https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66112
--- Comment #8 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Jakub Jelinek from comment #5)
> Created attachment 35524 [details]
> gcc6-pr66112-2.patch
>
> And i386 mulvhi4 and umulvhi4 support. For umulvhi4, I haven't found
> corresponding i386.md instruction that would emit mul{w}, so just changed
> SWI48 to SWI248 iterator in that case.
No, mulw outputs to %ax:%dx pair of HImode registers! Please see [1]
[1] http://x86.renejeschke.de/html/file_module_x86_id_210.html
