[PATCH v4 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

[Adrian Ratiu]

From: Nathan Chancellor <natechancellor at gmail.com>

Drop warning because kernel now requires GCC >= v4.9 after
commit 6ec4476ac825 ("Raise gcc version requirement to 4.9")
and clarify that -ftree-vectorize now always needs enabling
for GCC by directly testing the presence of CONFIG_CC_IS_GCC.

Another reason to remove the warning is that Clang exposes
itself as GCC < 4.6 so it triggers the warning about GCC
which doesn't make much sense and misleads Clang users by
telling them to update GCC.

Because Clang is now supported by the kernel print a clear
Clang-specific warning.

Link: https://github.com/ClangBuiltLinux/linux/issues/496
Link: https://github.com/ClangBuiltLinux/linux/issues/503
Reported-by: Nick Desaulniers <ndesaulniers at google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>
Signed-off-by: Nathan Chancellor <natechancellor at gmail.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu at collabora.com>
---
 arch/arm/lib/xor-neon.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
index b99dd8e1c93f..f9f3601cc2d1 100644
--- a/arch/arm/lib/xor-neon.c
+++ b/arch/arm/lib/xor-neon.c
@@ -14,20 +14,22 @@ MODULE_LICENSE("GPL");
 #error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
 #endif
 
+/*
+ * TODO: Even though -ftree-vectorize is enabled by default in Clang, the
+ * compiler does not produce vectorized code due to its cost model.
+ * See: https://github.com/ClangBuiltLinux/linux/issues/503
+ */
+#ifdef CONFIG_CC_IS_CLANG
+#warning Clang does not vectorize code in this file.
+#endif
+
 /*
  * Pull in the reference implementations while instructing GCC (through
  * -ftree-vectorize) to attempt to exploit implicit parallelism and emit
  * NEON instructions.
  */
-#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
+#ifdef CONFIG_CC_IS_GCC
 #pragma GCC optimize "tree-vectorize"
-#else
-/*
- * While older versions of GCC do not generate incorrect code, they fail to
- * recognize the parallel nature of these functions, and emit plain ARM code,
- * which is known to be slower than the optimized ARM code in asm-arm/xor.h.
- */
-#warning This code requires at least version 4.6 of GCC
 #endif
 
 #pragma GCC diagnostic ignored "-Wunused-variable"
-- 
2.30.0