From: David Laight <david.laight.linux@gmail.com>
To: kernel test robot <lkp@intel.com>
Cc: Eric Dumazet <edumazet@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
oe-kbuild-all@lists.linux.dev,
Linux Memory Management List <linux-mm@kvack.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
netdev@vger.kernel.org, Jakub Kicinski <kuba@kernel.org>,
Eric Dumazet <eric.dumazet@gmail.com>,
Paolo Abeni <pabeni@redhat.com>,
Nicolas Pitre <npitre@baylibre.com>
Subject: Re: [PATCH] compiler_types: Introduce inline_for_performance
Date: Sun, 18 Jan 2026 22:33:18 +0000 [thread overview]
Message-ID: <20260118223318.7a3e3837@pumpkin> (raw)
In-Reply-To: <202601190247.dDAvbbMH-lkp@intel.com>
On Mon, 19 Jan 2026 02:36:18 +0800
kernel test robot <lkp@intel.com> wrote:
> Hi Eric,
...
> vim +/__arch_xprod_64 +138 include/asm-generic/div64.h
>
> 461a5e51060c93 Nicolas Pitre 2015-10-30 125
> f682b27c57aec2 Nicolas Pitre 2015-10-30 126 #ifndef __arch_xprod_64
> f682b27c57aec2 Nicolas Pitre 2015-10-30 127 /*
> f682b27c57aec2 Nicolas Pitre 2015-10-30 128 * Default C implementation for __arch_xprod_64()
> f682b27c57aec2 Nicolas Pitre 2015-10-30 129 *
> f682b27c57aec2 Nicolas Pitre 2015-10-30 130 * Prototype: uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
> f682b27c57aec2 Nicolas Pitre 2015-10-30 131 * Semantic: retval = ((bias ? m : 0) + m * n) >> 64
> f682b27c57aec2 Nicolas Pitre 2015-10-30 132 *
> f682b27c57aec2 Nicolas Pitre 2015-10-30 133 * The product is a 128-bit value, scaled down to 64 bits.
> 00a31dd3acea0f Nicolas Pitre 2024-10-03 134 * Hoping for compile-time optimization of conditional code.
> f682b27c57aec2 Nicolas Pitre 2015-10-30 135 * Architectures may provide their own optimized assembly implementation.
> f682b27c57aec2 Nicolas Pitre 2015-10-30 136 */
> 5f712d70e20a46 Eric Dumazet 2026-01-18 137 static inline_for_performance
> d533cb2d2af400 Nicolas Pitre 2024-10-03 @138 uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
> f682b27c57aec2 Nicolas Pitre 2015-10-30 139 {
> f682b27c57aec2 Nicolas Pitre 2015-10-30 140 uint32_t m_lo = m;
> f682b27c57aec2 Nicolas Pitre 2015-10-30 141 uint32_t m_hi = m >> 32;
> f682b27c57aec2 Nicolas Pitre 2015-10-30 142 uint32_t n_lo = n;
> f682b27c57aec2 Nicolas Pitre 2015-10-30 143 uint32_t n_hi = n >> 32;
> 00a31dd3acea0f Nicolas Pitre 2024-10-03 144 uint64_t x, y;
> f682b27c57aec2 Nicolas Pitre 2015-10-30 145
> 00a31dd3acea0f Nicolas Pitre 2024-10-03 146 /* Determine if overflow handling can be dispensed with. */
> 00a31dd3acea0f Nicolas Pitre 2024-10-03 147 bool no_ovf = __builtin_constant_p(m) &&
> 00a31dd3acea0f Nicolas Pitre 2024-10-03 148 ((m >> 32) + (m & 0xffffffff) < 0x100000000);
Can that ever have got compiled?
Won't the compiler complain about 0x100000000 being out of range?
Lots of alternatives...
If u128 exists this should probably just be:
return ((u128)m * n + (bias ? m : 0)) >> 64;
Which is probably the only alternative an architecture might provide (none do AFAICT).
David
next prev parent reply other threads:[~2026-01-18 22:33 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20260118152448.2560414-1-edumazet@google.com>
2026-01-18 18:36 ` kernel test robot
2026-01-18 22:33 ` David Laight [this message]
2026-01-18 21:04 ` kernel test robot
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