[PATCH, C++, PR58282] Handle noexcept on transactions with -fno-exceptions

[Tom de Vries <Tom_deVries at mentor dot com>]

Jason,

Consider this testcase reduced from g++.dg/tm/noexcept-1.C:
...
struct TrueFalse
{
  static constexpr bool v() { return true; }
};

int global;

template<typename T> int foo()
{
  __transaction_atomic noexcept(T::v()) { global += 1; }
  return __transaction_atomic noexcept(T::v()) (global + 2);
}

int f1()
{
  return foo<TrueFalse>();
}
...

It fails with a SIGSEGV when run with -fno-exceptions:
...
$ g++ noexcept-1.C -fno-exceptions -fgnu-tm -std=c++0x -S
noexcept-1.C: In function ‘int foo() [with T = TrueFalse]’:
noexcept-1.C:10:43: internal compiler error: Segmentation fault
   __transaction_atomic noexcept(T::v()) { global += 1; }
                                           ^
...

The noexcept(t::v()) operator evaluates to true, so the noexcept specifier on
the transaction is set to true. Then cp/semantics.c:build_transaction_expr
builds a MUST_NOT_THROW_EXPR to safe-guard the property at runtime:
...
  if (noex)
    {
      expr = build_must_not_throw_expr (expr, noex);
      SET_EXPR_LOCATION (expr, loc);
      TREE_SIDE_EFFECTS (expr) = 1;
    }
...

During gimplification of the MUST_NOT_THROW_EXPR in
cp/cp-gimplify.c:gimplify_must_not_throw_expr, we're trying to translate into a
try/catch:
...
  gimplify_and_add (body, &try_);
  mnt = gimple_build_eh_must_not_throw (terminate_node);
  gimple_seq_add_stmt_without_update (&catch_, mnt);
  mnt = gimple_build_try (try_, catch_, GIMPLE_TRY_CATCH);
...

However, terminate_node is NULL at that point because of -fno-exceptions, so in
gimple.c:gimple_build_eh_must_not_throw, this line causes a SIGSEGV because decl
== terminate_node == NULL:
...
  gcc_assert (TREE_CODE (decl) == FUNCTION_DECL);
...

This patch fixes the SIGSEGV by not generating a MUST_NOT_THROW_EXPR with
-fno-exceptions.

Bootstrapped and reg-tested on x86_64.

OK for trunk?

Thanks,
- Tom

2013-09-02  Tom de Vries  <tom@codesourcery.com>

	PR c++/58282
	* semantics.c (finish_transaction_stmt, build_transaction_expr): Handle
	flag_exceptions.

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index ee3503c..c8b328c 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -5189,7 +5189,7 @@ finish_transaction_stmt (tree stmt, tree compound_stmt, int flags, tree noex)
 
   /* noexcept specifications are not allowed for function transactions.  */
   gcc_assert (!(noex && compound_stmt));
-  if (noex)
+  if (noex && flag_exceptions)
     {
       tree body = build_must_not_throw_expr (TRANSACTION_EXPR_BODY (stmt),
 					     noex);
@@ -5210,7 +5210,7 @@ tree
 build_transaction_expr (location_t loc, tree expr, int flags, tree noex)
 {
   tree ret;
-  if (noex)
+  if (noex && flag_exceptions)
     {
       expr = build_must_not_throw_expr (expr, noex);
       SET_EXPR_LOCATION (expr, loc);