ebabcf17bc
GCC7 is a bit too eager to generate suboptimal __multi3 calls (128bit
multiply with 128bit result) for MIPS64r6 builds, even in code which
doesn't explicitly use 128bit types, such as the following:
unsigned long func(unsigned long a, unsigned long b)
{
return a > (~0UL) / b;
}
Which GCC rearanges to:
return (unsigned __int128)a * (unsigned __int128)b > 0xffffffffffffffff;
Therefore implement __multi3, but only for MIPS64r6 with GCC7 as under
normal circumstances we wouldn't expect any calls to __multi3 to be
generated from kernel code.
Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: James Hogan <jhogan@kernel.org>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Maciej W. Rozycki <macro@mips.com>
Cc: Matthew Fortune <matthew.fortune@mips.com>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17890/
55 lines
1.3 KiB
C
55 lines
1.3 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/export.h>
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#include "libgcc.h"
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/*
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* GCC 7 suboptimally generates __multi3 calls for mips64r6, so for that
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* specific case only we'll implement it here.
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*
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* See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82981
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*/
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#if defined(CONFIG_64BIT) && defined(CONFIG_CPU_MIPSR6) && (__GNUC__ == 7)
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/* multiply 64-bit values, low 64-bits returned */
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static inline long long notrace dmulu(long long a, long long b)
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{
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long long res;
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asm ("dmulu %0,%1,%2" : "=r" (res) : "r" (a), "r" (b));
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return res;
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}
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/* multiply 64-bit unsigned values, high 64-bits of 128-bit result returned */
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static inline long long notrace dmuhu(long long a, long long b)
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{
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long long res;
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asm ("dmuhu %0,%1,%2" : "=r" (res) : "r" (a), "r" (b));
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return res;
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}
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/* multiply 128-bit values, low 128-bits returned */
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ti_type notrace __multi3(ti_type a, ti_type b)
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{
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TWunion res, aa, bb;
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aa.ti = a;
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bb.ti = b;
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/*
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* a * b = (a.lo * b.lo)
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* + 2^64 * (a.hi * b.lo + a.lo * b.hi)
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* [+ 2^128 * (a.hi * b.hi)]
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*/
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res.s.low = dmulu(aa.s.low, bb.s.low);
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res.s.high = dmuhu(aa.s.low, bb.s.low);
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res.s.high += dmulu(aa.s.high, bb.s.low);
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res.s.high += dmulu(aa.s.low, bb.s.high);
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return res.ti;
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}
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EXPORT_SYMBOL(__multi3);
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#endif /* 64BIT && CPU_MIPSR6 && GCC7 */
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