Frame pointer for Thumb-2 non-leaf functions

[Denis Khalikov <d dot khalikov at partner dot samsung dot com>]

Hello everyone,
I have a question about frame pointer for the thumb mode.

This is my example:

$cat a.cc
__attribute__((noinline)) int bar(int a) { return a + 1; }
__attribute__((noinline)) int foo(int a) { return bar(a + 1); }
int main(int argc, char **argv) { return foo(argc); }

$armv7l-linux-gnueabi-g++ --version
armv7l-linux-gnueabi-g++ (GCC) 9.0.0 20180709 (experimental)

$armv7l-linux-gnueabi-g++ -o a.S a.cc -fno-omit-frame-pointer -S -marm

The prologue for the foo () function is:

  push	{fp, lr}
  add	fp, sp, #4
  sub	sp, sp, #8

Current fp points to lr on the stack, so it's easy to find previous fp 
on the stack.

But, if we look at the same function with thumb mode, we will see the 
difference:

$armv7l-linux-gnueabi-g++ -o a.S a.cc -fno-omit-frame-pointer -S -mthumb

  push	{r7, lr}
  sub	sp, sp, #8
  add	r7, sp, #0

In this case the frame pointer points to the top of the stack (in case 
stack grows downward) or whatever thumb_set_frame_pointer() function 
will emit. So, looks like it's impossible to find the previous frame 
pointer at the runtime, because we should know the amount of the local 
vars and the size of them, in my example.

Also, as far as I understood, clang produce code with frame pointer 
which points to the predictable location for the arm and thumb modes.

$clang++ -o a.S a.cc -fno-omit-frame-pointer -mthumb -S -target armv7l

  push	{r7, lr}
  mov	r7, sp
  sub	sp, #8

$clang++ -o a.S a.cc -fno-omit-frame-pointer -marm -S -target armv7l

  push	{r11, lr}
  mov	r11, sp
  sub	sp, sp, #8

So, my question is, does GCC has any restrictions about frame layout 
generation for the thumb mode or it could be rewrited ?
Thanks.