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Doc: Discourage the use of -ffloat-store [PR14708]

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gcc/ChangeLog
	PR middle-end/14708
	* doc/invoke.texi (Optimize Options): List -fexcess-precision
	before -ffloat-store, moving some background discussion to the
	former from the latter.  Recommend using -fexcess-precision=standard
	instead of -ffloat-store.
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Sandra Loosemore 2025-04-11 00:06:17 +00:00
parent 2fb903387b
commit 0c210fbbd7
1 changed files with 23 additions and 19 deletions
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gcc/doc/invoke.texi
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@ -15318,32 +15318,24 @@ arithmetic. These options trade off between speed and
correctness. All must be specifically enabled.
@table @gcctabopt
@opindex ffloat-store
@item -ffloat-store
Do not store floating-point variables in registers, and inhibit other
options that might change whether a floating-point value is taken from a
register or memory.
@cindex floating-point precision
This option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a @code{double} is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use @option{-ffloat-store} for such programs, after modifying
them to store all pertinent intermediate computations into variables.
@opindex fexcess-precision
@item -fexcess-precision=@var{style}
This option allows further control over excess precision on machines
This option allows control over excess precision on machines
where floating-point operations occur in a format with more precision or
range than the IEEE standard and interchange floating-point types. By
default, @option{-fexcess-precision=fast} is in effect; this means that
range than the IEEE standard and interchange floating-point types.
An example of such a target is x87 floating point on x86 processors,
which uses an 80-bit representation internally instead of the 64-bit
IEEE format. For most programs, the excess precision is harmless,
but some programs may rely on the
requirements of the C or C++ language standards for handling IEEE values.
By default, @option{-fexcess-precision=fast} is in effect; this means that
operations may be carried out in a wider precision than the types specified
in the source if that would result in faster code, and it is unpredictable
when rounding to the types specified in the source code takes place.
When compiling C or C++, if @option{-fexcess-precision=standard} is specified
then excess precision follows the rules specified in ISO C99 or C++; in particular,
then excess precision follows the rules specified in ISO C99 or C++;
in particular,
both casts and assignments cause values to be rounded to their
semantic types (whereas @option{-ffloat-store} only affects
assignments). This option is enabled by default for C or C++ if a strict
@ -15361,6 +15353,18 @@ or @option{-mfpmath=sse+387} is specified; in the former case, IEEE
semantics apply without excess precision, and in the latter, rounding
is unpredictable.
@opindex ffloat-store
@item -ffloat-store
Do not store floating-point variables in registers, and inhibit other
options that might change whether a floating-point value is taken from a
register or memory. This option has generally been subsumed by
@option{-fexcess-precision=standard}, which is more general. If you do use
@option{-ffloat-store}, you may need to modify your program to explicitly
store intermediate computations in temporary variables since
@option{-ffloat-store} handles rounding to IEEE format
only on assignments and not casts as @option{-fexcess-precision=standard}
does.
@opindex ffast-math
@item -ffast-math
Sets the options @option{-fno-math-errno}, @option{-funsafe-math-optimizations},