Re: [Xenomai-core] gcc-4.6 issue

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 08/11/2011 10:52 PM, Gilles Chanteperdrix wrote:
> On 08/11/2011 07:31 PM, Gilles Chanteperdrix wrote:
>> On 08/11/2011 07:21 PM, Roland Stigge wrote:
>>> Hi,
>>>
>>> On 08/11/2011 04:48 PM, Daniele Nicolodi wrote:
>>>> I compiled linux 2.6.38.8 and xenomai-head with gcc-4.6. The obtained
>>>> kernel boots fine but xenomai services do not: latency hangs right after
>>>> the sched_setscheduler system call. With the same kernel I compiled user
>>>> space with gcc-4.4 and xenomia services work just fine.
>>>
>>> I can confirm this now for Debian. Sorry for the delay.
>>>
>>> Will use gcc-4.4 for building the Debian package for now.
>>>
> 
> The gcc 4.6 version is compiled with frame pointer disabled, whereas the
> 4.4 version is compiled with frame pointers. Enabling the
> -fomit-frame-pointer option with gcc 4.4 seems to cause issues as well.
> 
> With gcc 4.4, the difference is very visible in rt_task_start. The
> correct syscall chunk is:
> 
>   11:	0d 2b 02 00 02	     	or     $0x200022b,%eax
> 
> Computes the syscall number, put the result in eax.
> 
>   16:	89 45 fc	     	mov    %eax,-0x4(%ebp)
> 
> Move the result from eax to temporary space on stack.
> 
>   19:	8b 45 08	     	mov    0x8(%ebp),%eax
> 
> Load syscall first argument value into eax
> 
>   1c:	53		     	push   %ebx
>   1d:	89 c3		     	mov    %eax,%ebx
> 
> Push ebx, move syscall first argument from eax to ebx
> 
>   1f:	8b 45 fc	     	mov    -0x4(%ebp),%eax
> 
> Reload the syscall number from its temporary storage to eax.
> 
>   22:	65 ff 15 10 00 00 00 	call   *%gs:0x10
>   29:	5b		     	pop    %ebx
> 
> Syscall done.
> 
> The version without frame pointer:
>     4e3d:	8b 00		     	mov    (%eax),%eax
>     4e3f:	0d 2b 02 00 02	     	or     $0x200022b,%eax
>     4e44:	89 44 24 0c	     	mov    %eax,0xc(%esp)
>     4e48:	8b 44 24 18	     	mov    0x18(%esp),%eax
>     4e4c:	53		     	push   %ebx
>     4e4d:	89 c3		     	mov    %eax,%ebx
>     4e4f:	8b 44 24 0c	     	mov    0xc(%esp),%eax
>     4e53:	65 ff 15 10 00 00 00 	call   *%gs:0x10
>     4e5a:	5b		     	pop    %ebx
> 
> This is essentially the same sequence, except that the temporary storage
> used to store the syscall number is obtained via a relative offset of
> the stack pointer (esp) instead of a relative offset of the frame
> pointer, since we are compiling without frame pointers, the problem is
> that between the time this value is stored, and the time it is restored,
> the stack pointer changed since we pushed %ebx.
> 
> So, I do not really know what to make of it. The simple solution seems
> to me to continue compiling with frame pointers. I.e. add
> -fno-omit-frame-pointer with gcc 4.6.
> 

The following patch seems to do the trick. It makes the assumption that 
when compiling with -fomit-frame-pointer, we have one more register, so
the "R" constraint will always be able to avoid choosing eax, and eax 
will be free for the muxcode, so that the compiler will not choose the 
"m" alternative.

diff --git a/include/asm-x86/syscall.h b/include/asm-x86/syscall.h
index 3beb1ca..e51aa86 100644
--- a/include/asm-x86/syscall.h
+++ b/include/asm-x86/syscall.h
@@ -157,8 +157,8 @@ static inline void __xn_get_ebp(void **dest)
 		"movl %1, %%eax\n\t"				\
 		DOSYSCALL					\
 		RESTOREARGS_##nr				\
-		: "=a" (__resultvar)				\
-		: "i" (__xn_mux_code(0, op)) ASMFMT_##nr(args)	\
+		: "=a,a" (__resultvar)				\
+		: "i,i" (__xn_mux_code(0, op)) ASMFMT_##nr(args)	\
 		: "memory", "cc");				\
 	(int) __resultvar;					\
 })
@@ -171,8 +171,8 @@ static inline void __xn_get_ebp(void **dest)
 		"movl %1, %%eax\n\t"				\
 		DOSYSCALLSAFE					\
 		RESTOREARGS_##nr				\
-		: "=a" (__resultvar)				\
-		: "i" (__xn_mux_code(0, op)) ASMFMT_##nr(args)	\
+		: "=a,a" (__resultvar)				\
+		: "i,i" (__xn_mux_code(0, op)) ASMFMT_##nr(args)	\
 		: "memory", "cc");				\
 	(int) __resultvar;					\
 })
@@ -186,8 +186,8 @@ static inline void __xn_get_ebp(void **dest)
 		"movl %1, %%eax\n\t"				\
 		DOSYSCALL					\
 		RESTOREARGS_##nr				\
-		: "=a" (__resultvar)				\
-		: "m" (__muxcode) ASMFMT_##nr(args)		\
+		: "=a,a" (__resultvar)				\
+		: "a,m" (__muxcode) ASMFMT_##nr(args)		\
 		: "memory", "cc");				\
 	(int) __resultvar;					\
 })
@@ -211,15 +211,15 @@ static inline void __xn_get_ebp(void **dest)
 
 #define ASMFMT_0()
 #define ASMFMT_1(arg1) \
-	, "acdSD" (arg1)
+	, "R,R" (arg1)
 #define ASMFMT_2(arg1, arg2) \
-	, "adSD" (arg1), "c" (arg2)
+	, "R,R" (arg1), "c,c" (arg2)
 #define ASMFMT_3(arg1, arg2, arg3) \
-	, "aSD" (arg1), "c" (arg2), "d" (arg3)
+	, "R,R" (arg1), "c,c" (arg2), "d,d" (arg3)
 #define ASMFMT_4(arg1, arg2, arg3, arg4) \
-	, "aD" (arg1), "c" (arg2), "d" (arg3), "S" (arg4)
+	, "R,R" (arg1), "c,c" (arg2), "d,d" (arg3), "S,S" (arg4)
 #define ASMFMT_5(arg1, arg2, arg3, arg4, arg5) \
-	, "a" (arg1), "c" (arg2), "d" (arg3), "S" (arg4), "D" (arg5)
+	, "R,R" (arg1), "c,c" (arg2), "d,d" (arg3), "S,S" (arg4), "D,D" (arg5)
 
 #define XENOMAI_SYSBIND(a1,a2,a3,a4) \
 	XENOMAI_SYS_MUX_SAFE(4,__xn_sys_bind,a1,a2,a3,a4)


-- 
                                                                Gilles.