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

[Richard Sandiford <rdsandiford@googlemail.com>]

Sandra Loosemore <sandra@codesourcery.com> writes:
> The MIPS back end has an option -mno-float that is supported by 
> bare-metal configs using the SDE library.  However, this option is not 
> properly documented in the manual, and MIPS_ARCH_FLOAT_SPEC doesn't know 
> about it as one of the explicit floating-point configuration changes 
> that should override architecture defaults.  This patch addresses both 
> problems.  OK to commit?

OK, you're touching a sore spot here, but...

> +@item -mno-float
> +@opindex mno-float
> +Prevents the use of all floating-point operations.  This option is presently 
> +supported only by some bare-metal MIPS configurations.

...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.

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.

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

Richard