Hi!
The excess precision support broke building skia (dependency of firefox)
on ia32 (it has something like the a constexpr variable), but as the other
cases show, it is actually a preexisting problem if one uses casts from
constants with wider floating point types.
The problem is that cxx_eval_constant_expression tries to short-cut
processing of TREE_CONSTANT CONSTRUCTORs if they satisfy
reduced_constant_expression_p - instead of calling cxx_eval_bare_aggregate
on them it just verifies flags and if they are TREE_CONSTANT even after
that, just fold.
Now, on the testcase we have a TREE_CONSTANT CONSTRUCTOR containing
TREE_CONSTANT NOP_EXPR of REAL_CST. And, fold, which isn't recursive,
doesn't optimize that into VECTOR_CST, while later on we are only able
to optimize VECTOR_CST arithmetics, not arithmetics with vector
CONSTRUCTORs.
The following patch fixes that by only returning what fold returned
if for vector types it returned VECTOR_CST, otherwise let us
call cxx_eval_bare_aggregate. That function will try to constant
evaluate all the elements and if anything changes, return a CONSTRUCTOR,
in the vector type cases with fold called on it at the end.
Now, just calling cxx_eval_bare_aggregate for vector types doesn't work
either (e.g. constexpr-builtin4.C breaks), because cxx_eval_bare_aggregate
if nothing changes (like all elts are already REAL_CSTs or INTEGER_CSTs)
will return the old CONSTRUCTOR and nothing folds it into a VECTOR_CST.
Also, the reason for the short-cutting is I think trying to avoid
allocating a new CONSTRUCTOR when nothing changes and we just create
GC garbage by it.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2022-10-19 Jakub Jelinek <ja...@redhat.com>
PR c++/107295
* constexpr.cc (cxx_eval_constant_expression) <case CONSTRUCTOR>:
Don't short-cut TREE_CONSTANT vector ctors if fold doesn't turn them
into VECTOR_CST.
* g++.dg/ext/vector42.C: New test.
--- gcc/cp/constexpr.cc.jj 2022-10-17 12:29:33.518016420 +0200
+++ gcc/cp/constexpr.cc 2022-10-19 01:29:28.761935708 +0200
@@ -7391,7 +7391,12 @@ cxx_eval_constant_expression (const cons
VECTOR_CST if applicable. */
verify_constructor_flags (t);
if (TREE_CONSTANT (t))
- return fold (t);
+ {
+ r = fold (t);
+ if (TREE_CODE (TREE_TYPE (t)) != VECTOR_TYPE
+ || TREE_CODE (r) == VECTOR_CST)
+ return r;
+ }
}
r = cxx_eval_bare_aggregate (ctx, t, lval,
non_constant_p, overflow_p);
--- gcc/testsuite/g++.dg/ext/vector42.C.jj 2022-10-18 12:33:42.938510483
+0200
+++ gcc/testsuite/g++.dg/ext/vector42.C 2022-10-18 12:32:27.448544476 +0200
@@ -0,0 +1,12 @@
+// PR c++/107295
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct A {
+ typedef T __attribute__((vector_size (sizeof (int)))) V;
+};
+template <int, typename T> using B = typename A<T>::V;
+template <typename T> using V = B<4, T>;
+using F = V<float>;
+constexpr F a = F () + 0.0f;
+constexpr F b = F () + (float) 0.0;
+constexpr F c = F () + (float) 0.0L;
Jakub
