[v1] Fix redundant judgment in WARN_ONCE with clang

[PATCH] Fix redundant judgment in WARN_ONCE with clang

Posted by Xie Yuanbin 1 month, 1 week ago

For c code:
```c
extern int xx;
void test(void)
{
	if (WARN_ONCE(xx, "x"))
		__asm__ volatile ("nop":::);
}
```

Clang will generate the following assembly code:
```assemble
test:
	movl	xx(%rip), %eax // Assume xx == 0 (likely case)
	testl	%eax, %eax // judge once
	je	.LBB0_3    // jump to .LBB0_3
	testb	$1, test.__already_done(%rip)
	je	.LBB0_2
.LBB0_3:
	testl	%eax, %eax // judge again
	je	.LBB0_5    // jump to .LBB0_5
.LBB0_4:
	nop
.LBB0_5:
	retq
	// omit
```

In the above code, `xx == 0` should be a likely case, but in this case,
xx has been judged twice.

Test info:
1. kernel source:
linux-next
commit 9c0826a5d9aa4d52206d ("Add linux-next specific files for 20251107")
2. compiler:
clang: Debian clang version 21.1.4 (8) with
Debian LLD 21.1.4 (compatible with GNU linkers)
3. config:
base on default x86_64_defconfig, and setting:
CONFIG_MITIGATION_RETHUNK=n
CONFIG_STACKPROTECTOR=n

Add unlikely to __ret_cond to help the compiler optimize correctly.

Signed-off-by: Xie Yuanbin <qq570070308@gmail.com>
---
 include/linux/once_lite.h | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/include/linux/once_lite.h b/include/linux/once_lite.h
index 27de7bc32a06..194c5ebbfbc4 100644
--- a/include/linux/once_lite.h
+++ b/include/linux/once_lite.h
@@ -10,17 +10,17 @@
 #define DO_ONCE_LITE(func, ...)						\
 	DO_ONCE_LITE_IF(true, func, ##__VA_ARGS__)
 
-#define __ONCE_LITE_IF(condition)					\
-	({								\
-		static bool __section(".data..once") __already_done;	\
-		bool __ret_cond = !!(condition);			\
-		bool __ret_once = false;				\
-									\
-		if (unlikely(__ret_cond && !__already_done)) {		\
-			__already_done = true;				\
-			__ret_once = true;				\
-		}							\
-		unlikely(__ret_once);					\
+#define __ONCE_LITE_IF(condition)						\
+	({									\
+		static bool __section(".data..once") __already_done;		\
+		bool __ret_cond = !!(condition);				\
+		bool __ret_once = false;					\
+										\
+		if (unlikely(__ret_cond) && unlikely(!__already_done)) {	\
+			__already_done = true;					\
+			__ret_once = true;					\
+		}								\
+		unlikely(__ret_once);						\
 	})
 
 #define DO_ONCE_LITE_IF(condition, func, ...)				\
-- 
2.51.0

Re: [PATCH] Fix redundant judgment in WARN_ONCE with clang

Posted by Andrew Morton 1 month, 1 week ago

On Sun,  9 Nov 2025 16:37:15 +0800 Xie Yuanbin <qq570070308@gmail.com> wrote:

> For c code:
> ```c
> extern int xx;
> void test(void)
> {
> 	if (WARN_ONCE(xx, "x"))
> 		__asm__ volatile ("nop":::);
> }
> ```
> 
> Clang will generate the following assembly code:
> ```assemble
> test:
> 	movl	xx(%rip), %eax // Assume xx == 0 (likely case)
> 	testl	%eax, %eax // judge once
> 	je	.LBB0_3    // jump to .LBB0_3
> 	testb	$1, test.__already_done(%rip)
> 	je	.LBB0_2
> .LBB0_3:
> 	testl	%eax, %eax // judge again
> 	je	.LBB0_5    // jump to .LBB0_5
> .LBB0_4:
> 	nop
> .LBB0_5:
> 	retq
> 	// omit
> ```
> 
> In the above code, `xx == 0` should be a likely case, but in this case,
> xx has been judged twice.
>
> ...
>
> --- a/include/linux/once_lite.h
> +++ b/include/linux/once_lite.h
> @@ -10,17 +10,17 @@
>  #define DO_ONCE_LITE(func, ...)						\
>  	DO_ONCE_LITE_IF(true, func, ##__VA_ARGS__)
>  
> -#define __ONCE_LITE_IF(condition)					\
> -	({								\
> -		static bool __section(".data..once") __already_done;	\
> -		bool __ret_cond = !!(condition);			\
> -		bool __ret_once = false;				\
> -									\
> -		if (unlikely(__ret_cond && !__already_done)) {		\
> -			__already_done = true;				\
> -			__ret_once = true;				\
> -		}							\
> -		unlikely(__ret_once);					\
> +#define __ONCE_LITE_IF(condition)						\
> +	({									\
> +		static bool __section(".data..once") __already_done;		\
> +		bool __ret_cond = !!(condition);				\
> +		bool __ret_once = false;					\
> +										\
> +		if (unlikely(__ret_cond) && unlikely(!__already_done)) {	\

Sure, !__already_done is unlikely here.

> +			__already_done = true;					\
> +			__ret_once = true;					\
> +		}								\
> +		unlikely(__ret_once);						\

It's a shame you messed with the whitespace.  And I don't think it was
necessary anyway.  Here's what it would have looked like:

--- a/include/linux/once_lite.h~fix-redundant-judgment-in-warn_once-with-clang
+++ a/include/linux/once_lite.h
@@ -16,7 +16,7 @@
 		bool __ret_cond = !!(condition);			\
 		bool __ret_once = false;				\
 									\
-		if (unlikely(__ret_cond && !__already_done)) {		\
+		if (unlikely(__ret_cond) && unlikely(!__already_done)) {\
 			__already_done = true;				\
 			__ret_once = true;				\
 		}							\
_

Re: [PATCH] Fix redundant judgment in WARN_ONCE with clang

Posted by Xie Yuanbin 1 month, 1 week ago

On Sun, 9 Nov 2025 09:03:17 -0800, Andrew Morton wrote:
> It's a shame you messed with the whitespace.  And I don't think it was
> necessary anyway.  Here's what it would have looked like:
>
> --- a/include/linux/once_lite.h~fix-redundant-judgment-in-warn_once-with-clang
> +++ a/include/linux/once_lite.h
> @@ -16,7 +16,7 @@
> 		bool __ret_cond = !!(condition);			\
> 		bool __ret_once = false;				\
> 									\
> -		if (unlikely(__ret_cond && !__already_done)) {		\
> +		if (unlikely(__ret_cond) && unlikely(!__already_done)) {\
> 			__already_done = true;				\
> 			__ret_once = true;				\
> 		}							\

Okay, thanks, I will send the v2 patch.

Xie Yuanbin