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[arm][doc] Document changes to -mcpu, -mtune and -mfpu.

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This patch adds the remainder of the main documentation changes.  It
adds the changes for -mcpu, -mtune and -mfpu.  I've chosen to document
the extension options under -mcpu rather than under -mtune because,
while they are permitted with -mtune, they do not affect the behaviour
of the tuning done by the compiler.

I've also inverted the sense of the table (making the primary index
the extension name and then listing the CPU names to which it applies.
This is because the extensions are much more orthoganal in meaning
here and having a primary entry via the CPU name would lead to
enormous duplication.

Finally, it adds the relevant changes to -mfpu.  I haven't stated yet
that any setting of -mfpu other than 'auto' is deprecated, but that is
certainly the long-term goal of this patch series.

	* doc/invoke.texi (ARM Options, -mcpu): Document supported
	extension options.
	(ARM Options, -mtune): Document that this accepts the same
	extension options as -mcpu.
	(ARM Options, -mfpu): Document addition of -mfpu=auto.

From-SVN: r249308
This commit is contained in:
Richard Earnshaw 2017-06-16 21:07:03 +00:00 committed by Richard Earnshaw
parent 6d2f99aae1
commit f54133eaab
2 changed files with 71 additions and 7 deletions
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8
gcc/ChangeLog
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@ -1,3 +1,11 @@
2017-06-16 Richard Earnshaw <rearnsha@arm.com>
* doc/invoke.texi (ARM Options, -mcpu): Document supported
extension options.
(ARM Options, -mtune): Document that this accepts the same
extension options as -mcpu.
(ARM Options, -mfpu): Document addition of -mfpu=auto.
2017-06-16 Richard Earnshaw <rearnsha@arm.com>
* doc/invoke.texi (ARM Options, -march=): Document new syntax and

70
gcc/doc/invoke.texi
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@ -15489,8 +15489,6 @@ Permissible names are: @samp{arm2}, @samp{arm250},
@samp{cortex-m0.small-multiply},
@samp{cortex-m0plus.small-multiply},
@samp{exynos-m1},
@samp{falkor},
@samp{qdf24xx},
@samp{marvell-pj4},
@samp{xscale}, @samp{iwmmxt}, @samp{iwmmxt2}, @samp{ep9312},
@samp{fa526}, @samp{fa626},
@ -15510,12 +15508,15 @@ processors, balancing between optimizations that benefit some CPUs in the
range, and avoiding performance pitfalls of other CPUs. The effects of
this option may change in future GCC versions as CPU models come and go.
@option{-mtune} permits the same extension options as @option{-mcpu}, but
the extension options do not affect the tuning of the generated code.
@option{-mtune=native} causes the compiler to auto-detect the CPU
of the build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
@item -mcpu=@var{name}
@item -mcpu=@var{name@r{[}+extension@dots{}@r{]}}
@opindex mcpu
This specifies the name of the target ARM processor. GCC uses this name
to derive the name of the target ARM architecture (as if specified
@ -15524,9 +15525,62 @@ performance (as if specified by @option{-mtune}). Where this option
is used in conjunction with @option{-march} or @option{-mtune},
those options take precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural
extensions. Where this is so the architectural extensions are
normally enabled by default. If implementations that lack the
extension exist, then the extension syntax can be used to disable
those extensions that have been omitted. For floating-point and
Advanced SIMD (Neon) instructions, the settings of the options
@option{-mfloat-abi} and @option{-mfpu} must also be considered:
floating-point and Advanced SIMD instructions will only be used if
@option{-mfloat-abi} is not set to @samp{soft}; and any setting of
@option{-mfpu} other than @samp{auto} will override the available
floating-point and SIMD extension instructions.
For example, @samp{cortex-a9} can be found in three major
configurations: integer only, with just a floating-point unit or with
floating-point and Advanced SIMD. The default is to enable all the
instructions, but the extensions @samp{+nosimd} and @samp{+nofp} can
be used to disable just the SIMD or both the SIMD and floating-point
instructions respectively.
Permissible names for this option are the same as those for
@option{-mtune}.
The following extension options are common to the listed CPUs:
@table @samp
@item +nofp
Disables the floating-point instructions on @samp{arm9e},
@samp{arm946e-s}, @samp{arm966e-s}, @samp{arm968e-s}, @samp{arm10e},
@samp{arm1020e}, @samp{arm1022e}, @samp{arm926ej-s},
@samp{arm1026ej-s}, @samp{cortex-r5}, @samp{cortex-r7}, @samp{cortex-r8},
@samp{cortex-m4}, @samp{cortex-m7} and @samp{cortex-m33}.
Disables the floating-point and SIMD instructions on
@samp{generic-armv7-a}, @samp{cortex-a5}, @samp{cortex-a7},
@samp{cortex-a8}, @samp{cortex-a9}, @samp{cortex-a12},
@samp{cortex-a15}, @samp{cortex-a17}, @samp{cortex-a15.cortex-a7},
@samp{cortex-a17.cortex-a7}, @samp{cortex-a32}, @samp{cortex-a35}
and @samp{cortex-a53}.
@item +nofp.dp
Disables the double-precision component of the floating-point instructions
on @samp{cortex-r5} and @samp{cortex-m7}.
@item +nosimd
Disables the SIMD (but not floating-point) instructions on
@samp{generic-armv7-a}, @samp{cortex-a5}, @samp{cortex-a7}
and @samp{cortex-a9}.
@end table
Additionally the @samp{generic-armv7-a} pseudo target defaults to
VFPv3 with 16 double-precision registers. It supports the following
extension options: @samp{vfpv3-d16}, @samp{vfpv3},
@samp{vfpv3-d16-fp16}, @samp{vfpv3-fp16}, @samp{vfpv4-d16},
@samp{vfpv4}, @samp{neon}, @samp{neon-vfpv3}, @samp{neon-fp16},
@samp{neon-vfpv4}. The meanings are the same as for the extensions to
@option{-march=armv7-a}.
@option{-mcpu=generic-@var{arch}} is also permissible, and is
equivalent to @option{-march=@var{arch} -mtune=generic-@var{arch}}.
See @option{-mtune} for more information.
@ -15539,7 +15593,8 @@ is unsuccessful the option has no effect.
@item -mfpu=@var{name}
@opindex mfpu
This specifies what floating-point hardware (or hardware emulation) is
available on the target. Permissible names are: @samp{vfpv2}, @samp{vfpv3},
available on the target. Permissible names are: @samp{auto}, @samp{vfpv2},
@samp{vfpv3},
@samp{vfpv3-fp16}, @samp{vfpv3-d16}, @samp{vfpv3-d16-fp16}, @samp{vfpv3xd},
@samp{vfpv3xd-fp16}, @samp{neon-vfpv3}, @samp{neon-fp16}, @samp{vfpv4},
@samp{vfpv4-d16}, @samp{fpv4-sp-d16}, @samp{neon-vfpv4},
@ -15548,11 +15603,12 @@ available on the target. Permissible names are: @samp{vfpv2}, @samp{vfpv3},
Note that @samp{neon} is an alias for @samp{neon-vfpv3} and @samp{vfp}
is an alias for @samp{vfpv2}.
If @option{-msoft-float} is specified this specifies the format of
floating-point values.
The setting @samp{auto} is the default and is special. It causes the
compiler to select the floating-point and Advanced SIMD instructions
based on the settings of @option{-mcpu} and @option{-march}.
If the selected floating-point hardware includes the NEON extension
(e.g. @option{-mfpu}=@samp{neon}), note that floating-point
(e.g. @option{-mfpu=neon}), note that floating-point
operations are not generated by GCC's auto-vectorization pass unless
@option{-funsafe-math-optimizations} is also specified. This is
because NEON hardware does not fully implement the IEEE 754 standard for