Mulx: Illegal Instruction
tg at gmplib.org
Wed Dec 2 08:07:35 UTC 2015
Paul Peet <peetpaul69 at gmail.com> writes:
Distro: Arch Linux (x86_64)
CPU: Intel Pentium G4400 (Skylake)
Kernel: Linux Kernel 4.2.5-1-ARCH
I discovered this bug while trying to compiling the linux kernel but
it ultimately failed.
The issue which occured was an SIGILL one. I traced down the issue
back to the assembly code and the result was that
it was executing a mulx instruction which apparently is invalid here
on my cpu (Pentium G4400).
A member suggested that it might be an issue with the detection logic in gmp.
For more detailed overview of the issue:
Disassembly of the illegal instruction:
Dump of assembler code for function __gmpn_sqr_basecase_coreibwl:
0x00007ffff7708000 <+0>: cmp $0x2,%rdx
0x00007ffff7708004 <+4>: jae 0x7ffff7708016
0x00007ffff7708006 <+6>: mov (%rsi),%rdx
=> 0x00007ffff7708009 <+9>: mulx %rdx,%rax,%rdx
I would expect skylake to come with MULX (which were introduced with
haswell) as well as ADX (which were introduced with broadwell)..
Perhaps Intel fused them out in an attempt to make a low-end product
less attractive. I'll ask.
We encountered a similar situation with AVX challenged haswells.
It will be too much work for us to support a matrix of
(microarchitectures x fused instructions). We will handle this new
world by dropping back to an older microarchitecture where the fused
instruction exists. E.g., a skylake without MULX will be treated as an
Please encrypt, key id 0xC8601622
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