On Fri, Jan 26, 2018 at 02:11:19PM +0100, Richard Biener wrote:
> >> POINTER_PLUS_EXPR offets are to be interpreted as signed (ptrdiff_t)
> >> so using uhwi and then performing an unsigned division is wrong code.
> >> See mem_ref_offset how to deal with this (ugh - poly-ints...). Basically
> >> you have to force the thing to signed. Like just use
> >>
> >> HOST_WIDE_INT offset = TREE_INT_CST_LOW (op01);
> >
> > Does it really matter here though? Any negative offsets there are UB, we
> > should punt on them rather than try to optimize them.
> > As we known op01 is unsigned, if we check that it fits into shwi_p, it means
> > it will be 0 to shwi max and then we can handle it in uhwi too.
>
> Ah, of course. Didn't look up enough context to see what this code
> does in the end ;)
>
> > /* ((foo*)&vectorfoo)[1] => BIT_FIELD_REF<vectorfoo,...> */
> > if (VECTOR_TYPE_P (op00type)
> > && (same_type_ignoring_top_level_qualifiers_p
> > - (type, TREE_TYPE (op00type))))
> > + (type, TREE_TYPE (op00type)))
> > + && tree_fits_shwi_p (op01))
>
> nevertheless this appearant "mismatch" deserves a comment (checking
> shwi and using uhwi).
So like this?
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2018-01-26 Marek Polacek <pola...@redhat.com>
Jakub Jelinek <ja...@redhat.com>
PR c++/83659
* constexpr.c (cxx_fold_indirect_ref): Use unsigned HOST_WIDE_INT
when computing offsets. Verify first that tree_fits_shwi_p (op01).
Formatting fix.
* g++.dg/torture/pr83659.C: New test.
--- gcc/cp/constexpr.c.jj 2018-01-24 17:18:42.392392254 +0100
+++ gcc/cp/constexpr.c 2018-01-26 18:54:43.991828743 +0100
@@ -3070,7 +3070,8 @@ cxx_fold_indirect_ref (location_t loc, t
{
tree part_width = TYPE_SIZE (type);
tree index = bitsize_int (0);
- return fold_build3_loc (loc, BIT_FIELD_REF, type, op, part_width,
index);
+ return fold_build3_loc (loc, BIT_FIELD_REF, type, op, part_width,
+ index);
}
/* Also handle conversion to an empty base class, which
is represented with a NOP_EXPR. */
@@ -3109,12 +3110,22 @@ cxx_fold_indirect_ref (location_t loc, t
/* ((foo*)&vectorfoo)[1] => BIT_FIELD_REF<vectorfoo,...> */
if (VECTOR_TYPE_P (op00type)
- && (same_type_ignoring_top_level_qualifiers_p
- (type, TREE_TYPE (op00type))))
+ && same_type_ignoring_top_level_qualifiers_p
+ (type, TREE_TYPE (op00type))
+ /* POINTER_PLUS_EXPR second operand is sizetype, unsigned,
+ but we want to treat offsets with MSB set as negative.
+ For the code below negative offsets are invalid and
+ TYPE_SIZE of the element is something unsigned, so
+ check whether op01 fits into HOST_WIDE_INT, which implies
+ it is from 0 to INTTYPE_MAXIMUM (HOST_WIDE_INT), and
+ then just use tree_to_uhwi because we want to treat the
+ value as unsigned. */
+ && tree_fits_shwi_p (op01))
{
- HOST_WIDE_INT offset = tree_to_shwi (op01);
+ unsigned HOST_WIDE_INT offset = tree_to_uhwi (op01);
tree part_width = TYPE_SIZE (type);
- unsigned HOST_WIDE_INT part_widthi = tree_to_shwi
(part_width)/BITS_PER_UNIT;
+ unsigned HOST_WIDE_INT part_widthi
+ = tree_to_uhwi (part_width) / BITS_PER_UNIT;
unsigned HOST_WIDE_INT indexi = offset * BITS_PER_UNIT;
tree index = bitsize_int (indexi);
--- gcc/testsuite/g++.dg/torture/pr83659.C.jj 2018-01-26 18:46:40.077572013
+0100
+++ gcc/testsuite/g++.dg/torture/pr83659.C 2018-01-26 18:56:36.822888606
+0100
@@ -0,0 +1,18 @@
+// PR c++/83659
+// { dg-do compile }
+
+typedef int V __attribute__ ((__vector_size__ (16)));
+V a;
+V b[2];
+
+int
+foo ()
+{
+ return reinterpret_cast <int *> (&a)[-1] += 1;
+}
+
+int
+bar ()
+{
+ return reinterpret_cast <int *> (&a[1])[-1];
+}
Jakub
