From ea74eb771ebeff4d8d9f8612b089cd07a8ed8941 Mon Sep 17 00:00:00 2001
From: Stephen Williams <steve@icarus.com>
Date: Tue, 25 Mar 2008 21:35:08 -0700
Subject: [PATCH] Fix a couple problems with signed multiply

Problems with signed expressions that are set to parameters and
that include multipliciation exposed a few bugs in the calculation
of signed multiply. Fix this and add some improved diagnostics.
---
 eval_tree.cc | 19 +++++++++++++++----
 verinum.cc   | 12 ++++++++----
 verinum.h    | 13 +++++++++++++
 3 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/eval_tree.cc b/eval_tree.cc
index 1dd138421..93d6d949e 100644
--- a/eval_tree.cc
+++ b/eval_tree.cc
@@ -808,13 +808,18 @@ NetExpr* NetEBDiv::eval_tree(int prune_to_width)
 	    verinum lval = lc->value();
 	    verinum rval = rc->value();
 
+	    NetExpr*tmp = 0;
 	    switch (op_) {
 		case '/':
-		  return new NetEConst(lval / rval);
-
+		  tmp = new NetEConst(lval / rval);
+		  break;
 		case '%':
-		  return new NetEConst(lval % rval);
+		  tmp = new NetEConst(lval % rval);
+		  break;
 	    }
+	    ivl_assert(*this, tmp);
+	    tmp->set_line(*this);
+	    return tmp;
       }
 
       return 0;
@@ -919,7 +924,13 @@ NetExpr* NetEBMult::eval_tree(int prune_to_width)
       verinum lval = lc->value();
       verinum rval = rc->value();
 
-      return new NetEConst(lval * rval);
+      NetEConst*tmp = new NetEConst(lval * rval);
+
+      if (debug_eval_tree)
+	    cerr << get_fileline() << ": debug: Evaluate "
+		 << lval << " * " << rval << " --> " << *tmp << endl;
+
+      return tmp;
 }
 
 NetExpr* NetEBPow::eval_tree_real_()
diff --git a/verinum.cc b/verinum.cc
index 05f2ef25c..d7ed2a25a 100644
--- a/verinum.cc
+++ b/verinum.cc
@@ -881,14 +881,18 @@ verinum operator * (const verinum&left, const verinum&right)
       verinum result(verinum::V0, left.len() + right.len(), has_len_flag);
       result.has_sign(left.has_sign() || right.has_sign());
 
-      for (unsigned rdx = 0 ;  rdx < right.len() ;  rdx += 1) {
+      verinum::V r_sign = sign_bit(right);
+      for (unsigned rdx = 0 ;  rdx < result.len() ;  rdx += 1) {
 
-	    if (right.get(rdx) == verinum::V0)
+	    verinum::V r_bit = rdx < right.len()? right.get(rdx) : r_sign;
+	    if (r_bit == verinum::V0)
 		  continue;
 
+	    verinum::V l_sign = sign_bit(left);
 	    verinum::V carry = verinum::V0;
-	    for (unsigned ldx = 0 ;  ldx < left.len() ;  ldx += 1) {
-		  result.set(ldx+rdx, add_with_carry(left[ldx],
+	    for (unsigned ldx = 0 ;  ldx < result.len()-rdx ;  ldx += 1) {
+		  verinum::V l_bit = ldx < left.len()? left[ldx] : l_sign;
+		  result.set(ldx+rdx, add_with_carry(l_bit,
 						     result[rdx+ldx],
 						     carry));
 	    }
diff --git a/verinum.h b/verinum.h
index 4670e45c9..f400a1a5c 100644
--- a/verinum.h
+++ b/verinum.h
@@ -104,6 +104,19 @@ class verinum {
       bool string_flag_;
 };
 
+/*
+ * This returns the sign bit of the verinum value. If the value is
+ * unsigned, then return an implicit sign bit of 0. Otherwise, return
+ * the high bit.
+ */
+inline verinum::V sign_bit(const verinum&val)
+{
+      if (val.has_sign())
+	    return val.get(val.len()-1);
+      else
+	    return verinum::V0;
+}
+
 /* Return a verinum that has the same value as the input, but is at
    least as wide as the requested width. This may involve sign
    extension, if the value is signed. */