[PATCH v10 1/4] x86/boot: comment out and document redundant "cc" clobber in memcmp()

[Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>]

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