[Bug c/52632] New: GCC compfail on O0

["vbyakovl23 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52632

             Bug #: 52632
           Summary: GCC compfail on O0
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vbyakovl23@gmail.com


Test case a.c gives following compfail when compiling with O0

gcc –O0 –c a.c
a.c: In function 'foo':
a.c:4:1: error: size of unnamed array is negative

For higher opt level it's ok.
gcc version 4.8.0 20120319 (experimental) (GCC)

The failure happened because FE on –O0 replaces builtin call by zero if it is
not folded. See gcc/builtins.c, fold_builtin_1(), line 10270
  switch (fcode)
    {
    case BUILT_IN_CONSTANT_P:
      { 
        tree val = fold_builtin_constant_p (arg0);

        /* Gimplification will pull the CALL_EXPR for the builtin out of
           an if condition.  When not optimizing, we'll not CSE it back.
           To avoid link error types of regressions, return false now.  */
        if (!val && !optimize)
          val = integer_zero_node;

        return val;
      }

It may be fixed by a patch that disabled error message in case of not optimize. 

diff --git a/gcc/c-decl.c b/gcc/c-decl.c
index 160d393..1ba3f51 100644
--- a/gcc/c-decl.c
+++ b/gcc/c-decl.c
@@ -5345,7 +5345,7 @@ grokdeclarator (const struct c_declarator *declarator,
                if (TREE_CODE (size) == INTEGER_CST && size_maybe_const)
                  {
                    constant_expression_warning (size);
-                   if (tree_int_cst_sgn (size) < 0)
+                   if ((pedantic || optimize) && tree_int_cst_sgn (size) < 0)
                      {
                        if (name)
                          error_at (loc, "size of array %qE is negative",
name);