From dffe1be819c76baeaa6e0cba7174d3a61608e16b Mon Sep 17 00:00:00 2001
From: Stephen Williams <steve@icarus.com>
Date: Tue, 15 Apr 2008 19:18:03 -0700
Subject: [PATCH] Trim results of unsized constant add to prevent width
 expansion

When constant values are added to get another constant value, and the
expression is not otherwise a fixed size, then trim the result to
prevent unbounded expansion of unsized expression with.

In the process, find and fix a bug in the vtrim function that caused
an assert if trimming were not possible for an unsigned value.
---
 eval_tree.cc | 14 ++++++++++----
 verinum.cc   | 10 +++++++++-
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/eval_tree.cc b/eval_tree.cc
index c0c9c5019..47d7d211f 100644
--- a/eval_tree.cc
+++ b/eval_tree.cc
@@ -95,12 +95,14 @@ NetExpr* NetEBAdd::eval_tree(int prune_to_width)
 		  return 0;
 	    }
 
+	    if (debug_eval_tree) {
+		  cerr << get_fileline() << ": debug: Evaluate expr=" << *this
+		       << " --- prune=" << prune_to_width
+		       << " has_width=" << (has_width()? "true" : "false") << endl;
+	    }
+
 	    /* Result might have known width. */
 	    if (has_width()) {
-		  if (debug_eval_tree) {
-			cerr << get_fileline() << ": debug: Evaluate expr=" << *this
-			     << " --- prune=" << prune_to_width << endl;
-		  }
 		  unsigned lwid = lc->expr_width();
 		  unsigned rwid = rc->expr_width();
 		  unsigned  wid = (rwid > lwid) ? rwid : lwid;
@@ -108,6 +110,10 @@ NetExpr* NetEBAdd::eval_tree(int prune_to_width)
 			wid += 1;
 		  verinum val2=verinum(val,wid);
 		  val=val2;
+	    } else {
+		    /* No fixed width, so trim the bits losslessly. */
+		  verinum val2 = trim_vnum(val);
+		  val = val2;
 	    }
 
 	    return new NetEConst(val);
diff --git a/verinum.cc b/verinum.cc
index 0f9907389..44e114731 100644
--- a/verinum.cc
+++ b/verinum.cc
@@ -505,12 +505,20 @@ verinum trim_vnum(const verinum&that)
 	    unsigned top = that.len()-1;
 	    while ((top > 0) && (that.get(top) == verinum::V0))
 		  top -= 1;
+
+	      /* Now top is the index of the highest non-zero bit. If
+		 that turns out to the highest bit in the vector, then
+		 tere is no trimming possible. */
+	    if (top+1 == that.len())
+		  return that;
+
+	      /* Make tlen wide enough to include the highest non-zero
+		 bit, plus one extra 0 bit. */
 	    tlen = top+2;
 
 	      /* This can only happen when the verinum is all zeros,
 		 so make it a single bit wide. */
 	    if (that.get(top) == verinum::V0) tlen -= 1;
-
       }
 
       verinum tmp (verinum::V0, tlen, false);