Re: [patch] Fix PR target/67265

[Bernd Schmidt <bschmidt at redhat dot com>]

On 11/11/2015 12:38 PM, Eric Botcazou wrote:
> this is an ICE on an asm statement requiring a lot of registers, when compiled
> in 32-bit mode on x86/Linux with -O -fstack-check -fPIC:
>
> pr67265.c:10:3: error: 'asm' operand has impossible constraints
>
> The issue is that, since stack checking defines STACK_CHECK_MOVING_SP on this
> platform, the frame pointer is necessary in order to be able to propagate
> exceptions raised on stack overflow.  But this is required only in Ada so we
> can certainly avoid doing it in C or C++.

>         /* We need the frame pointer to catch stack overflow exceptions
>  	  if the stack pointer is moving.  */
> -       || (flag_stack_check && STACK_CHECK_MOVING_SP)
> +       || (STACK_CHECK_MOVING_SP
> +	   && flag_stack_check
> +	   && flag_exceptions
> +	   && cfun->can_throw_non_call_exceptions)

This piece of code along doesn't tell me exactly why the frame pointer 
is needed. I was looking for an explicit use, but I now guess that if 
you have multiple adjusts of the frame pointer you can't easily undo 
them in the error case (the function behaves as-if using alloca). Is 
that it? And without exceptions I assume you just get a call to abort so 
it doesn't matter? If I understood all that right, then this is ok.

In i386.c I see a code block with a similar condition,

  /* If the only reason for frame_pointer_needed is that we conservatively
     assumed stack realignment might be needed, but in the end nothing that
     needed the stack alignment had been spilled, clear 
frame_pointer_needed
     and say we don't need stack realignment.  */

and the condition has

      && !(flag_stack_check && STACK_CHECK_MOVING_SP)

Should that be changed too?


Bernd