Re: port-macppc/54827: alloca() is broken in gcc 8 on powerpc

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

The following reply was made to PR port-macppc/54827; it has been noted by GNATS.

From: Valery Ushakov <uwe%stderr.spb.ru@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: port-macppc/54827: alloca() is broken in gcc 8 on powerpc
Date: Wed, 22 Jan 2020 17:23:19 +0300

 I think I found a real fix, though I have no idea why does it work and
 how the old code did ever work before :)
 
 As mentioned earlier, our in-tree rs6000/netbsd.h overrides
 STACK_BOUNDARY to always be 16 bytes:
 
   /* Override STACK_BOUNDARY to use Altivec compliant one.  */
   #undef STACK_BOUNDARY
   #define STACK_BOUNDARY	128
 
 
 Stock gcc rs6000/netbsd.h doesn't have that override, but still emits
 asm for stack frame aligned to 16 bytes.
 
 This is b/c the SYSV ABI that we use already mandates 16 bytes aligned
 stack frames (8 bytes alignment only seems to be used by the embedded
 EABI).  This is effected by rs6000/sysv4.h
 
   #undef PREFERRED_STACK_BOUNDARY
   #define PREFERRED_STACK_BOUNDARY 128
 
 It's included by tm.h before netbsd.h
 
 
 defaults.h takes care of defining PREFERRED_STACK_BOUNDARY if
 necessary
 
   /* If PREFERRED_STACK_BOUNDARY is not defined, set it to STACK_BOUNDARY.
      STACK_BOUNDARY is required.  */
   #ifndef PREFERRED_STACK_BOUNDARY
   #define PREFERRED_STACK_BOUNDARY STACK_BOUNDARY
   #endif
 
 
 Another relevant comment can be found in combine-stack-adj.c:
 
    [...] It is of significantly more value to targets that define
    PREFERRED_STACK_BOUNDARY more aligned than STACK_BOUNDARY [...]
 
 
 So it looks like netbsd.h 1) overrides wrong define, 2) that is
 already overriden by a "base" header.  And so we ended up with
 STACK_BOUNDARY == PREFERRED_STACK_BOUNDARY == 16 instead of stock
 version STACK_BOUNDARY == 8 and PREFERRED_STACK_BOUNDARY == 16.
 
 I'm not qualified to comment on why ppc code is written the way it is
 (using STACK_BOUNDARY that is not actually used by the ABI it emits
 the code for) and what broke between 7 and 8 to make our old
 arrangement fail.  Dropping the override from netbsd.h makes in-tree
 compiler emit the same asm for all the four -O{0,2} -m{no-,}altivec
 combinations as the stock gcc for the alloca-test.c above.  The stack
 frames are still 16 bytes aligned (as mandated by SYSV ABI).
 
 
 --- netbsd.h.~1.14.~	    2020-01-22 16:31:12.806472210 +0300
 +++ netbsd.h		    2020-01-22 16:31:28.902491784 +0300
 @@ -138,10 +138,6 @@
  #define TRAMPOLINE_SIZE 48
  #endif
  
 -/* Override STACK_BOUNDARY to use Altivec compliant one.  */
 -#undef STACK_BOUNDARY
 -#define STACK_BOUNDARY	128
 -
  /* Use standard DWARF numbering for DWARF debugging information.  */
  #define RS6000_USE_DWARF_NUMBERING
 
 -uwe