[PATCH 4/10, GAS/ARM] Rework selection of feature bits to base build attributes on

[Thomas Preudhomme]

Hi,

=== Context ===

This patch is part of a patch series to add support for ARMv8-R
architecture. Its purpose is to set the feature bits on which to decide
what the build attributes should be according to the mode
(autodetection, user specified architecture or CPU, or
-march/-mcpu=all).

=== Motivation ===

Currently, the flags variable which is used to determine the build
attribute is constructed from the instruction used (arm_arch_used and
thumb_arch_used) as well as the user specified architecture or CPU
(selected_cpu). This means when several .arch are specified the
resulting feature bits can be such that no architecture provide them
all and can thus result in incorrect Tag_CPU_arch. See for instance
what having both .arch armv8-a and .arch armv8-m.base would result in.
This is not caught by the testsuite because of further bugs in the
Tag_CPU_arch build attribute value selection logic (see next patch in
the series).

=== Patch description ===

As one would expect, this patch solves the problem by setting flags
from feature bits used if in autodetection mode [1] and from
selected_cpu otherwise. The logic to set arm_ext_v1, arm_ext_v4t and
arm_ext_os feature bits is also moved to only run in autodetection mode
since otherwise the architecture or CPU would have a consistent set of
feature bits already.

[1] No architecture or CPU was specified by the user

ChangeLog entry is as follows:

*** gas/ChangeLog ***

2017-01-26  Thomas Preud'homme  <thomas.preudhomme@arm.com>

	* config/tc-arm.c (aeabi_set_public_attributes): Populate flags from
	feature bits used or selected_cpu depending on whether a CPU was
	selected by the user.

=== Testing ===

Testsuite shows no regression when run for arm-none-eabi targets.

Is this ok for master branch?

Best regards,

Thomas
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