This is a note to let you know that I've just added the patch titled

MIPS: Implement __multi3 for GCC7 MIPS64r6 builds

to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: mips-implement-__multi3-for-gcc7-mips64r6-builds.patch and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From ebabcf17bcd7ce968b1631ebe08236275698f39b Mon Sep 17 00:00:00 2001

From: James Hogan jhogan@kernel.org Date: Thu, 7 Dec 2017 07:20:46 +0000 Subject: MIPS: Implement __multi3 for GCC7 MIPS64r6 builds

From: James Hogan jhogan@kernel.org

commit ebabcf17bcd7ce968b1631ebe08236275698f39b upstream.

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/ Cc: Guenter Roeck linux@roeck-us.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

--- arch/mips/lib/Makefile | 3 +- arch/mips/lib/libgcc.h | 17 +++++++++++++++ arch/mips/lib/multi3.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 73 insertions(+), 1 deletion(-)

--- a/arch/mips/lib/Makefile +++ b/arch/mips/lib/Makefile @@ -15,4 +15,5 @@ obj-$(CONFIG_CPU_R3000) += r3k_dump_tlb obj-$(CONFIG_CPU_TX39XX) += r3k_dump_tlb.o

# libgcc-style stuff needed in the kernel -obj-y += ashldi3.o ashrdi3.o bswapsi.o bswapdi.o cmpdi2.o lshrdi3.o ucmpdi2.o +obj-y += ashldi3.o ashrdi3.o bswapsi.o bswapdi.o cmpdi2.o lshrdi3.o multi3.o \ + ucmpdi2.o --- a/arch/mips/lib/libgcc.h +++ b/arch/mips/lib/libgcc.h @@ -9,10 +9,18 @@ typedef int word_type __attribute__ ((mo struct DWstruct { int high, low; }; + +struct TWstruct { + long long high, low; +}; #elif defined(__LITTLE_ENDIAN) struct DWstruct { int low, high; }; + +struct TWstruct { + long long low, high; +}; #else #error I feel sick. #endif @@ -22,4 +30,13 @@ typedef union { long long ll; } DWunion;

+#if defined(CONFIG_64BIT) && defined(CONFIG_CPU_MIPSR6) +typedef int ti_type __attribute__((mode(TI))); + +typedef union { + struct TWstruct s; + ti_type ti; +} TWunion; +#endif + #endif /* __ASM_LIBGCC_H */ --- /dev/null +++ b/arch/mips/lib/multi3.c @@ -0,0 +1,54 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/export.h> + +#include "libgcc.h" + +/* + * GCC 7 suboptimally generates __multi3 calls for mips64r6, so for that + * specific case only we'll implement it here. + * + * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82981 + */ +#if defined(CONFIG_64BIT) && defined(CONFIG_CPU_MIPSR6) && (__GNUC__ == 7) + +/* multiply 64-bit values, low 64-bits returned */ +static inline long long notrace dmulu(long long a, long long b) +{ + long long res; + + asm ("dmulu %0,%1,%2" : "=r" (res) : "r" (a), "r" (b)); + return res; +} + +/* multiply 64-bit unsigned values, high 64-bits of 128-bit result returned */ +static inline long long notrace dmuhu(long long a, long long b) +{ + long long res; + + asm ("dmuhu %0,%1,%2" : "=r" (res) : "r" (a), "r" (b)); + return res; +} + +/* multiply 128-bit values, low 128-bits returned */ +ti_type notrace __multi3(ti_type a, ti_type b) +{ + TWunion res, aa, bb; + + aa.ti = a; + bb.ti = b; + + /* + * a * b = (a.lo * b.lo) + * + 2^64 * (a.hi * b.lo + a.lo * b.hi) + * [+ 2^128 * (a.hi * b.hi)] + */ + res.s.low = dmulu(aa.s.low, bb.s.low); + res.s.high = dmuhu(aa.s.low, bb.s.low); + res.s.high += dmulu(aa.s.high, bb.s.low); + res.s.high += dmulu(aa.s.low, bb.s.high); + + return res.ti; +} +EXPORT_SYMBOL(__multi3); + +#endif /* 64BIT && CPU_MIPSR6 && GCC7 */

Patches currently in stable-queue which might be from jhogan@kernel.org are

queue-4.9/lib-mpi-fix-umul_ppmm-for-mips64r6.patch queue-4.9/mips-implement-__multi3-for-gcc7-mips64r6-builds.patch