Re: [PATCH v2 1/2] arm: lib: xor-neon: remove unnecessary GCC < 4.6 warning

Hi Ard,

On Fri, 13 Nov 2020, Ard Biesheuvel [off-list ref] wrote:

On Thu, 12 Nov 2020 at 22:23, 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"). 

Reported-by: Nick Desaulniers <ndesaulniers@google.com> 
Signed-off-by: Nathan Chancellor <redacted> 
Signed-off-by: Adrian Ratiu <redacted> 

Again, this does not do what it says on the tin. 

If you want to disable the pragma for Clang, call that out in 
the commit log, and don't hide it under a GCC version change.

I am not doing anything for Clang in this series.

The option to auto-vectorize in Clang is enabled by default but 
doesn't work for some reason (likely to do with how it computes 
the cost model, so maybe not even a bug at all) and if we enable 
it explicitely (eg via a Clang specific pragma) we get some 
warnings we currently do not understand, so I am not changing the 
Clang behaviour at the recommendation of Nick.

So this is only for GCC as the "tin" says :) We can fix clang 
separately as the Clang bug has always been present and is 
unrelated.

Without the pragma, the generated code is the same as the 
generic code, so it makes no sense to build xor-neon.ko at all, 
right? 

Yes that is correct and that is the reason why in v1 I opted to 
not build xor-neon.ko for Clang anymore, but that got NACKed, so 
here I'm fixing the low hanging fruit: the very obvious & clear 
GCC problems.

quoted

---
 arch/arm/lib/xor-neon.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
index b99dd8e1c93f..e1e76186ec23 100644
--- a/arch/arm/lib/xor-neon.c
+++ b/arch/arm/lib/xor-neon.c

@@ -19,15 +19,8 @@ MODULE_LICENSE("GPL");
  * -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"
--

2.29.2

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