Re: [PATCH] c++: build_new_1 and non-dep array size [PR111929]

[Patrick Palka <ppalka@redhat.com>]

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On Tue, 24 Oct 2023, Jason Merrill wrote:

> On 10/24/23 13:03, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > like the right approach?
> > 
> > -- >8 --
> > 
> > This PR is another instance of NON_DEPENDENT_EXPR having acted as an
> > "analysis barrier" for middle-end routines, and now that it's gone we
> > may end up passing weird templated trees (that have a generic tree code)
> > to the middle-end which leads to an ICE.  In the testcase below the
> > non-dependent array size 'var + 42' is expressed as an ordinary
> > PLUS_EXPR, but whose operand types have different precisions -- long and
> > int respectively -- naturally because templated trees encode only the
> > syntactic form of an expression devoid of e.g. implicit conversions
> > (typically).  This type incoherency triggers a wide_int assert during
> > the call to size_binop in build_new_1 which requires the operand types
> > have the same precision.
> > 
> > This patch fixes this by replacing our incremental folding of 'size'
> > within build_new_1 with a single call to cp_fully_fold (which is a no-op
> > in template context) once 'size' is fully built.
> 
> This is OK, but we could probably also entirely skip a lot of the calculation
> in a template, since we don't care about any values.  Can we skip the entire
> if (array_p) block?

That seems to be safe correctness-wise, but QOI-wise it'd mean we'd no
longer diagnose a too large array size ahead of time:

  template<class>
  void f() {
    new int[__SIZE_MAX__ / sizeof(int)];
  }

  <stdin>: In function ‘void f()’:
  <stdin>:3:37: error: size ‘(((sizetype)(18446744073709551615 / sizeof (int))) * 4)’ of array exceeds maximum object size ‘9223372036854775807’

(That we diagnose this ahead of time is thanks to the NON_DEPENDENT_EXPR
removal; previously 'nelts' was wrapped in NON_DEPENDENT_EXPR which
ironically prevented fold_non_dependent_expr from folding it to a
constant...)

> 
> > 	PR c++/111929
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* init.cc (build_new_1): Use convert, build2, build3 instead of
> > 	fold_convert, size_binop and fold_build3 when building 'size'.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/template/non-dependent28.C: New test.
> > ---
> >   gcc/cp/init.cc                                  | 9 +++++----
> >   gcc/testsuite/g++.dg/template/non-dependent28.C | 6 ++++++
> >   2 files changed, 11 insertions(+), 4 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/template/non-dependent28.C
> > 
> > diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> > index d48bb16c7c5..56c1b5e9f5e 100644
> > --- a/gcc/cp/init.cc
> > +++ b/gcc/cp/init.cc
> > @@ -3261,7 +3261,7 @@ build_new_1 (vec<tree, va_gc> **placement, tree type,
> > tree nelts,
> >         max_outer_nelts = wi::udiv_trunc (max_size, inner_size);
> >         max_outer_nelts_tree = wide_int_to_tree (sizetype, max_outer_nelts);
> >   -      size = size_binop (MULT_EXPR, size, fold_convert (sizetype,
> > nelts));
> > +      size = build2 (MULT_EXPR, sizetype, size, convert (sizetype, nelts));
> >           if (TREE_CODE (cst_outer_nelts) == INTEGER_CST)
> >   	{
> > @@ -3344,7 +3344,7 @@ build_new_1 (vec<tree, va_gc> **placement, tree type,
> > tree nelts,
> >         /* Use a class-specific operator new.  */
> >         /* If a cookie is required, add some extra space.  */
> >         if (array_p && TYPE_VEC_NEW_USES_COOKIE (elt_type))
> > -	size = size_binop (PLUS_EXPR, size, cookie_size);
> > +	size = build2 (PLUS_EXPR, sizetype, size, cookie_size);
> >         else
> >   	{
> >   	  cookie_size = NULL_TREE;
> > @@ -3358,8 +3358,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type,
> > tree nelts,
> >         if (cxx_dialect >= cxx11 && flag_exceptions)
> >   	errval = throw_bad_array_new_length ();
> >         if (outer_nelts_check != NULL_TREE)
> > -	size = fold_build3 (COND_EXPR, sizetype, outer_nelts_check,
> > -			    size, errval);
> > +	size = build3 (COND_EXPR, sizetype, outer_nelts_check, size, errval);
> > +      size = cp_fully_fold (size);
> >         /* Create the argument list.  */
> >         vec_safe_insert (*placement, 0, size);
> >         /* Do name-lookup to find the appropriate operator.  */
> > @@ -3418,6 +3418,7 @@ build_new_1 (vec<tree, va_gc> **placement, tree type,
> > tree nelts,
> >         /* If size is zero e.g. due to type having zero size, try to
> >   	 preserve outer_nelts for constant expression evaluation
> >   	 purposes.  */
> > +      size = cp_fully_fold (size);
> >         if (integer_zerop (size) && outer_nelts)
> >   	size = build2 (MULT_EXPR, TREE_TYPE (size), size, outer_nelts);
> >   diff --git a/gcc/testsuite/g++.dg/template/non-dependent28.C
> > b/gcc/testsuite/g++.dg/template/non-dependent28.C
> > new file mode 100644
> > index 00000000000..3e45154f61d
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/non-dependent28.C
> > @@ -0,0 +1,6 @@
> > +// PR c++/111929
> > +
> > +template<class>
> > +void f(long var) {
> > +  new int[var + 42];
> > +}
> 
>