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From: Richard Biener <richard.guenther@gmail.com>
To: Marc Glisse <marc.glisse@inria.fr>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [RFC PATCH] Fix pointer diff (was: -fsanitize=pointer-overflow support (PR sanitizer/80998))
Date: Thu, 19 Oct 2017 15:11:00 -0000	[thread overview]
Message-ID: <CAFiYyc0m9RsAeWGA=uPMwqE8CvKpjO8yHykg-Ng+Eq-XC=6LRg@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1710082250230.3077@stedding.saclay.inria.fr>

On Mon, Oct 9, 2017 at 12:55 PM, Marc Glisse <marc.glisse@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

  reply	other threads:[~2017-10-19 14:53 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-19 18:25 [RFC PATCH] -fsanitize=pointer-overflow support (PR sanitizer/80998) Jakub Jelinek
2017-06-20  7:41 ` Richard Biener
2017-06-20  8:14   ` Jakub Jelinek
2017-06-20  8:18     ` Richard Biener
2017-06-21  7:58       ` Jakub Jelinek
2017-06-21  8:04         ` Richard Biener
2017-06-21 14:40       ` [RFC PATCH] Fix pointer diff (was: -fsanitize=pointer-overflow support (PR sanitizer/80998)) Jakub Jelinek
2017-06-21 15:17         ` Jakub Jelinek
2017-06-21 16:27         ` Marc Glisse
2017-06-22  8:31           ` Richard Biener
2017-06-22  9:29             ` Marc Glisse
2017-06-22  9:46               ` Richard Biener
2017-07-01 16:41                 ` Marc Glisse
2017-07-03 12:37                   ` Richard Biener
2017-10-09 11:01                     ` Marc Glisse
2017-10-19 15:11                       ` Richard Biener [this message]
2017-10-28 13:04                         ` Marc Glisse
2017-10-28 17:13                           ` Richard Biener
2017-07-04  8:53       ` [RFC PATCH] -fsanitize=pointer-overflow support (PR sanitizer/80998) Jakub Jelinek
2017-06-21  8:00   ` Jakub Jelinek
2017-06-21  8:05     ` Richard Biener

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