diff --git a/include/Makefile b/include/Makefile
index b33a8b9f1..a50426d6c 100644
--- a/include/Makefile
+++ b/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 \
diff --git a/include/minix/compiler-ack.h b/include/minix/compiler-ack.h
new file mode 100644
index 000000000..a5ab4db10
--- /dev/null
+++ b/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 */
diff --git a/include/minix/compiler.h b/include/minix/compiler.h
new file mode 100644
index 000000000..25732d156
--- /dev/null
+++ b/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 */
diff --git a/lib/libc/arch/i386/int64/add64.S b/lib/libc/arch/i386/int64/add64.S
index 0518b8d43..573bd7ff8 100644
--- a/lib/libc/arch/i386/int64/add64.S
+++ b/lib/libc/arch/i386/int64/add64.S
@@ -1,5 +1,7 @@
 /*	add64() - 64 bit addition			Author: Kees J. Bot */
 /*								7 Dec 1995 */
+#include <minix/compiler.h>
+
 .text
 .globl	_add64
 
@@ -12,7 +14,7 @@ _add64:
 	movl	12(%esp), %edx
 	adcl	20(%esp), %edx
 	movl	%edx, 4(%eax)
-	ret
+	ret BYTES_TO_POP_ON_STRUCT_RETURN
 
 /* */
 /* $PchId: add64.ack.s,v 1.2 1996/04/11 18:59:57 philip Exp $ */
diff --git a/lib/libc/arch/i386/int64/add64u.S b/lib/libc/arch/i386/int64/add64u.S
index 5a63df8fd..41a0a93a6 100644
--- a/lib/libc/arch/i386/int64/add64u.S
+++ b/lib/libc/arch/i386/int64/add64u.S
@@ -1,5 +1,7 @@
 /*	add64u() - unsigned to 64 bit addition		Author: Kees J. Bot */
 /*								7 Dec 1995 */
+#include <minix/compiler.h>
+
 .text
 .globl	_add64u, _add64ul
 
@@ -14,7 +16,7 @@ _add64ul:
 	movl	12(%esp), %edx
 	adcl	$0, %edx
 	movl	%edx, 4(%eax)
-	ret
+	ret BYTES_TO_POP_ON_STRUCT_RETURN
 
 /* */
 /* $PchId: add64u.ack.s,v 1.2 1996/04/11 18:59:57 philip Exp $ */
diff --git a/lib/libc/arch/i386/int64/cvu64.S b/lib/libc/arch/i386/int64/cvu64.S
index 13fa832c1..8513f9d0e 100644
--- a/lib/libc/arch/i386/int64/cvu64.S
+++ b/lib/libc/arch/i386/int64/cvu64.S
@@ -1,5 +1,7 @@
 /*	cvu64() - unsigned converted to 64 bit		Author: Kees J. Bot */
 /*								7 Dec 1995 */
+#include <minix/compiler.h>
+
 .text
 .globl	_cvu64, _cvul64
 
@@ -11,7 +13,7 @@ _cvul64:
 	movl	8(%esp), %edx
 	movl	%edx, (%eax)
 	movl	$0, 4(%eax)
-	ret
+	ret BYTES_TO_POP_ON_STRUCT_RETURN
 
 /* */
 /* $PchId: cvu64.ack.s,v 1.2 1996/04/11 18:59:57 philip Exp $ */
diff --git a/lib/libc/arch/i386/int64/make64.S b/lib/libc/arch/i386/int64/make64.S
index c53474bb1..27b5a1d98 100644
--- a/lib/libc/arch/i386/int64/make64.S
+++ b/lib/libc/arch/i386/int64/make64.S
@@ -1,6 +1,8 @@
 /*	make64() - make a 64 bit number from two 32 bit halves */
 /*							Author: Kees J. Bot */
 /*								7 Dec 1995 */
+#include <minix/compiler.h>
+
 .text
 .globl	_make64
 
@@ -11,7 +13,7 @@ _make64:
 	movl	%edx, (%eax)
 	movl	12(%esp), %edx
 	movl	%edx, 4(%eax)
-	ret
+	ret BYTES_TO_POP_ON_STRUCT_RETURN
 
 /* */
 /* $PchId: make64.ack.s,v 1.2 1996/04/11 18:59:57 philip Exp $ */
diff --git a/lib/libc/arch/i386/int64/mul64u.S b/lib/libc/arch/i386/int64/mul64u.S
index 60e8c487c..fd693a62e 100644
--- a/lib/libc/arch/i386/int64/mul64u.S
+++ b/lib/libc/arch/i386/int64/mul64u.S
@@ -1,6 +1,8 @@
 /*	mul64u() - unsigned long by unsigned multiply giving 64 bit result */
 /*							Author: Kees J. Bot */
 /*								7 Dec 1995 */
+#include <minix/compiler.h>
+
 .text
 .globl	_mul64u
 
@@ -12,7 +14,7 @@ _mul64u:
 	movl	%eax, (%ecx)
 	movl	%edx, 4(%ecx)
 	movl	%ecx, %eax
-	ret
+	ret BYTES_TO_POP_ON_STRUCT_RETURN
 
 /* */
 /* $PchId: mul64u.ack.s,v 1.2 1996/04/11 18:59:57 philip Exp $ */
diff --git a/lib/libc/arch/i386/int64/sub64.S b/lib/libc/arch/i386/int64/sub64.S
index 5100311be..fc1d80ad7 100644
--- a/lib/libc/arch/i386/int64/sub64.S
+++ b/lib/libc/arch/i386/int64/sub64.S
@@ -1,5 +1,7 @@
 /*	sub64() - 64 bit subtraction			Author: Kees J. Bot */
 /*								7 Dec 1995 */
+#include <minix/compiler.h>
+
 .text
 .globl	_sub64
 
@@ -12,7 +14,7 @@ _sub64:
 	movl	12(%esp), %edx
 	sbbl	20(%esp), %edx
 	movl	%edx, 4(%eax)
-	ret
+	ret BYTES_TO_POP_ON_STRUCT_RETURN
 
 /* */
 /* $PchId: sub64.ack.s,v 1.2 1996/04/11 18:59:57 philip Exp $ */
diff --git a/lib/libc/arch/i386/int64/sub64u.S b/lib/libc/arch/i386/int64/sub64u.S
index 84314f275..2f5c2d45f 100644
--- a/lib/libc/arch/i386/int64/sub64u.S
+++ b/lib/libc/arch/i386/int64/sub64u.S
@@ -1,5 +1,7 @@
 /*	sub64() - unsigned from 64 bit subtraction	Author: Kees J. Bot */
 /*								7 Dec 1995 */
+#include <minix/compiler.h>
+
 .text
 .globl	_sub64u, _sub64ul
 
@@ -14,7 +16,7 @@ _sub64ul:
 	movl	12(%esp), %edx
 	sbbl	$0, %edx
 	movl	%edx, 4(%eax)
-	ret
+	ret BYTES_TO_POP_ON_STRUCT_RETURN
 
 /* */
 /* $PchId: sub64u.ack.s,v 1.2 1996/04/11 18:59:57 philip Exp $ */