MIPS: lib: Provide MIPS64r6 __multi3() for GCC < 7

Some versions of GCC suboptimally generate calls to the __multi3()
intrinsic for MIPS64r6 builds, resulting in link failures due to the
missing fun

MIPS: lib: Provide MIPS64r6 __multi3() for GCC < 7

Some versions of GCC suboptimally generate calls to the __multi3()
intrinsic for MIPS64r6 builds, resulting in link failures due to the
missing function:

LD vmlinux.o
MODPOST vmlinux.o
kernel/bpf/verifier.o: In function `kmalloc_array':
include/linux/slab.h:631: undefined reference to `__multi3'
fs/select.o: In function `kmalloc_array':
include/linux/slab.h:631: undefined reference to `__multi3'
...

We already have a workaround for this in which we provide the
instrinsic, but we do so selectively for GCC 7 only. Unfortunately the
issue occurs with older GCC versions too - it has been observed with
both GCC 5.4.0 & GCC 6.4.0.

MIPSr6 support was introduced in GCC 5, so all major GCC versions prior
to GCC 8 are affected and we extend our workaround accordingly to all
MIPS64r6 builds using GCC versions older than GCC 8.

Signed-off-by: Paul Burton <[email protected]>
Reported-by: Vladimir Kondratiev <[email protected]>
Fixes: ebabcf17bcd7 ("MIPS: Implement __multi3 for GCC7 MIPS64r6 builds")
Patchwork: https://patchwork.linux-mips.org/patch/20297/
Cc: James Hogan <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: [email protected]
Cc: [email protected] # 4.15+

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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 e

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: Thomas Petazzoni <[email protected]>
Signed-off-by: James Hogan <[email protected]>
Tested-by: Waldemar Brodkorb <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Maciej W. Rozycki <[email protected]>
Cc: Matthew Fortune <[email protected]>
Cc: Florian Fainelli <[email protected]>
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/17890/

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