sysv_386: make stack aligned in stackman_call

Incoming stack pointer must be adjusted to make this happen.
Also adjust old stack pointer to make it correspond to incoming stack pointer.

Author: kristjanvalur

lib/sysv_i386/libstackman.a

@@ -82,22 +82,25 @@ stackman_call:
 # 108 "switch_x86_gcc.h" 1
 	movl %esp, %ebx
 # 0 "" 2
-# 109 "switch_x86_gcc.h" 1
-	movl %esp, %eax
+# 110 "switch_x86_gcc.h" 1
+	leal 4(%esp), %eax
 # 0 "" 2
 #NO_APP
 	movl	16(%ebp), %edx
 #APP
-# 112 "switch_x86_gcc.h" 1
+# 113 "switch_x86_gcc.h" 1
 	movl %edx, %esp
 # 0 "" 2
+# 115 "switch_x86_gcc.h" 1
+	subl $4, %esp
+# 0 "" 2
 #NO_APP
 	pushl	%eax
 	pushl	$2
 	pushl	12(%ebp)
 	call	*8(%ebp)
 #APP
-# 116 "switch_x86_gcc.h" 1
+# 119 "switch_x86_gcc.h" 1
 	movl %ebx, %esp
 # 0 "" 2
 #NO_APP

src/platforms/switch_x86_gcc.S

@@ -106,10 +106,13 @@ void *stackman_call(stackman_cb_t callback, void *context, void *stack_pointer)
 
   /* sp = store stack pointer in ebx */
   __asm__ ("movl %%esp, %%ebx" : : : "ebx");
-  __asm__ ("movl %%esp, %[sp]" : [sp] "=r" (old_sp));
+  /* save old stack pointer at same offset as new stack pointer */
+  __asm__ ("leal 4(%%esp), %[sp]" : [sp] "=r" (old_sp));
 
   /* set stack pointer from provided using assembly */
   __asm__ ("movl %[sp], %%esp" :: [sp] "r" (stack_pointer));
+  /* subtract 4 bytes to make it 16 byte alighed after pushing 3 args */
+  __asm__ ("subl $4, %esp");
 
   result = callback(context, STACKMAN_OP_CALL, old_sp);
   /* restore stack pointer */