It was only GCC 10 that fixed a MIPS64r6 code generation issue with a
`__multi3' libcall inefficiently produced to perform 64-bit widening
multiplication while suitable machine instructions exist to do such a
calculation. The fix went in with GCC commit 48b2123f6336 ("re PR
target/82981 (unnecessary __multi3 call for mips64r6 linux kernel)").
Adjust our code accordingly, removing build failures such as:
mips64-linux-ld: lib/math/div64.o: in function `mul_u64_add_u64_div_u64':
div64.c:(.text+0x84): undefined reference to `__multi3'
with the GCC versions affected.
Fixes: ebabcf17bc ("MIPS: Implement __multi3 for GCC7 MIPS64r6 builds")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202601140146.hMLODc6v-lkp@intel.com/
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Cc: stable@vger.kernel.org # v4.15+
Reviewed-by: David Laight <david.laight.linux@gmail.com.
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
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 <paul.burton@mips.com>
Reported-by: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
Fixes: ebabcf17bc ("MIPS: Implement __multi3 for GCC7 MIPS64r6 builds")
Patchwork: https://patchwork.linux-mips.org/patch/20297/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: stable@vger.kernel.org # 4.15+
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/
