: [PATCH, MIPS] -mno-float odds and ends

[Sandra Loosemore <sandra@codesourcery.com>]

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On 07/30/2012 01:38 PM, Richard Sandiford wrote:
>
> ...unfortunately, it doesn't prevent the use floating-point operations.
> That's why it's such a bad option.  The only difference from the compiler
> proper's point of view between -msoft-float and -mno-float is that they
> define different preprocessor macros.
>
> The onus is instead on the programmer to avoid writing anything that
> might tempt the compiler into using floating-point operations.  If the
> user gets it wrong, they get (at best) a link-time error rather than a
> compile-time error.
>
> I think we should document it that way.  E.g. something like:
>
> @item -mno-float
> @opindex mno-float
> Equivalent to @option{-msoft-float}, but asserts that the user is
> trying to avoid all floating-point operations.  This option is presently
> supported only by some bare-metal MIPS configurations, where it selects
> a special set of libraries that lack all floating-point support
> (including, for example, the floating-point @code{printf} formats).
> If code compiled with @code{-mno-float} accidentally contains
> floating-point operations, it is likely to suffer a link-time
> or run-time failure.
>
> but you're better at the wordsmithing than I am.

OK, I've gone with a slightly tweaked version of your wording.

> Perhaps we should document the __mips_no_float preprocessor macro too,
> since that's how things like printf() know that they don't need the
> floating-point stuff.

Hmmm, I don't think that's necessary, at least as part of this patch; we 
don't document the related __mips_hard_float or __mips_soft_float 
preprocessor definitions, either.

> The mips.h part is OK though, thanks.  Feel free to apply it separately
> if that's more convenient than keeping the patch together.

I've checked in the attached version of the patch.  Thanks for the 
speedy review!  :-)

-Sandra


2012-07-30  Sandra Loosemore  <sandra@codesourcery.com>
	    Julian Brown  <julian@codesourcery.com>

	gcc/
	* doc/invoke.texi (MIPS Options): Document -mno-float.
	* config/mips/mips.h (MIPS_ARCH_FLOAT_SPEC): Make it know
	about -mno-float.


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Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 189950)
+++ gcc/doc/invoke.texi	(working copy)
@@ -733,7 +733,8 @@ Objective-C and Objective-C++ Dialects}.
 -mabi=@var{abi}  -mabicalls  -mno-abicalls @gol
 -mshared  -mno-shared  -mplt  -mno-plt  -mxgot  -mno-xgot @gol
 -mgp32  -mgp64  -mfp32  -mfp64  -mhard-float  -msoft-float @gol
--msingle-float  -mdouble-float  -mdsp  -mno-dsp  -mdspr2  -mno-dspr2 @gol
+-mno-float -msingle-float  -mdouble-float  @gol
+-mdsp  -mno-dsp  -mdspr2  -mno-dspr2 @gol
 -mmcu -mmno-mcu @gol
 -mfpu=@var{fpu-type} @gol
 -msmartmips  -mno-smartmips @gol
@@ -15633,6 +15634,18 @@ Use floating-point coprocessor instructi
 Do not use floating-point coprocessor instructions.  Implement
 floating-point calculations using library calls instead.
 
+@item -mno-float
+@opindex mno-float
+Equivalent to @option{-msoft-float}, but additionally asserts that the
+program being compiled does not perform any floating-point operations.
+This option is presently supported only by some bare-metal MIPS
+configurations, where it may select a special set of libraries
+that lack all floating-point support (including, for example, the
+floating-point @code{printf} formats).  
+If code compiled with @code{-mno-float} accidentally contains
+floating-point operations, it is likely to suffer a link-time
+or run-time failure.
+
 @item -msingle-float
 @opindex msingle-float
 Assume that the floating-point coprocessor only supports single-precision
Index: gcc/config/mips/mips.h
===================================================================
--- gcc/config/mips/mips.h	(revision 189950)
+++ gcc/config/mips/mips.h	(working copy)
@@ -713,7 +713,7 @@ struct mips_cpu_info {
    link-compatible.  */
 
 #define MIPS_ARCH_FLOAT_SPEC \
-  "%{mhard-float|msoft-float|march=mips*:; \
+  "%{mhard-float|msoft-float|mno-float|march=mips*:; \
      march=vr41*|march=m4k|march=4k*|march=24kc|march=24kec \
      |march=34kc|march=74kc|march=1004kc|march=5kc \
      |march=octeon|march=xlr: -msoft-float;		  \