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[AArch64][doc] Clarify -msve-vector-bits=128 behaviour

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We've received reports about the -msve-vector-bits=128 bits being somewhat ambiguous.
It isn't clear whether -msve-vector-bits=128 forces vector-length-agnostic code or whether
-msve-vector-bits=scalable forces 128-bit vector-lengh-specific code.
The latter is a, perhaps unintuitive, reading that we want to exclude.

This patch makes it more explicit that -msve-vector-bits=128 is special and produces
vector-length *agnostic* code. In the end, I've rewritten the whole option documentation.

Checked make pdf that the output looks reasonable.

    * doc/invoke.texi (-msve-vector-bits): Clarify -msve-vector-bits=128
    behavior.

From-SVN: r267081
This commit is contained in:
Kyrylo Tkachov 2018-12-13 10:37:15 +00:00 committed by Kyrylo Tkachov
parent 7cab07f089
commit ee03c51c0b
2 changed files with 17 additions and 9 deletions
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gcc/ChangeLog
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@ -1,3 +1,8 @@
2018-12-13 Kyrylo Tkachov <kyrylo.tkachov@arm.com>
* doc/invoke.texi (-msve-vector-bits): Clarify -msve-vector-bits=128
behavior.
2018-12-13 Wei Xiao <wei3.xiao@intel.com>
* common/config/i386/i386-common.c (processor_names): Add cascadelake.

21
gcc/doc/invoke.texi
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@ -15675,16 +15675,19 @@ an effect when SVE is enabled.
GCC supports two forms of SVE code generation: ``vector-length
agnostic'' output that works with any size of vector register and
``vector-length specific'' output that only works when the vector
registers are a particular size. Replacing @var{bits} with
@samp{scalable} selects vector-length agnostic output while
replacing it with a number selects vector-length specific output.
The possible lengths in the latter case are: 128, 256, 512, 1024
and 2048. @samp{scalable} is the default.
At present, @samp{-msve-vector-bits=128} produces the same output
as @samp{-msve-vector-bits=scalable}.
``vector-length specific'' output that allows GCC to make assumptions
about the vector length when it is useful for optimization reasons.
The possible values of @samp{bits} are: @samp{scalable}, @samp{128},
@samp{256}, @samp{512}, @samp{1024} and @samp{2048}.
Specifying @samp{scalable} selects vector-length agnostic
output. At present @samp{-msve-vector-bits=128} also generates vector-length
agnostic output. All other values generate vector-length specific code.
The behavior of these values may change in future releases and no value except
@samp{scalable} should be relied on for producing code that is portable across
different hardware SVE vector lengths.
The default is @samp{-msve-vector-bits=scalable}, which produces
vector-length agnostic code.
@end table
@subsubsection @option{-march} and @option{-mcpu} Feature Modifiers