[gcc r13-6473] c++: Don't defer local statics initialized with constant expressions [PR108702]

[gcc r13-6473] c++: Don't defer local statics initialized with constant expressions [PR108702]

[Jakub Jelinek]

https://gcc.gnu.org/g:9d5730dee4f42e94004b38f8f4862c0b1f4d964c

commit r13-6473-g9d5730dee4f42e94004b38f8f4862c0b1f4d964c
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sat Mar 4 09:51:31 2023 +0100

    c++: Don't defer local statics initialized with constant expressions [PR108702]
    
    The stmtexpr19.C testcase used to be rejected as it has a static
    variable in statement expression in constexpr context, but as that
    static variable is initialized by constant expression, when P2647R1
    was implemented we agreed to make it valid.
    
    Now, as reported, the testcase compiles fine, but doesn't actually link
    because the static variable isn't defined anywhere, and with -flto ICEs
    because of this problem.  This is because we never
    varpool_node::finalize_decl those vars, the constant expression in which
    the DECL_EXPR is present for the static VAR_DECL is folded (constant
    evaluated) into just the address of the VAR_DECL.
    Now, similar testcase included below (do we want to include it in the
    testsuite too?) works fine, because in
    cp_finish_decl -> make_rtl_for_nonlocal_decl
    we have since PR70353 fix:
      /* We defer emission of local statics until the corresponding
         DECL_EXPR is expanded.  But with constexpr its function might never
         be expanded, so go ahead and tell cgraph about the variable now.  */
      defer_p = ((DECL_FUNCTION_SCOPE_P (decl)
                  && !var_in_maybe_constexpr_fn (decl))
                 || DECL_VIRTUAL_P (decl));
    and so don't defer them in constexpr/consteval functions.  The following
    patch calls rest_of_decl_compilation which make_rtl_for_nonlocal_decl didn't
    call when encountering DECL_EXPRs of such vars during constant evaluation
    if they weren't finalized yet.
    
    2023-03-04  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/108702
            * constexpr.cc: Include toplev.h.
            (cxx_eval_constant_expression) <case DECL_EXPR>: When seeing a local
            static initialized by constant expression outside of a constexpr
            function which has been deferred by make_rtl_for_nonlocal_decl,
            call rest_of_decl_compilation on it.
    
            * g++.dg/ext/stmtexpr19.C: Use dg-do link rather than dg-do compile.

Diff:
---
 gcc/cp/constexpr.cc                   | 19 +++++++++++++++++++
 gcc/testsuite/g++.dg/ext/stmtexpr19.C |  2 +-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index acf9847a4d1..364695b762c 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -40,6 +40,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "attribs.h"
 #include "fold-const.h"
 #include "intl.h"
+#include "toplev.h"
 
 static bool verify_constant (tree, bool, bool *, bool *);
 #define VERIFY_CONSTANT(X)						\
@@ -7127,6 +7128,24 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    break;
 	  }
 
+	/* make_rtl_for_nonlocal_decl could have deferred emission of
+	   a local static var, but if it appears in a statement expression
+	   which is constant expression evaluated to e.g. just the address
+	   of the variable, its DECL_EXPR will never be seen during
+	   gimple lowering's record_vars_into as the statement expression
+	   will not be in the IL at all.  */
+	if (VAR_P (r)
+	    && TREE_STATIC (r)
+	    && !DECL_REALLY_EXTERN (r)
+	    && DECL_FUNCTION_SCOPE_P (r)
+	    && !var_in_maybe_constexpr_fn (r)
+	    && decl_constant_var_p (r))
+	  {
+	    varpool_node *node = varpool_node::get (r);
+	    if (node == NULL || !node->definition)
+	      rest_of_decl_compilation (r, 0, at_eof);
+	  }
+
 	if (AGGREGATE_TYPE_P (TREE_TYPE (r))
 	    || VECTOR_TYPE_P (TREE_TYPE (r)))
 	  {
diff --git a/gcc/testsuite/g++.dg/ext/stmtexpr19.C b/gcc/testsuite/g++.dg/ext/stmtexpr19.C
index be395b69065..5355c15df9a 100644
--- a/gcc/testsuite/g++.dg/ext/stmtexpr19.C
+++ b/gcc/testsuite/g++.dg/ext/stmtexpr19.C
@@ -1,6 +1,6 @@
 // PR c++/81073
 // { dg-options "" }
-// { dg-do compile { target c++11 } }
+// { dg-do link { target c++11 } }
 
 struct test { const int *addr; };