[Bug gas/27099] New: [ARM] ISB instruction inside an IT block

[kordalski.wojciech at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=27099

            Bug ID: 27099
           Summary: [ARM] ISB instruction inside an IT block
           Product: binutils
           Version: 2.35.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: kordalski.wojciech at gmail dot com
  Target Milestone: ---

Created attachment 13068
  --> https://sourceware.org/bugzilla/attachment.cgi?id=13068&action=edit
The minimal example

During research of ARM Cortex-M3-based processor for our master thesis, we
found a bug (using a tool that generates random-but-correct sequences of
assembly instructions).

Minimal example:

    .syntax unified
    .cpu cortex-m3
    .thumb

    .align 2
    .type f1, %function
    .thumb_func
    f1:
        it eq
        isbeq

The example compiles successfully (arm-none-eabi-as -o test.o test.s
-mcpu=cortex-m3) without any message emitted, but it should not.

The ARMv7-M Architecture Reference Manual states that ISB instruction inside an
IT block is unpredictable (see Section A7.7.38 IT, page A7-236).

The ARMv7-M Architecture Reference Manual can be downloaded from:
https://static.docs.arm.com/ddi0403/ed/DDI0403E_d_armv7m_arm.pdf

Expected behavior: assembler should emit a warning or error message.

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