cdecl calling convention expects the callee to pop the hidden pointer on

struct return. For example, GCC and LLVM comply with this (tested on IA32).

ACK doesn't seem to follow this convention and expects the caller to clean up
the stack. Compiling hand-written ACK-compliant assembly code (returning a 
struct) with GCC or LLVM used to break things (4-bytes misaligned stack).

The patch fixes this problem.

include/Makefile

@@ -21,7 +21,8 @@ INCS+=	minix/a.out.h minix/bitmap.h minix/callnr.h minix/cdrom.h \
 	minix/rs.h minix/safecopies.h minix/sef.h minix/sound.h \
 	minix/sys_config.h minix/sysinfo.h minix/syslib.h \
 	minix/sysutil.h minix/tty.h minix/type.h minix/types.h \
-	minix/u64.h minix/vfsif.h minix/vm.h
+	minix/u64.h minix/vfsif.h minix/vm.h \
+	minix/compiler.h minix/compiler-ack.h
 INCS+=	net/hton.h net/if.h net/ioctl.h net/netlib.h
 INCS+=	net/gen/arp_io.h net/gen/dhcp.h net/gen/ether.h \
 	net/gen/eth_hdr.h net/gen/eth_io.h net/gen/icmp.h \

include/minix/compiler-ack.h

@@ -0,0 +1,9 @@
+/* Definitions for ACK-specific features. */
+
+#ifndef _MINIX_COMPILER_ACK_H
+#define _MINIX_COMPILER_ACK_H
+
+/* ACK expects the caller to pop the hidden pointer on struct return. */
+#define BYTES_TO_POP_ON_STRUCT_RETURN
+
+#endif /* _MINIX_COMPILER_ACK_H */

include/minix/compiler.h

@@ -0,0 +1,25 @@
+/* Definitions for compiler-specific features. */
+
+#ifndef _MINIX_COMPILER_H
+#define _MINIX_COMPILER_H
+
+/*===========================================================================*
+ *			      Compiler overrides			     *
+ *===========================================================================*/
+/* ACK */
+#ifdef __ACK__
+#include <minix/compiler-ack.h>
+#endif
+
+/*===========================================================================*
+ *				Default values				     *
+ *===========================================================================*/
+/*
+ * cdecl calling convention expects the callee to pop the hidden pointer on
+ * struct return. For example, GCC and LLVM comply with this (tested on IA32).
+ */
+#ifndef BYTES_TO_POP_ON_STRUCT_RETURN
+#define BYTES_TO_POP_ON_STRUCT_RETURN $4
+#endif
+
+#endif /* _MINIX_COMPILER_H */