x86, boot: #undef memcpy etc in string.c

undef memcpy and friends in boot/string.c so that the functions defined here will have the correct names, otherwise we end up up trying to redefine __builtin_memcpy etc. Surprisingly, gcc allows this (and, helpfully, discards the --builtin_ prefix from the function name when compiling it), but clang does not.

Adding these #undef's appears to preserve what I assume was the original intent of the code.

---
 arch/x86/boot/string.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 5457b02fc050..b40266850869 100644
@@ -16,6 +16,15 @@
 #include "ctype.h"
 #include "string.h"
 
+/*
+ * Undef these macros so that the functions that we provide
+ * here will have the correct names regardless of how string.h
+ * may have chosen to #define them.
+ */
+#undef memcpy
+#undef memset
+#undef memcmp
+
 int memcmp(const void *s1, const void *s2, size_t len)
 {
 	bool diff;

2.12.0.367.g23dc2f6d3c-goog

Use the standard regparm=0 calling convention for memcpy and memset when building with clang.

This is a work around for a long standing clang bug (see https://llvm.org/bugs/show_bug.cgi?id=3997) where clang always uses the standard regparm=0 calling convention for any implcit calls to memcpy and memset that it generates (eg for structure assignments and initialization) even if an alternate calling convention such as regparm=3 has been specified.

---
 arch/x86/boot/copy.S   | 15 +++++++++++++--
 arch/x86/boot/string.h | 13 +++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

index 1eb7d298b47d..57142d1ad0d2 100644
@@ -18,6 +18,12 @@
 	.text
 
 GLOBAL(memcpy)
+#ifdef	__clang__	/* Use normal ABI calling conventions */
+	movw	4(%esp), %ax
+	movw	8(%esp), %dx
+	movw	12(%esp), %cx
+#endif
+_memcpy:
 	pushw	%si
 	pushw	%di
 	movw	%ax, %di

@@ -34,6 +40,11 @@ GLOBAL(memcpy)
 ENDPROC(memcpy)
 
 GLOBAL(memset)
+#ifdef	__clang__	/* Use normal ABI calling conventions */
+	movw	4(%esp), %ax
+	movw	8(%esp), %dx
+	movw	12(%esp), %cx
+#endif
 	pushw	%di
 	movw	%ax, %di
 	movzbl	%dl, %eax

@@ -52,7 +63,7 @@ GLOBAL(copy_from_fs)
 	pushw	%ds
 	pushw	%fs
 	popw	%ds
-	calll	memcpy
+	calll	_memcpy
 	popw	%ds
 	retl
 ENDPROC(copy_from_fs)

@@ -61,7 +72,7 @@ GLOBAL(copy_to_fs)
 	pushw	%es
 	pushw	%fs
 	popw	%es
-	calll	memcpy
+	calll	_memcpy
 	popw	%es
 	retl
 ENDPROC(copy_to_fs)

index 113588ddb43f..e735cccb3fc8 100644
@@ -6,8 +6,21 @@
 #undef memset
 #undef memcmp
 
+/*
+ * Use normal ABI calling conventions - i.e. regparm(0) -
+ * for memcpy() and memset() if we are building the real
+ * mode setup code with clang since clang may make implicit
+ * calls to these functions that assume regparm(0).
+ */
+#if defined(_SETUP) && defined(__clang__)
+void __attribute__((regparm(0))) *memcpy(void *dst, const void *src,
+					 size_t len);
+void __attribute__((regparm(0))) *memset(void *dst, int c, size_t len);
+#else
 void *memcpy(void *dst, const void *src, size_t len);
 void *memset(void *dst, int c, size_t len);
+#endif
+
 int memcmp(const void *s1, const void *s2, size_t len);
 
 /*

2.12.0.367.g23dc2f6d3c-goog

Apologies for the lack of context. This change, and the #undef of mem* in x86/boot/string.c
are actually the only patches that are absolutely needed to get an x86_64 kernel to
build and boot successfully with clang provided that you also arrange for clang
be invoked with the -no-integrated-as option. In practice it is also necessary to disable
a few more clang warnings to make the build tolerable.

I will send out a more complete set of patches later today which deal with those other
issues.

As far as the specific issue of this particular clang bug is concerned I did also consider
adding a configuration option to simply not build the real mode boot code at all but
that looked like it would require much more extensive surgery.