On Tue, 22 Oct 2024, Dave Hansen wrote:
> So I think Peter's version was the best. It shuts up clang and also
> preserves the existing (good) gcc 'sub' behavior. I'll send it out for
> real in a bit, but I'm thinking of something like the attached patch.
The desired behavior is a "dec". "sub" has a longer op code AFAICT.
From d63bcd350e1a3ba6196dadb26cb2f36f0ba1e182 Mon Sep 17 00:00:00 2001
From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Date: Fri, 18 Oct 2024 11:07:47 -0700
Subject: [PATCH] x86/percpu: Avoid comparing unsigned types to -1
clang warns when comparing an unsinged type to -1 since the comparison
is always false.
This can be quickly reproduced by setting CONFIG_WERROR=y and running:
make W=1 CC=clang-14 net/ipv4/tcp_output.o
net/ipv4/tcp_output.c:187:3: error: result of comparison of constant -1 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
187 | NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
188 | tp->compressed_ack);
| ~~~~~~~~~~~~~~~~~~~
...
arch/x86/include/asm/percpu.h:238:31: note: expanded from macro 'percpu_add_op'
238 | ((val) == 1 || (val) == -1)) ? \
| ~~~~~ ^ ~~
Fix this by avoiding a comparison of an uncast -1 to 'val'.
Doing this in addition to the existing 'pao_ID__' calculation would make it
even more unreadable. Remove 'pao_ID__' and replace it with the three
components of its calculation.
This preserves some unintuitive but useful behavior. For instance, gcc sees:
percpu_add_op(..., var, (u8)-1);
and can transform that into a "dec". Clang, on the other hand, sees the 'u8'
type and assumes that "(val) == -1" is false, which was the root of the
warning.
This is useful gcc behavior because:
#define this_cpu_sub(pcp, val) this_cpu_add(pcp, -(typeof(pcp))(val))
so any code that does:
this_cpu_sub(A, 1)
where 'A' is an unsigned type generates a "dec". Clang, on the other
hand generates a less-efficient "add".
Reported-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---
arch/x86/include/asm/percpu.h | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index c55a79d5feae..57d9759c692e 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -234,18 +234,19 @@ do { \
*/
#define percpu_add_op(size, qual, var, val) \
do { \
- const int pao_ID__ = (__builtin_constant_p(val) && \
- ((val) == 1 || (val) == -1)) ? \
- (int)(val) : 0; \
+ const int pao_const__ = __builtin_constant_p(val); \
+ const int pao_inc__ = (val) == 1; \
+ const int pao_dec__ = (typeof(var))(val) == \
+ (typeof(var))-1; \
\
if (0) { \
typeof(var) pao_tmp__; \
pao_tmp__ = (val); \
(void)pao_tmp__; \
} \
- if (pao_ID__ == 1) \
+ if (pao_const__ && pao_inc__) \
percpu_unary_op(size, qual, "inc", var); \
- else if (pao_ID__ == -1) \
+ else if (pao_const__ && pao_dec__) \
percpu_unary_op(size, qual, "dec", var); \
else \
percpu_binary_op(size, qual, "add", var, val); \
--
2.34.1
