Commit ebabcf17 authored by James Hogan's avatar James Hogan Committed by Ralf Baechle
Browse files

MIPS: Implement __multi3 for GCC7 MIPS64r6 builds 



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: default avatarThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: default avatarJames Hogan <jhogan@kernel.org>
Tested-by: default avatarWaldemar 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/
parent ccf85c74

arch/mips/lib/Makefile

+2 −1
Original line number Original line Diff line number Diff line
@@ -16,4 +16,5 @@ obj-$(CONFIG_CPU_R3000) += r3k_dump_tlb.o 
obj-$(CONFIG_CPU_TX39XX)	+= r3k_dump_tlb.o

obj-$(CONFIG_CPU_TX39XX)	+= r3k_dump_tlb.o





# libgcc-style stuff needed in the kernel

# 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

arch/mips/lib/libgcc.h

+17 −0
Original line number Original line Diff line number Diff line
@@ -10,10 +10,18 @@ typedef int word_type __attribute__ ((mode (__word__))); 
struct DWstruct {

struct DWstruct {

	int high, low;

	int high, low;

};

};



struct TWstruct {

	long long high, low;

};

#elif defined(__LITTLE_ENDIAN)

#elif defined(__LITTLE_ENDIAN)

struct DWstruct {

struct DWstruct {

	int low, high;

	int low, high;

};

};



struct TWstruct {

	long long low, high;

};

#else

#else

#error I feel sick.

#error I feel sick.

#endif

#endif
@@ -23,4 +31,13 @@ typedef union { 
	long long ll;

	long long ll;

} DWunion;

} 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 */
 
#endif /* __ASM_LIBGCC_H */

arch/mips/lib/multi3.c

0 → 100644
+54 −0
Original line number Original line Diff line number Diff line 
// 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 */

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