Question about DRAP register and reserving hard registers

[Steve Ellcey]

On Mon, 2015-06-29 at 11:10 +0100, Richard Henderson wrote:

> > I also need the drap pointer in the MIPS epilogue but I would like to
> > avoid having to get it from memory.  Ideally I would like to restore it
> > from the virtual register that the prologue code / get_drap_rtx code put
> > it into.  I tried just doing a move from the virtual drap register to
> > the real one in expand_epilogue but that didn't work because it looks
> > like you can't access virtual registers from expand_prologue or
> > expand_epilogue.  I guess that is why the code to copy the hard drap reg
> > to the virtual drap_reg is done in get_drap_reg and not in
> > expand_prologue.  I thought about putting code in get_drap_reg to do
> > this copying but I don't see how to access the end of a function.  The
> > hard drap reg to virtual drap reg copy is inserted into the beginning of
> > a function with:
> >
> > insn = emit_insn_before (seq, NEXT_INSN (entry_of_function ()));
> >
> > Is there an equivalent method to insert code to the end of a function?
> > I don't see an 'end_of_function ()' routine anywhere.
> 
> Because, while generating initial rtl for a function, the beginning of a 
> function has already been emitted, while the end of the function hasn't.
> 
> You'd need to hook into expand_function_end, right at the bottom, before the 
> call to use_return_register.
> 
> 
> r~

I ran into an interesting issue while doing this.  Right now the expand
pass calls construct_exit_block (which calls expand_function_end) before
it calls expand_stack_alignment.  That means that crtl->drap_reg, etc
are not yet set up when in expand_function_end.  I moved the
expand_stack_alignment call up before construct_exit_block to fix that.
I hope moving it up doesn't break anything.

Steve Ellcey
sellcey@imgtec.com