On Mon, Oct 9, 2017 at 12:55 PM, Marc Glisse <marc.gli...@inria.fr> wrote:
> On Mon, 3 Jul 2017, Richard Biener wrote:
>
>> On Sat, 1 Jul 2017, Marc Glisse wrote:
>>
>>> I wrote a quick prototype to see what the fallout would look like.
>>> Surprisingly, it actually passes bootstrap+testsuite on ppc64el with all
>>> languages with no regression. Sure, it is probably not a complete
>>> migration, there are likely a few places still converting to ptrdiff_t
>>> to perform a regular subtraction, but this seems to indicate that the
>>> work isn't as bad as using a signed type in pointer_plus_expr for
>>> instance.
>>
>>
>> The fold_binary_loc hunk looks dangerous (it'll generate MINUS_EXPR
>> from POINTER_MINUS_EXPR in some cases I guess).
>>
>> The tree code needs documenting in tree.def and generic.texi.
>>
>> Otherwise ok(*).
>>
>> Thanks,
>> Richard.
>>
>> (*) ok, just kidding -- or maybe not
>
>
> I updated the prototype a bit. Some things I noticed:
>
> - the C front-end has support for address spaces that seems to imply that
> pointer subtraction (and division by the size) may be done in a type larger
> than ptrdiff_t. Currently, it generates
> (ptrdiff_t)(((inttype)q-(inttype)p)/size) for q-p where inttype is some type
> potentially larger than ptrdiff_t.
It uses a ptrdiff_t corresponding to the respective address space, yes.
That we use sizetype elsewhere unconditionally is a bug :/
I am thus generating a POINTER_DIFF_EXPR
> with that type, while I was originally hoping its type would always be
> ptrdiff_t. It complicates code and I am not sure I didn't break address
> spaces anyway... (expansion likely needs to do the inttype dance)
I think that's fine. Ideally targets would provide a type to use for each
respective address space given we have targets that have sizetype smaller
than ptr_mode.
> Are ptrdiff_t (what POINTER_DIFF_EXPR should return) and size_t (what
> POINTER_PLUS_EXPR takes as second argument) always the same type
> signed/unsigned?
POINTER_PLUS_EXPR takes 'sizetype', not size_t. So the answer is clearly
no. And yes, that's ugly and broken and should be fixed.
> Counter-examples: m32c (when !TARGET_A16), visium, darwin,
> powerpcspe, s390, vms... and it isn't even always the same that is bigger
> than the other. That's quite messy.
Eh. Yeah, targets are free to choose 'sizetype' and they do so for
efficiency. IMHO we should get rid of this "feature".
> - I had to use @@ in match.pd, not for constants, but because in GENERIC we
> sometimes ended up with pointers where operand_equal_p said yes but
> types_match disagreed.
>
> - It currently regresses 2 go tests: net/http runtime/debug
Those are flaky for me and fail sometimes and sometimes not.
+@item POINTER_DIFF_EXPR
+This node represents pointer subtraction. The two operands always
+have pointer/reference type. The second operand is always an unsigned
+integer type compatible with sizetype. It returns a signed integer.
the 2nd sentence looks bogusly copied.
+ /* FIXME. */
+ if (code == POINTER_DIFF_EXPR)
+ return int_const_binop (MINUS_EXPR,
+ fold_convert (ptrdiff_type_node, arg1),
+ fold_convert (ptrdiff_type_node, arg2));
wide_int_to_tree (type, wi::to_widest (arg1) - wi::to_widest (arg2));
?
+ case POINTER_DIFF_EXPR:
+ {
+ if (!POINTER_TYPE_P (rhs1_type)
+ || !POINTER_TYPE_P (rhs2_type)
+ // || !useless_type_conversion_p (rhs2_type, rhs1_type)
types_compatible_p (rhs1_type, rhs2_type)?
+ // || !useless_type_conversion_p (ptrdiff_type_node, lhs_type))
+ || TREE_CODE (lhs_type) != INTEGER_TYPE
+ || TYPE_UNSIGNED (lhs_type))
+ {
+ error ("type mismatch in pointer diff expression");
+ debug_generic_stmt (lhs_type);
+ debug_generic_stmt (rhs1_type);
+ debug_generic_stmt (rhs2_type);
+ return true;
there's also verify_expr which could want adjustment for newly created
tree kinds.
So if the precision of the result is larger than that of the pointer operands
we sign-extend the result, right? So the subtraction is performed in precision
of the pointer operands and then sign-extended/truncated to the result type?
Which means it is _not_ a widening subtraction to get around the
half-address-space
issue. The tree.def documentation should reflect this semantic
detail. Not sure
if the RTL expansion follows it.
I think that we'd ideally have a single legal INTEGER_TYPE precision
per pointer type precision and that those precisions should match...
we don't have to follow the mistakes of POINTER_PLUS_EXPR.
So ... above verify TYPE_PRECISION (rhs1_type) == TYPE_PRECISION (lhs_type)?
Some targets have 24bit ptr_mode but no 24bit integer type which means the
FE likely chooses 32bit int for the difference computation. But the middle-end
should be free to create a 24bit precision type with SImode.
Otherwise as said before - go ahead, I think this would be great to
have for GCC 8. I'd say
ask the maintainers of the above list of targets to do some testing.
"Fixing" POINTER_PLUS_EXPR would be very nice as well. Again, matching
precision - I'm not sure if we need to force a signed operand, having either
might be nice given all sizes are usually unsigned.
Thanks and sorry for the delay,
Richard.
> --
> Marc Glisse
