[PATCH v10 1/4] x86/boot: comment out and document redundant "cc" clobber in memcmp()
From: Mauricio Faria de Oliveira
Date: Mon Sep 21 2026 - 21:36:35 EST
The "cc" clobber remains recognized for source compatibility, but it has
no meaning anymore; it is automatically generated without condition-code
constraints (explanation in [1]; related code in gcc [2] and clang [3]).
Comment out the redundant "cc" clobber for documentation purposes.
Reported-by: "H. Peter Anvin" <hpa@xxxxxxxxx>
Link: https://lore.kernel.org/all/5e19b195-0ca2-4510-81cb-497b40e4aaf5@xxxxxxxxx/
Link: https://lore.kernel.org/all/57b0d188-b256-bde4-43e6-99dae4f59d60@xxxxxxx/ [1]
Link: https://github.com/gcc-mirror/gcc/blob/78d4ac73dd391005b895a6148cd9831e28e1208b/gcc/config/i386/i386.cc#L25355-L25363 [2]
Link: https://github.com/llvm/llvm-project/blob/6dfe1677ab8dffbc6ec13d53a1e0215d75147689/clang/lib/Basic/Targets/X86.h#L299-L301 [3]
Fixes: a8c171c107c0 ("x86/boot: Add volatile, clobbers and zero-length test in memcmp()")
Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
---
arch/x86/boot/string.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index 1632d40e1f545ae0665597069b568ea6b6c263e5..e10260ed58b69fc734a4e63f15785a643f414441 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -36,11 +36,15 @@ int memcmp(const void *s1, const void *s2, size_t len)
/*
* Make sure ZF is properly set in the len==0 case because in it,
* RCX==0 and the REPE; CMPSB won't get executed.
+ *
+ * The "cc" clobber has no meaning anymore, just source compatibility.
+ * On x86 the flag status bits are automatically added to the clobber
+ * set when there are no =@ccXY constraints. Keep it as documentation.
*/
asm volatile("test %3, %3\n\t"
"repe cmpsb"
: "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len)
- : : "cc", "memory");
+ : : /* "cc", */ "memory");
return diff;
}
--
2.47.3