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

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

Hi all,

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.

Ok for trunk and GCC 8?

Thanks,
Kyrill

2018-12-13  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

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

commit 4eb1316700234b7e1be6d9ea84dc96e18c294048
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date:   Wed Dec 12 13:32:59 2018 +0000

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

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index dd262b60d88..04d55829528 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -15716,16 +15716,19 @@ 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