lib/xor: make xor prototypes more friendly to compiler vectorization

Modern compilers are perfectly capable of extracting parallelism from
the XOR routines, provided that the prototypes reflect the nature of the
input accurately, in particular, the fact that the input vectors are
expected not to overlap. This is not documented explicitly, but is
implied by the interchangeability of the various C routines, some of
which use temporary variables while others don't: this means that these
routines only behave identically for non-overlapping inputs.

So let's decorate these input vectors with the __restrict modifier,
which informs the compiler that there is no overlap. While at it, make
the input-only vectors pointer-to-const as well.

Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/563
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 14 insertions, 7 deletions

diff --git a/arch/x86/include/asm/xor_avx.h b/arch/x86/include/asm/xor_avx.h
index 0c4e5b5e3852..7f81dd5897f4 100644
--- a/arch/x86/include/asm/xor_avx.h
+++ b/arch/x86/include/asm/xor_avx.h
@@ -26,7 +26,8 @@
 		BLOCK4(8) \
 		BLOCK4(12)
 
-static void xor_avx_2(unsigned long bytes, unsigned long *p0, unsigned long *p1)
+static void xor_avx_2(unsigned long bytes, unsigned long * __restrict p0,
+		      const unsigned long * __restrict p1)
 {
 	unsigned long lines = bytes >> 9;
 
@@ -52,8 +53,9 @@ do { \
 	kernel_fpu_end();
 }
 
-static void xor_avx_3(unsigned long bytes, unsigned long *p0, unsigned long *p1,
-	unsigned long *p2)
+static void xor_avx_3(unsigned long bytes, unsigned long * __restrict p0,
+		      const unsigned long * __restrict p1,
+		      const unsigned long * __restrict p2)
 {
 	unsigned long lines = bytes >> 9;
 
@@ -82,8 +84,10 @@ do { \
 	kernel_fpu_end();
 }
 
-static void xor_avx_4(unsigned long bytes, unsigned long *p0, unsigned long *p1,
-	unsigned long *p2, unsigned long *p3)
+static void xor_avx_4(unsigned long bytes, unsigned long * __restrict p0,
+		      const unsigned long * __restrict p1,
+		      const unsigned long * __restrict p2,
+		      const unsigned long * __restrict p3)
 {
 	unsigned long lines = bytes >> 9;
 
@@ -115,8 +119,11 @@ do { \
 	kernel_fpu_end();
 }
 
-static void xor_avx_5(unsigned long bytes, unsigned long *p0, unsigned long *p1,
-	unsigned long *p2, unsigned long *p3, unsigned long *p4)
+static void xor_avx_5(unsigned long bytes, unsigned long * __restrict p0,
+	     const unsigned long * __restrict p1,
+	     const unsigned long * __restrict p2,
+	     const unsigned long * __restrict p3,
+	     const unsigned long * __restrict p4)
 {
 	unsigned long lines = bytes >> 9;