Woon yung Liu <ysai...@yahoo.com> writes: > On Wednesday, June 1, 2016 5:45 AM, Richard Henderson <r...@redhat.com> > wrote: > > This is almost always incorrect, and certainly before reload. > > You need to use gen_lowpart. There are examples in the code > > > fragments that I sent the other week. > > The problem is that gen_lowpart() doesn't seem to support casting to > other modes of the same size. > When I use it, the assert within gen_lowpart_general() will fail due to > gen_lowpart_common() rejecting the operation (new mode size is not > smaller than the old).
The conclusion we came to when developing MSA is that simplify_gen_subreg is the way to go for converting between vector modes: simplify_gen_subreg (new_mode, rtx, old_mode, 0) I'm not sure there is much need to change modes after reload so do it upfront at expand time or when splitting and you should be OK. See trunk mips.c for a number of uses of this when converting vector modes. > > You need to read the gcc internals documentation. They are all three > different > > > uses, though there is some overlap between define_insn and > define_expand. > > I actually read the GCC internals documentation before I even begun any > attempt at this, but there was still a lot that I did not successfully > grasp. > > I'll go with define_expand then. define_expand only provides a way to generate an instruction or sequence of instructions to achieve the overall goal. You must also have define_insn definitions for any pattern you emit or the generated code will fail to match. A define_insn_and_split is just shorthand for a define_insn where one or more output patterns are '#' (split) and you want to define the split alongside the instruction rather than as a separate define_split. As far as I understand the difference is syntactic sugar. > But I am already doubting that I will complete this port as I can no > longer see a favourable conclusion. It may take time but I'm sure we can help talk through the problems. As a new GCC developer you are a welcome addition to the community. Thanks, Matthew
