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

@@ -25,12 +25,10 @@
  */
 
 /*      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>

@@ -44,10 +42,36 @@
  * 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, ...)
 {

@@ -58,26 +82,31 @@
 
         ucp->uc_mcontext.gregs[EIP] = (greg_t)func;
 
         size = sizeof (long) * (argc + 1);
 
-        sp = (long *)(((uintptr_t)ucp->uc_stack.ss_sp +
+        tsp = (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);
+        /*
+         * 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)