patch feedback
6507 i386 makecontext(3c) needs to 16-byte align the stack

*** 25,36 ****
   */
  
  /*      Copyright (c) 1988 AT&T */
  /*        All Rights Reserved   */
  
- #pragma ident   "%Z%%M% %I%     %E% SMI"
- 
  #pragma weak _makecontext = makecontext
  
  #include "lint.h"
  #include <stdarg.h>
  #include <ucontext.h>
--- 25,34 ----
*** 44,53 ****
--- 42,77 ----
   * When makecontext() returns, the ucontext_t will be set to run the
   * given function with the given parameters on the stack specified by
   * uc_stack, and which will return to the ucontext_t specified by uc_link.
   */
  
+ /*
+  * The original i386 ABI said that the stack pointer need be only 4-byte
+  * aligned before a function call (STACK_ALIGN == 4).  The ABI supplement
+  * version 1.0 changed the required alignment to 16-byte for the benefit of
+  * floating point code compiled using sse2.  The compiler assumes this
+  * alignment and maintains it for calls it generates.  If the stack is
+  * initially properly aligned, it will continue to be so aligned.  If it is
+  * not initially so aligned, it will never become so aligned.
+  *
+  * One slightly confusing detail to keep in mind is that the 16-byte
+  * alignment (%esp & 0xf == 0) is true just *before* the call instruction.
+  * The call instruction will then push a return value, decrementing %esp by
+  * 4.  Therefore, if one dumps %esp at the at the very first instruction in
+  * a function, it will end with a 0xc.  The compiler expects this and
+  * compensates for it properly.
+  *
+  * Note: If you change this value, you need to change it in the following
+  * files as well:
+  *
+  *  - lib/libc/i386/threads/machdep.c
+  *  - lib/common/i386/crti.s
+  *  - lib/common/i386/crt1.s
+  */
+ #undef  STACK_ALIGN
+ #define STACK_ALIGN     16
+ 
  static void resumecontext(void);
  
  void
  makecontext(ucontext_t *ucp, void (*func)(), int argc, ...)
  {
*** 58,83 ****
  
          ucp->uc_mcontext.gregs[EIP] = (greg_t)func;
  
          size = sizeof (long) * (argc + 1);
  
!         sp = (long *)(((uintptr_t)ucp->uc_stack.ss_sp +
              ucp->uc_stack.ss_size - size) & ~(STACK_ALIGN - 1));
  
!         tsp = sp + 1;
! 
!         va_start(ap, argc);
! 
!         while (argc-- > 0) {
!                 *tsp++ = va_arg(ap, long);
!         }
! 
!         va_end(ap);
  
          *sp = (long)resumecontext;              /* return address */
  
          ucp->uc_mcontext.gregs[UESP] = (greg_t)sp;
  }
  
  
  static void
  resumecontext(void)
--- 82,112 ----
  
          ucp->uc_mcontext.gregs[EIP] = (greg_t)func;
  
          size = sizeof (long) * (argc + 1);
  
!         tsp = (long *)(((uintptr_t)ucp->uc_stack.ss_sp +
              ucp->uc_stack.ss_size - size) & ~(STACK_ALIGN - 1));
  
!         /*
!          * Since we're emulating the call instruction, we must push the
!          * return address (which involves adjusting the stack pointer to
!          * have the proper 4-byte bias).
!          */
!         sp = tsp - 1;
  
          *sp = (long)resumecontext;              /* return address */
  
          ucp->uc_mcontext.gregs[UESP] = (greg_t)sp;
+ 
+         /*
+          * "push" all the arguments
+          */
+         va_start(ap, argc);
+         while (argc-- > 0)
+                 *tsp++ = va_arg(ap, long);
+         va_end(ap);
  }
  
  
  static void
  resumecontext(void)