linux-kernel - [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED

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Date:   Mon, 16 Apr 2018 10:54:26 +0200
From:   Philipp Klocke <Phil_K97@....de>
To:     unlisted-recipients:; (no To-header on input)
Cc:     lukas.bulwahn@...il.com, kernelnewbies@...nelnewbies.org,
        llvmlinux@...ts.linuxfoundation.org, sil2review@...ts.osadl.org,
        der.herr@...r.at, Philipp Klocke <Phil_K97@....de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: Change sched_feat(x) in !CONFIG_SCHED_DEBUG case

Make sched_feat(x) return 1 or 0 instead of 2**x or 0 when
sysctl_sched_features is constant, by changing the left shift of the
mask-bit to a right shift of the bitmap. The behaviour of the code
remains unchanged.
We prove this by showing that the compiler can evaluate this shift
during compile time, which results in generating the same
machine code as before.

This patch is motivated by the clang warning Wconstant-logical-operand,
issued when logically comparing a variable to a constant integer that is
neither 1 nor 0.  It happens for sched_feat(x) when sysctl_sched_features
is constant, i.e., CONFIG_SCHED_DEBUG is not set.

kernel/sched/fair.c:3927:14: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]
        if (initial && sched_feat(START_DEBIT))
                    ^  ~~~~~~~~~~~~~~~~~~~~~~~
kernel/sched/fair.c:3927:14: note: use '&' for a bitwise operation
        if (initial && sched_feat(START_DEBIT))
                    ^~
                    &
kernel/sched/fair.c:3927:14: note: remove constant to silence this warning
        if (initial && sched_feat(START_DEBIT))
                   ~^~~~~~~~~~~~~~~~~~~~~~~~~~

This resolves the warning, leaves the code base largely as is.

Tested with gcc 7.3.1 and clang 6.0.0 on x86_64, comparing resulting
binaries with diff.
Applicable to all modern compilers and architectures

Signed-off-by: Philipp Klocke <Phil_K97@....de>
Suggested-by: Lukas Bulwahn <lukas.bulwahn@...il.com>
---
 kernel/sched/sched.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index fb5fc458547f..d2aedee6ab0f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1305,7 +1305,11 @@ static const_debug __maybe_unused unsigned int sysctl_sched_features =
 	0;
 #undef SCHED_FEAT

+#ifdef CONFIG_SCHED_DEBUG
 #define sched_feat(x) (sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
+#else
+#define sched_feat(x) ((sysctl_sched_features >> __SCHED_FEAT_##x) & 1UL)
+#endif

 #endif /* SCHED_DEBUG && HAVE_JUMP_LABEL */

-- 
2.17.0