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Diffstat (limited to 'gas/doc/c-mips.texi')
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diff --git a/gas/doc/c-mips.texi b/gas/doc/c-mips.texi index 741237a..8a25a76 100644 --- a/gas/doc/c-mips.texi +++ b/gas/doc/c-mips.texi @@ -30,6 +30,7 @@ Assembly Language Programming'' in the same work. * MIPS assembly options:: Directives to control code generation * MIPS autoextend:: Directives for extending MIPS 16 bit instructions * MIPS insn:: Directive to mark data as an instruction +* MIPS NaN Encodings:: Directives to record which NaN encoding is being used * MIPS Option Stack:: Directives to save and restore options * MIPS ASE Instruction Generation Overrides:: Directives to control generation of MIPS ASE instructions @@ -416,6 +417,16 @@ Also no MIPS16 branches are ever relaxed. By default @samp{--no-relax-branch} is selected, causing any out-of-range branches to produce an error. +@cindex @option{-mnan=} command line option, MIPS +@item -mnan=@var{encoding} +This option indicates whether the source code uses the IEEE 2008 +NaN encoding (@option{-mnan=2008}) or the original MIPS encoding +(@option{-mnan=legacy}). It is equivalent to adding a @code{.nan} +directive to the beginning of the source file. @xref{MIPS NaN Encodings}. + +@option{-mnan=legacy} is the default if no @option{-mnan} option or +@code{.nan} directive is used. + @item --trap @itemx --no-break @c FIXME! (1) reflect these options (next item too) in option summaries; @@ -722,6 +733,47 @@ baz: @end example +@node MIPS NaN Encodings +@section Directives to record which NaN encoding is being used + +@cindex MIPS IEEE 754 NaN data encoding selection +@cindex @code{.nan} directive, MIPS +The IEEE 754 floating-point standard defines two types of not-a-number +(NaN) data: ``signalling'' NaNs and ``quiet'' NaNs. The original version +of the standard did not specify how these two types should be +distinguished. Most implementations followed the i387 model, in which +the first bit of the significand is set for quiet NaNs and clear for +signalling NaNs. However, the original MIPS implementation assigned the +opposite meaning to the bit, so that it was set for signalling NaNs and +clear for quiet NaNs. + +The 2008 revision of the standard formally suggested the i387 choice +and as from Sep 2012 the current release of the MIPS architecture +therefore optionally supports that form. Code that uses one NaN encoding +would usually be incompatible with code that uses the other NaN encoding, +so MIPS ELF objects have a flag (@code{EF_MIPS_NAN2008}) to record which +encoding is being used. + +Assembly files can use the @code{.nan} directive to select between the +two encodings. @samp{.nan 2008} says that the assembly file uses the +IEEE 754-2008 encoding while @samp{.nan legacy} says that the file uses +the original MIPS encoding. If several @code{.nan} directives are given, +the final setting is the one that is used. + +The command-line options @option{-mnan=legacy} and @option{-mnan=2008} +can be used instead of @samp{.nan legacy} and @samp{.nan 2008} +respectively. However, any @code{.nan} directive overrides the +command-line setting. + +@samp{.nan legacy} is the default if no @code{.nan} directive or +@option{-mnan} option is given. + +Note that @sc{gnu} @code{@value{AS}} does not produce NaNs itself and +therefore these directives do not affect code generation. They simply +control the setting of the @code{EF_MIPS_NAN2008} flag. + +Traditional MIPS assemblers do not support these directives. + @node MIPS Option Stack @section Directives to save and restore options |