When forcing a constant of mode MODE into memory, force_const_mem asks the frontend to provide the type associated with that mode. In principle type_for_mode is allowed to return null, and although one use site correctly handled that, the other didn't.
I think there's agreement that it's bogus to use type_for_mode for this kind of thing, since it forces frontends to handle types that don't exist in that language. See e.g. http://gcc.gnu.org/PR46805 where the Go frontend was forced to handle vector types even though Go doesn't have vector types. Also, the frontends use code like: else if (VECTOR_MODE_P (mode)) { machine_mode inner_mode = GET_MODE_INNER (mode); tree inner_type = c_common_type_for_mode (inner_mode, unsignedp); if (inner_type != NULL_TREE) return build_vector_type_for_mode (inner_type, mode); } and there's no guarantee that every vector mode M used by backend rtl has an associated vector type whose TYPE_MODE is M. I think really the type_for_mode hook should only return trees that _do_ have the requested TYPE_MODE, but PR46805 linked above shows that this is likely to have too many knock-on consequences. It doesn't make sense for force_const_mem to ask about vector modes that aren't valid for vector types, so this patch handles the condition there instead. This is needed for SVE multi-register modes, which are modelled as vector modes but are not usable as vector types. Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linus-gnu. OK to install? Richard 2017-09-20 Richard Sandiford <richard.sandif...@linaro.org> Alan Hayward <alan.hayw...@arm.com> David Sherwood <david.sherw...@arm.com> gcc/ * varasm.c (force_const_mem): Don't ask the front end about vector modes that are not supported as vector types by the target. Index: gcc/varasm.c =================================================================== --- gcc/varasm.c 2017-09-12 14:28:56.402824780 +0100 +++ gcc/varasm.c 2017-09-20 13:33:15.942547232 +0100 @@ -3785,10 +3785,17 @@ force_const_mem (machine_mode mode, rtx desc = ggc_alloc<constant_descriptor_rtx> (); *slot = desc; + tree type = NULL_TREE; + if (mode != VOIDmode + /* Don't ask the frontend about vector modes if there cannot be a + VECTOR_TYPE whose TYPE_MODE is MODE. */ + && (!VECTOR_MODE_P (mode) + || targetm.vector_mode_supported_p (mode))) + type = lang_hooks.types.type_for_mode (mode, 0); + /* Align the location counter as required by EXP's data type. */ align = GET_MODE_ALIGNMENT (mode == VOIDmode ? word_mode : mode); - tree type = lang_hooks.types.type_for_mode (mode, 0); if (type != NULL_TREE) align = CONSTANT_ALIGNMENT (make_tree (type, x), align); @@ -3832,7 +3839,8 @@ force_const_mem (machine_mode mode, rtx /* Construct the MEM. */ desc->mem = def = gen_const_mem (mode, symbol); - set_mem_attributes (def, lang_hooks.types.type_for_mode (mode, 0), 1); + if (type) + set_mem_attributes (def, type, 1); set_mem_align (def, align); /* If we're dropping a label to the constant pool, make sure we
