[Bug gas/20364] New: Data-based padding causes current address evaluation to be non-constant on aarch64

[contact at paulk dot fr]

https://sourceware.org/bugzilla/show_bug.cgi?id=20364

            Bug ID: 20364
           Summary: Data-based padding causes current address evaluation
                    to be non-constant on aarch64
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: contact at paulk dot fr
  Target Milestone: ---

It appears that starting with binutils 2.26, evaluating the current address
(using the dot symbol) after a .align directive using data-based padding (e.g.
.align 6, 0) makes it non-constant, resulting in errors such as: "Error:
non-constant expression in ".if" statement" when evaluating "." inside an .if
statement. This is on aarch64.

The issue appeared when build the ARM Trusted Firmware:
https://github.com/ARM-software/arm-trusted-firmware

The related assembly is a combination of the vector_entry/vector_base (that
does the alignment) and check_vector_size (that uses the dot symbol) macros,
defined at:
https://github.com/ARM-software/arm-trusted-firmware/blob/master/include/common/asm_macros.S#L69
and used at
https://github.com/ARM-software/arm-trusted-firmware/blob/master/bl31/aarch64/runtime_exceptions.S#L171

The assembly is correct and is apparently not the problem. However, switching
the ".align x, 0" directives to ".align x" solves the issue, but upstream is
not going to make that change.

The issue was discussed in depth at
https://www.mail-archive.com/address@hidden/msg05685.html
and an issue was opened on the ARM Trusted Firmware tracker at
https://github.com/ARM-software/tf-issues/issues/401

A test case was provided at
https://www.mail-archive.com/address@hidden/msg05688/tmp.s

The issue was bisected down to (bad) commit
c1baaddf8861aea666b84baeb4746caff51a579d

commit c1baaddf8861aea666b84baeb4746caff51a579d
Author: Renlin Li <address@hidden>
Date:   Thu Apr 2 14:59:45 2015 +0100

    [AArch64] Emit DATA_MAP in order within text section

    2015-03-27  Renlin Li  <address@hidden>

    gas/
      * config/tc-aarch64.c (mapping_state): Emit MAP_DATA within text
section in order.
      (mapping_state_2): Don't emit MAP_DATA here.
      (s_aarch64_inst): Align frag during state transition.
      (md_assemble): Likewise.

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