Created attachment 42571 [details]
somewhat reduced testcase

Seen with current glibc "make check", gcc 20171109 

during RTL pass: expand
/home/amodra/build/glibc/math/test-tgmath3.c: In function 'test_log10_17':
/home/amodra/build/glibc/math/test-tgmath3.c:150331:23: internal compiler error: in as_a, at machmode.h:345
   vol_var___CFLOAT128 = log10 (vol_var___CFLOAT128);
0x1050cc0f scalar_mode as_a<scalar_mode>(machine_mode)
	/home/amodra/src/gcc/gcc/machmode.h:345
0x1050cc0f convert_mode_scalar
	/home/amodra/src/gcc/gcc/expr.c:280
0x1050cc0f convert_move(rtx_def*, rtx_def*, int)
	/home/amodra/src/gcc/gcc/expr.c:271
0x105156c7 store_expr_with_bounds(tree_node*, rtx_def*, int, bool, bool, tree_node*)
	/home/amodra/src/gcc/gcc/expr.c:5631
0x105172c3 expand_assignment(tree_node*, tree_node*, bool)
	/home/amodra/src/gcc/gcc/expr.c:5322
0x103879af expand_gimple_stmt_1
	/home/amodra/src/gcc/gcc/cfgexpand.c:3675
0x103879af expand_gimple_stmt
	/home/amodra/src/gcc/gcc/cfgexpand.c:3773
0x1038ba83 expand_gimple_basic_block
	/home/amodra/src/gcc/gcc/cfgexpand.c:5774
0x10391b67 execute
	/home/amodra/src/gcc/gcc/cfgexpand.c:6375

gcc -c -O1 -mcpu=power8 to reproduce with attached testcase