[Bug c++/17972] [3.4 Regression] const/pure functions result in bad asm

["ebotcazou at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From ebotcazou at gcc dot gnu dot org  2005-02-24 16:49 -------
Hum... again a consequence of the C++ front-end not setting TREE_SIDE_EFFECTS on
every CALL_EXPR, like the C front-end.  The sequence of events is as follows:

1. fn1()->s -= 1 is expanded to the 3-operand form in build_modify_expr

	{
	  /* A binary op has been requested.  Combine the old LHS
     	     value with the RHS producing the value we should actually
     	     store into the LHS.  */

	  my_friendly_assert (!PROMOTES_TO_AGGR_TYPE (lhstype, REFERENCE_TYPE),
			      978652);
	  lhs = stabilize_reference (lhs);
	  newrhs = cp_build_binary_op (modifycode, lhs, rhs);
	  if (newrhs == error_mark_node)
	    {
	      error ("  in evaluation of `%Q(%#T, %#T)'", modifycode,
		     TREE_TYPE (lhs), TREE_TYPE (rhs));
	      return error_mark_node;
	    }
	  
	  /* Now it looks like a plain assignment.  */
	  modifycode = NOP_EXPR;
	}

Note the call to stabilize_reference.  However, it doesn't stabilize anything
here because the lhs is advertised as having no side-effects.

2. The tree-inliner inlines the call.  Since the same tree is referenced twice
in the expression, the inlined body is also referenced twice is the expression
and, therefore, expanded twice to RTL.  However labels are not expanded multiple
times but reused, so the second block of RTL ends up referencing the first and
all hell breaks loose.

At this point my conclusion is that the only safe approach is that of the C
front-end.

Mark, do you recall having reviewed the initial patch (see comment #22)?  What
do you think?  Thanks in advance.


-- 
           What    |Removed                     |Added
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                 CC|                            |mark at codesourcery dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17972