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Subject: Re: [PATCH] [ARM] minor opt in thumb_stack_frame_destroyed_p
To: Yao Qi <qiyaoltc@gmail.com>, gdb-patches@sourceware.org
References: <1460730578-28723-1-git-send-email-yao.qi@linaro.org>
From: Pedro Alves <palves@redhat.com>
Message-ID: <571115C3.8010500@redhat.com>
Date: Fri, 15 Apr 2016 16:24:00 -0000
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On 04/15/2016 03:29 PM, Yao Qi wrote:

> --- a/gdb/arm-tdep.c
> +++ b/gdb/arm-tdep.c
> @@ -3135,6 +3135,14 @@ thumb_stack_frame_destroyed_p (struct gdbarch *gdbarch, CORE_ADDR pc)
>    if (!find_pc_partial_function (pc, NULL, &func_start, &func_end))
>      return 0;
>  
> +  if (func_end - pc > 4 * 4)
> +    {
> +      /* There shouldn't be more than four instructions in epilogue.
> +	 If PC is still 16 bytes away from FUNC_END, it isn't in
> +	 epilogue.  */

This assumes epilogue sequences are always at the end of the function.

I suspected this isn't a safe assumption, and I asked gcc folks.  

The answer I got was that gcc can and does emit epilogues in the
middle of functions.  Below's an example I was given, and the
corresponding x86-64 and ARM disassembly.  Note the 'retq' / 'bx lr'
in the middle.

#include <stdlib.h>

void f (int x)
{
  if (x == 0)
    abort();
}

Compile with -O2.

x86-64:

0000000000000000 <f>:
   0:   85 ff                   test   %edi,%edi
   2:   74 02                   je     6 <f+0x6>
   4:   f3 c3                   repz retq 
   6:   50                      push   %rax
   7:   e8 00 00 00 00          callq  c <f+0xc>

ARM:

00000000 <f>:
   0:   b100            cbz     r0, 4 <f+0x4>
   2:   4770            bx      lr
   4:   b508            push    {r3, lr}
   6:   f7ff fffe       bl      0 <abort>
   a:   bf00            nop

Thanks,
Pedro Alves