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

On Tue, 19 Jan 2021, Nick Desaulniers [off-list ref] 
wrote:

On Tue, Jan 19, 2021 at 5:17 AM Adrian Ratiu 
[off-list ref] wrote: 

quoted

From: Nathan Chancellor <redacted> 

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@google.com> 
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> 

This is not the version of the patch I had reviewed; please drop 
my reviewed-by tag when you change a patch significantly, as 
otherwise it looks like I approved this patch. 

Nacked-by: Nick Desaulniers [off-list ref] 

Sorry for not removing the reviewed-by tags from the previous 
versions in this v4. I guess the only way forward with this is to 
actually make clang vectorization work. Also thanks for the patch 
suggestion in the other e-mail!

quoted

Signed-off-by: Nathan Chancellor <redacted>
Signed-off-by: Adrian Ratiu <redacted>
---
 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

Arnd, remind me again why it's a bug that the compiler's cost model
says it's faster to not produce a vectorized version of these loops?
I stand by my previous comment: https://bugs.llvm.org/show_bug.cgi?id=40976#c8

quoted

+
 /*
  * 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

-- 
Thanks,
~Nick Desaulniers

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