[m-rev.] for review: make unchecked shifts by uint amounts builtin, step 1

[Zoltan Somogyi]

On Thu, 30 Apr 2020 17:06:27 +1000 (AEST), Julien Fischer <jfischer at opturion.com> wrote:
> > compiler/erl_call_gen.m:
> >     Don't treat unchecked shifts by uint amounts as builtins, since I (zs)
> >     don't know how this should be done in Erlang.
> 
> I wouldn't worry too much about the Erlang backend; it's currently lacking
> a lot of support for non-int integer types anyway.

I agree, though I didn't think this diff to erl_call_gen.m exhibited any such worry :-)

> > diff --git a/compiler/llds_out_data.m b/compiler/llds_out_data.m
> > index 92ba641fe..d0148ae2c 100644
> > --- a/compiler/llds_out_data.m
> > +++ b/compiler/llds_out_data.m
> > @@ -1178,16 +1178,22 @@ output_rval(Info, Rval, !IO) :-
> >              output_rval_as_type(Info, SubRvalB, lt_int(int_type_int), !IO),
> >              io.write_string(")", !IO)
> >          ;
> > -            % The second operand of the shift operatators always has type
> > -            % `int'.
> > -            ( Op = unchecked_left_shift(IntType), OpStr = "<<"
> > -            ; Op = unchecked_right_shift(IntType), OpStr = ">>"
> > +            % The second operand of the shift operators always has type
> > +            % `int' in C, but Mercury also allows it to be uint.
> 
> The second operand of a shift in C can have any integral type.

I suspected as much, but my copy of K&R is packed away, and I couldn't find
any confirmation of this online among the avalanche of search results that
all focused on explaining the difference between int and uint in the LEFT
operands of shift operators, but never discussed the issue in the RIGHT
operands.

> (Obviously,
> for the behaviour to be defined the second operand must have a value that
> is non-negative and within the bit size of the first operand.)  Or do
> you mean that we always generate shift expressions so that their second
> operand has type int?

That's what the code I have now. I guess I could undo all the code additions
that look like this:
 
> > +            ( Op = unchecked_left_shift(IntType, ShiftType), OpStr = "<<"
> > +            ; Op = unchecked_right_shift(IntType, ShiftType), OpStr = ">>"
> >              ),
> >              io.write_string("(", !IO),
> >              output_rval_as_type(Info, SubRvalA, lt_int(IntType), !IO),
> >              io.write_string(" ", !IO),
> >              io.write_string(OpStr, !IO),
> >              io.write_string(" ", !IO),
> > +            (
> > +                ShiftType = shift_by_int
> > +            ;
> > +                ShiftType = shift_by_uint,
> > +                io.write_string("(int) ", !IO)
> > +            ),
> >              output_rval_as_type(Info, SubRvalB, lt_int(int_type_int), !IO),
> >              io.write_string(")", !IO)

as unnecessary, but then shifts are so rare that it does not really matter.

Thanks for the review.

Zoltan.