[Bug c/87400] New: GCC doesn't produce valid frames for stack traces with Thumb-2 (Cortex-M3+)

[alexkalmuk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87400

            Bug ID: 87400
           Summary: GCC doesn't produce valid frames for stack traces with
                    Thumb-2 (Cortex-M3+)
           Product: gcc
           Version: 7.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alexkalmuk at gmail dot com
  Target Milestone: ---
            Target: arm-none-eabi

GCC generates special frames for doing stack traces on ARM. These frames can be
generated for ARM instruction set using `-mapcs-frame`:

Example of the produced frame:
$ arm-none-eabi-gcc -marm -mcpu=arm926ej-s -march=armv5te -mapcs-frame
--specs=nosys.specs -o test test.c

000081f8 <my_func>:
    81f8:   e1a0c00d    mov ip, sp
    81fc:   e92dd800    push    {fp, ip, lr, pc}

This type of prologue can be also generated for Cortex-M (thumb instruction
set) using `-mthumb -mcpu=cortex-m1 -mtpcs-frame -mtpcs-leaf-frame`:

$ arm-none-eabi-gcc -mthumb -mcpu=cortex-m1 -mtpcs-frame -mtpcs-leaf-frame
--specs=nosys.specs -o test test.c

In that case function prologue will look like:
00008104 <my_func>:
    8104:   b084        sub sp, #16
    8106:   b580        push    {r7, lr}
    8108:   aa06        add r2, sp, #24
    810a:   9203        str r2, [sp, #12]
    810c:   467a        mov r2, pc
    810e:   9205        str r2, [sp, #20]
    8110:   465a        mov r2, fp
    8112:   9202        str r2, [sp, #8]

It's the same thing like for ARM instruction set, but since Thumb doesn't allow
to push high registers, GCC produces it through mov/str instructions.

But in case of `-mcpu=cortex-m3` (instead of `-mcpu=cortex-m1`), we do not get
such prologue. After investigating gcc/config/arm/arm.c, I figured out that
these prologues are generated only for Thumb-1 (Cortex-M0/M1) and ARM
instruction sets, but ingnored for Thumb-2 (Cortex-M3+). It is a bug, or there
is some reason do not generate these prologues for Thumb-2?