Bug 27099

Summary: [ARM] ISB instruction inside an IT block
Product: binutils Reporter: Wojciech Kordalski <kordalski.wojciech>
Component: gasAssignee: Not yet assigned to anyone <unassigned>
Status: UNCONFIRMED ---    
Severity: minor    
Priority: P2    
Version: 2.35   
Target Milestone: ---   
Host: Target:
Build: Last reconfirmed:
Attachments: The minimal example

Description Wojciech Kordalski 2020-12-20 14:05:37 UTC 
Created attachment 13068 [details]
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.
Comment 1 Wojciech Kordalski 2020-12-20 22:02:39 UTC 
By accident, I did a mistake in the reference to the ARMv7-M Architecture Reference Manual.

The note that "ISB instruction inside an IT block is unpredictable" is in Section A7.7.37 ISB, page A7-235.