https://bugs.freedesktop.org/show_bug.cgi?id=103234
--- Comment #5 from Nicolai Hähnle nhaehnle@gmail.com --- Thank you for the report!
Can you try with Mesa from git master?
The issue is that in Mesa 17.2.2 (which you have according to glxinfo), si_state_draw.c:1305 corresponds to
1304 index_offset -= start_offset; 1305 } else if (sctx->b.chip_class <= CIK && 1306 r600_resource(indexbuf)->TC_L2_dirty) {
and it's basically impossible to get a crash there, so I suspect the Gentoo build actually applies some other patches.
Something random you could try is starting KWin with GALLIUM_THREAD=0.