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From: kenner@vlsi1.ultra.nyu.edu (Richard Kenner)
Message-Id: <10409071147.AA24235@vlsi1.ultra.nyu.edu>
To: dnovillo@redhat.com, rth@redhat.com
Subject: Serious problems with tree-ssa operand handling
Cc: gcc@gcc.gnu.org
X-SW-Source: 2004-09/txt/msg00281.txt.bz2

I sent http://gcc.gnu.org/ml/gcc/2004-09/msg00189.html on Saturday because
the breakage also happened on Friday.  I've done more work here, which has left
me more confused.  See also http://gcc.gnu.org/ml/gcc/2004-09/msg00196.html

The issue is the adding of virtual operands for a call.  It uses alias
information to do that and adds the operand with:

        tree var = referenced_var (i);
 
        add_stmt_operand (&var, ...);
 
As I said originally, this puts a pointer into the stack of that function
into the operands of the statement.  That causes an access of junk when
the operands are referenced.

I tried fixing that with:

	tree *var_p = &referenced_var (i);
	add_stmt_operand (var_p, stmt, opf_is_def);

That now references defined memory.  But it has the problem that we then
try to put an SSA_NAME into referenced_vars, which is wrong.

Moreover, we get a verification error:

g-exctra.adb: In function `GNAT.EXCEPTION_TRACES.DECORATOR_WRAPPER':
g-exctra.adb:69: error: Found a virtual definition for a GIMPLE register
while verifying SSA_NAME decorator_traceback_35 in statement
#   gnat__exception_traces__current_decorator<D449>_29 = V_MAY_DEF <gnat__except
ion_traces__current_decorator<D449>_18>;
#   T.9<D466>_34 = V_MAY_DEF <T.9<D466>_23>;
#   decorator_traceback<D456>_35 = V_MAY_DEF <decorator_traceback<D456>_17>;
#   T.11<D468>_30 = V_MAY_DEF <T.11<D468>_28>;
#   VUSE <T.8<D465>_27>;
T.11<D468> = gnat__exception_traces__current_decorator.7_19 (T.8<D465>);

This is confusing because an example in doc/tree-ssa.texi implies this sort
of thing is valid.

I disabled that check and "fixed" the clobbering of referenced_vars by trying

		tree *var_p = (tree *) ggc_alloc (sizeof (tree *));

		*var_p = referenced_var (i);
		add_stmt_operand (var_p, stmt, opf_is_def);

Now I get another check failure, saying that a variable appears both as a
real and virtual operand.  When I disable *that* check, I finally get
the Ada RTS to compile, but I get new ACATS failures (possibly related, but
maybe not).

But I shouldn't have to put in that kludge and disable both checks.

Something looks very badly wrong here but I don't understand how this is
supposed to work, so can't say what.