Re: [PATCH] Add __builtin_stack_top

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, Aug 19, 2015 at 9:58 AM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> On Wed, Aug 19, 2015 at 08:25:49AM -0700, H.J. Lu wrote:
>> Here is a patch to add __builtin_argument_pointer.  I only have
>
> Sorry to be a pain but...  all the other builtins use _address
> instead of _pointer, it's probably best to follow that.
>
>>  -- Built-in Function: void * __builtin_argument_pointer (void)
>>      This function returns the argument pointer.
>>
>> as documentation.  Can you suggest a better description so that it can
>> be implemented also by other compilers?
>
> Maybe something like (heavily cut'n'pasted):
>
>
> @deftypefn {Built-in Function} {void *} __builtin_argument_address (void)
> This function is similar to @code{__builtin_frame_address} with an
> argument of 0, but it returns the address of the incoming arguments to
> the current function rather than the address of its frame.

This doesn't make senses when there is no argument or arguments
are passed in registers.  To me, argument pointer is a virtual concept
and an implementation detail internal to GCC.  I am not sure if another
compiler can implement it based on this description.

> The exact definition of this address depends upon the processor and the
> calling convention.  Usually some arguments are passed in registers and
> the rest on the stack, and this builtin returns the address of the first
> argument that is on the stack.
>
>
>> +      /* Can't use DRAP if the stack address has been taken.  */
>> +      if (cfun->argument_pointer_taken)
>> +     sorry ("%<__builtin_argument_pointer%> not supported with stack"
>> +            " realignment.  This may be worked around by adding"
>> +            " -maccumulate-outgoing-args.");
>
> This doesn't work with DRAP?  Pity :-(

With DRAP,  we do

      /* Replicate the return address on the stack so that return
         address can be reached via (argp - 1) slot.  This is needed
         to implement macro RETURN_ADDR_RTX and intrinsic function
         expand_builtin_return_addr etc.  */
      t = plus_constant (Pmode, crtl->drap_reg, -UNITS_PER_WORD);
      t = gen_frame_mem (word_mode, t);
      insn = emit_insn (gen_push (t));
      RTX_FRAME_RELATED_P (insn) = 1;

      /* For the purposes of frame and register save area addressing,
         we've started over with a new frame.  */
      m->fs.sp_offset = INCOMING_FRAME_SP_OFFSET;
      m->fs.realigned = true;

which doesn't work for __builtin_argument_pointer.

> The patch looks plausible, but I of course can not approve it.
>

Thanks.


-- 
H.J.