Patch "MIPS: Implement __multi3 for GCC7 MIPS64r6 builds" has been added to the 4.9-stable tree

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;a=summary

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 <[email protected]> know about it.

From: James Hogan <[email protected]>
Date: Thu, 7 Dec 2017 07:20:46 +0000
Subject: MIPS: Implement __multi3 for GCC7 MIPS64r6 builds

From: James Hogan <[email protected]>

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.

Patchwork: https://patchwork.linux-mips.org/patch/17890/

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

@@ -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

@@ -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 */

Patches currently in stable-queue which might be from [email protected] are

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