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Re: [PATCH 6/10, GAS/ARM] Rework Tag_CPU_arch build attribute value selection

Hi Richard,

On 21/06/17 15:33, Richard Earnshaw (lists) wrote:
On 21/06/17 11:12, Thomas Preudhomme wrote:

=== Context ===

This patch is part of a patch series to add support for ARMv8-R
architecture. Its purpose is to rework the Tag_CPU_arch build attribute
value selection to (i) match architecture or CPU if specified by user
without any need for hack and (ii) match an architecture with all the
features used if in autodetection mode or return an error.

=== Motivation ===

Currently, Tag_CPU_arch build attribute value selection assumes that an
architecture is always a superset of architectures released earlier. As
such, the logic is to browse architectures in chronological order of
release and selecting the Tag_CPU_arch value of the last one to
contribute a feature used[1]/requested[2] not contributed by earlier

[1] in case of autodetection mode
[2] otherwise, ie. in case of -mcpu/-march or associated directives

This logic fails the two objectives mentionned in the Context section.
First, due to the assumption it does an architecture can be selected
while not having all the features used/requested which fails the second
objective. Second, not doing an exact match when an architecture or CPU
is selected by the user means the wrong value is chosen when a later
architecture provides a subset of the feature bits of an earlier
architecture. This is the case for the implementation of ARMv8-R (see
later patch).

An added benefit of this patch is that it is possible to easily generate
more consistent build attribute by setting the feature bits from the
architecture matched in aeabi_set_public_attributes in autodetection
mode. This is better done as a separate patch because lots of testcase'
expected results must then be updated accordingly.

=== Patch description ===

The patch changes the main logic for Tag_CPU_arch and Tag_CPU_arch_profile
values selection to:
- look for an exact match in case an architecture or CPU was specified
  on the command line or in a directive
- select the first released architecture that provides a superset of the
  feature used in the autodetection case
- select the most featureful architecture in case of -march=all
The array cpu_arch_ver is updated to include all architectures in order
to make the first point work.

Note that when looking for an exact match, the architecture with
selected extension is tried first and then only the architecture. This
is because some architectures are exactly equivalent to an earlier
architecture with its extensions selected. ARMv6S-M (= ARMv6-M + OS
extension) and ARMv6KZ (ARMv6K + security extension) are two such

Other adjustments are also necessary in aeabi_set_public_attributes to
make this change work.

1) The logic to set Tag_ARM_ISA_use and Tag_THUMB_ISA_use must check the
absence of feature bit used/requested to decide whether to give the
default value for empty files (see EABI attribute defaults test). It was
previously checking that arch == 0 which would only happen if no feature
bit could be matched by any architecture, ie there is no feature bit to

2) A fallback to a superset match must exist when no_cpu_selected ()
returns true. This is because aeabi_set_public_attributes is called
again after relaxation and at this point selected_cpu is set from the
previous execution of that function. There is therefore no way to check
whether the user specified an architecture or CPU.

ChangeLog entries are as follow:

*** gas/ChangeLog ***

2017-06-13  Thomas Preud'homme  <>

    * config/tc-arm.c (fpu_any): Defined from FPU_ANY.
    (cpu_arch_ver): Add all architectures and sort by release date.
    (have_ext_for_needed_feat_p): New.
    (get_aeabi_cpu_arch_from_fset): New.
    (aeabi_set_public_attributes): Call above function to determine
    Tag_CPU_arch and Tag_CPU_arch_profile values.  Adapt Tag_ARM_ISA_use
    and Tag_THUMB_ISA_use selection logic to check absence of feature bit
    * testsuite/gas/arm/attr-march-armv1.d: Fix expected Tag_CPU_arch build
    attribute value.
    * testsuite/gas/arm/attr-march-armv2.d: Likewise.
    * testsuite/gas/arm/attr-march-armv2a.d: Likewise.
    * testsuite/gas/arm/attr-march-armv2s.d: Likewise.
    * testsuite/gas/arm/attr-march-armv3.d: Likewise.
    * testsuite/gas/arm/attr-march-armv3m.d: Likewise.
    * testsuite/gas/arm/pr12198-2.d: Likewise.

*** include/ChangeLog ***

2017-01-26  Thomas Preud'homme  <>

    * opcode/arm.h (FPU_ANY): New macro.

=== Testing ===

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

Is this ok for master branch?

I have two issues with this patch...

+/* Mapping from CPU features to EABI CPU arch values.  Table must be
+   chronologically for architectures, with an exception for ARMv6-M and
+   ARMv6S-M due to legacy reasons.  No new architecture should have a
+   special case.  */

This comment warrants some justification.  We should explain that the
reason for maintaining chronological order is to avoid the build
attributes of object files changing as new variant architectures get added.

Indeed. I'll improve the comments in code and cover letter for that patch.

 File Attributes
   Tag_CPU_name: "1"
-  Tag_CPU_arch: v4
   Tag_ARM_ISA_use: Yes

I think we should always emit a Tag_CPU_arch directive.  It's wrong to
assume that the consumer of an object file will be able to deduce the
base architecture from a simple CPU name - especially an odd-ball one
like '1' which isn't an official product name.

Tag_CPU_arch = v4 is probably the right thing to emit here - we don't
have tags for architectures older than that.

The document "Addenda to, and Errata in, the ABI® for the ARM Architecture"(document ID: ARM IHI 0045D) issue E r2.10 says that a value of 0 is for pre-v4 architectures. Since 0 is the default value this at least follows the documentation.

Best regards,


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