linux-kernel - [PATCH v2 5/5] hash.h: Silence a clang -Wshorten-64-to-32 warning

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Message-ID: <20250430171534.132774-6-irogers@google.com>
Date: Wed, 30 Apr 2025 10:15:34 -0700
From: Ian Rogers <irogers@...gle.com>
To: Yury Norov <yury.norov@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Ian Rogers <irogers@...gle.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Jakub Kicinski <kuba@...nel.org>, Jacob Keller <jacob.e.keller@...el.com>, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, Leo Yan <leo.yan@....com>
Subject: [PATCH v2 5/5] hash.h: Silence a clang -Wshorten-64-to-32 warning

The clang warning -Wshorten-64-to-32 can be useful to catch
inadvertent truncation. In some instances this truncation can lead to
changing the sign of a result, for example, truncation to return an
int to fit a sort routine. Silence the warning by making the implicit
truncation explicit. This isn't to say the code is currently incorrect
but without silencing the warning it is hard to spot the erroneous
cases.

Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 include/linux/hash.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/hash.h b/include/linux/hash.h
index 38edaa08f862..ecc8296cb397 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -75,7 +75,7 @@ static __always_inline u32 hash_64_generic(u64 val, unsigned int bits)
 {
 #if BITS_PER_LONG == 64
 	/* 64x64-bit multiply is efficient on all 64-bit processors */
-	return val * GOLDEN_RATIO_64 >> (64 - bits);
+	return (u32)(val * GOLDEN_RATIO_64 >> (64 - bits));
 #else
 	/* Hash 64 bits using only 32x32-bit multiply. */
 	return hash_32((u32)val ^ __hash_32(val >> 32), bits);
-- 
2.49.0.906.g1f30a19c02-goog