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[PATCH] x86/string: correct memmove()'s forwarding to memcpy()

With memcpy() expanding to the compiler builtin, we may not hand it
overlapping source and destination. We strictly mean to forward to our
own implementation (a few lines up in the same source file).

Fixes: 78825e1c60fa ("x86/string: Clean up x86/string.h")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
An alternative would be to "#undef memcpy" near the top of the file. But
I think the way it's done now is more explicit to the reader. An #undef
would be the only way if the macro was an object-like one.

At least with gcc10 this does alter generated code: The builtin gets
expanded into a tail call, while after this change our memcpy() gets
inlined into memmove(). This would change again once we separate the 3
functions here into their own CUs for placing them in an archive.

--- a/xen/arch/x86/string.c
+++ b/xen/arch/x86/string.c
@@ -43,7 +43,7 @@ void *(memmove)(void *dest, const void *
         return dest;
     if ( dest < src )
-        return memcpy(dest, src, n);
+        return (memcpy)(dest, src, n);
     asm volatile (
         "   std         ; "



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