[Bug tree-optimization/21236] New: force_gimple_operand destroys trees passed to it

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

While fixing bug 21173 and 20963, it was noticed that force_gimple_operand
destroys trees passed to it.
This is a very surprising result, and was the actual underlying cause of those bugs.

(the testcase in g++.dg/tree-ssa/pr20963.C shows this)

a debugging session:

 Breakpoint 3, create_expression_by_pieces (block=0x400c4870,
expr=0x88403f0, stmts=0x400c6510) at tree-ssa-pre.c:1322

expr came from a node in ANTIC_IN, we *can't* modify it, because it's
not a copy.

(gdb) p debug_generic_stmt (expr)
(charD.3 *) &0B->typeD.1681;
... 
1388            newexpr = force_gimple_operand (expr,
(gdb) n
1390            if (forced_stmts)
(gdb) p debug_generic_stmt (expr)
(charD.3 *) &D.1708_3->typeD.1681;

-- 
           Summary: force_gimple_operand destroys trees passed to it
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dberlin at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21236