[Bug target/57932] Aligned stack wastes more than k bytes (as needed), if preferred stack boundary k=2**n, n>=4

[meisenmann....@fh-salzburg.ac.at]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57932

Markus Eisenmann <meisenmann....@fh-salzburg.ac.at> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|IA-32/x86-64                |i386

--- Comment #1 from Markus Eisenmann <meisenmann....@fh-salzburg.ac.at> ---
After further investigations within [gcc-4.6.4\]gcc\config\i386\i386.c I have
found (maybe) the issue:

Within function ix86_compute_frame_layout there are 2 stack-realignments:
a) the local stack is aligned first, w/o including stack-needs (see b
afterwards)

  /* Align start of frame for local function.  */
  if (stack_realign_fp
      || offset != frame->sse_reg_save_offset
      || size != 0
      || !current_function_is_leaf
      || cfun->calls_alloca
      || ix86_current_function_calls_tls_descriptor)
    offset = (offset + stack_alignment_needed - 1) & -stack_alignment_needed;

b) after that a second re-calculation takes place, including the size of
alloca/pushed args/sub-calls:

  offset += size;

  ...

  /* Align stack boundary.  Only needed if we're calling another function
     or using alloca.  */
  if (!current_function_is_leaf || cfun->calls_alloca
      || ix86_current_function_calls_tls_descriptor)
    offset = (offset + preferred_alignment - 1) & -preferred_alignment;

Based on my understanding - the waste/gap of stack includes a) and b) together.
In my opinion - if stack_alignment_needed == preferred_alignment the first
alignment isn't nedded; I.e. a) is already included by b)

At leats, are there an further comments to my findings ... ?