[Bug bootstrap/77359] [7 Regression] AIX bootstrap failure due to alignment of stack pointer + STACK_DYNAMIC_OFFSET

["vogt at linux dot vnet.ibm.com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77359

--- Comment #5 from Dominik Vogt <vogt at linux dot vnet.ibm.com> ---
Sounds fair.  So, either the bug is that the stack pointer has 8 byte
alignment, or the formula for STACK_DYNAMIC_OFFSET results in the the wrong
amount:

-- rs6000.h --
/* Offset from the stack pointer register to an item dynamically 
   allocated on the stack, e.g., by `alloca'. 

   The default value for this macro is `STACK_POINTER_OFFSET' plus the 
   length of the outgoing arguments.  The default is correct for most 
   machines.  See `function.c' for details.  */ 
#define STACK_DYNAMIC_OFFSET(FUNDECL)                                   \ 
  (RS6000_ALIGN (crtl->outgoing_args_size,                              \ 
                 (TARGET_ALTIVEC || TARGET_VSX) ? 16 : 8)               \ 
   + (STACK_POINTER_OFFSET)) 
-- snip --

Or there is some place that doesn't properly use STACK_DYNAMIC_OFFSET().

--

Hm, if neither TARGET_ALTIVEC nor TARGET_VSX is set, we would get an 8 byte
aligned STACK_DYNAMIC_OFFSET while the STACK_BOUNDARY from aix.h is 128.  The
common code uses STACK_BOUNDARY as the assumed alignment, which is wrong =>
bang.

I don't know about the AIX Abi, but there are (at least) three ways to fix
that:

1) Provide a version in STACK_DYNAMIC_OFFSET in aix.h that always rounds up to
STACK_BOUNDARY.

2) In aix.h, use 128 bit STACK_BOUNDARY only if TARGET_ALITVEC or TARGET_VSX is
set (probably breaks the Abi?).

3) Introduce another macro into common code to override the hard wired
REGNO_POINTER_ALIGN (VIRTUAL_STACK_DYNAMIC_REGNUM) = STACK_BOUNDARY.  Use a new
macro that with the value "((TARGET_ALTIVEC || TARGET_VSX) ? 128 : 64)".

The latter is probably the most flexible solution, but I'm not sure if that's
really necessary.