[Bug c/47685] New: Tail call doesn't work for thumb2

[alexandre.nunes at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47685

           Summary: Tail call doesn't work for thumb2
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: alexandre.nunes@gmail.com


Created attachment 23298
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23298
testcase.

When compiling the attached test program for i.e. -mcpu=cortex-a9 -marm the
tail call mechanism works correctly ... if you replace -marm for -mthumb then
the result is a traditional stack frame, for which i couldn't find any switches
to turn it off.

For example, this was taken from the example code for -mthumb:


func_c:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        push    {lr}
        movs    r3, #33
        bl      func_a
        pop     {pc}
        .size   func_c, .-func_c

And this for -marm:


func_c:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        mov     r3, #33
        b       func_a
        .size   func_c, .-func_c


This is more relevant for example to -mcpu=cortex-m3 where we have no ram to
waste.