From 1a64c7d232b4965784c7c1f035631793054ebdae Mon Sep 17 00:00:00 2001
From: Geza Lore <gezalore@gmail.com>
Date: Wed, 15 Apr 2020 22:26:15 +0100
Subject: [PATCH] Fix run-time formatting of variable wider than 1023 bits
 (#2261)

---
 src/V3EmitC.cpp                          |  7 +++---
 test_regress/t/t_format_wide_decimal.out |  2 ++
 test_regress/t/t_format_wide_decimal.v   | 31 ++++++++++++++++++------
 3 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/src/V3EmitC.cpp b/src/V3EmitC.cpp
index f0c81b11d..ba95d9fee 100644
--- a/src/V3EmitC.cpp
+++ b/src/V3EmitC.cpp
@@ -2042,9 +2042,10 @@ void EmitCStmts::displayArg(AstNode* dispp, AstNode** elistp, bool isScan, const
     string pfmt;
     if ((fmtLetter == '#' || fmtLetter == 'd' || fmtLetter == 't') && !isScan
         && vfmt == "") {  // Size decimal output.  Spec says leading spaces, not zeros
-        double mantissabits = argp->widthMin() - ((fmtLetter == 'd') ? 1 : 0);
-        double maxval = pow(2.0, mantissabits);
-        double dchars = log10(maxval) + 1.0;
+        const double mantissabits = argp->widthMin() - ((fmtLetter == 'd') ? 1 : 0);
+        // This is log10(2**mantissabits) as log2(2**mantissabits)/log2(10),
+        // + 1.0 rounding bias.
+        double dchars = mantissabits / 3.321928094887362 + 1.0;
         if (fmtLetter == 'd') dchars++;  // space for sign
         int nchars = int(dchars);
         pfmt = string("%") + cvtToStr(nchars) + fmtLetter;
diff --git a/test_regress/t/t_format_wide_decimal.out b/test_regress/t/t_format_wide_decimal.out
index 4c5c7138a..8dd14a9b7 100644
--- a/test_regress/t/t_format_wide_decimal.out
+++ b/test_regress/t/t_format_wide_decimal.out
@@ -1,2 +1,4 @@
 179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137215
+ 89884656743115795386465259539451236680898848947115328636715040578866337902750481566354238661203768010560056939935696678829394884407208311246423715319737062188883946712432742638151109800623047059726541476042502884419075341171231440736956555270413618581675255342293149119973622969239858152417678164812112068607
+ 44942328371557897693232629769725618340449424473557664318357520289433168951375240783177119330601884005280028469967848339414697442203604155623211857659868531094441973356216371319075554900311523529863270738021251442209537670585615720368478277635206809290837627671146574559986811484619929076208839082406056034303
 *-* All Finished *-*
diff --git a/test_regress/t/t_format_wide_decimal.v b/test_regress/t/t_format_wide_decimal.v
index c901f959d..2b2a04835 100644
--- a/test_regress/t/t_format_wide_decimal.v
+++ b/test_regress/t/t_format_wide_decimal.v
@@ -6,15 +6,32 @@
 // Version 2.0.
 // SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
 
-module t_format_wide_decimal;
+module t_format_wide_decimal(/*AUTOARG*/
+   // Inputs
+   clk
+   );
+   input clk;
 
-   initial begin
-      // Format very wide constant number (which has more bits than can
-      // be counted in exponent of a double precision float), with %d.
-      $display("%d", 1024'hffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff);
+   int   cycle = 0;
+   bit [1023:0] x = '1;
 
-      $write("*-* All Finished *-*\n");
-      $finish;
+   always @(posedge clk) begin
+      if (cycle == 0) begin
+         // Format very wide constant number (which has more bits than can
+         // be counted in exponent of a double precision float), with %d.
+         $display("%d", 1024'hffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff);
+      end else begin
+         // Same, but for a variable with value only known at run-time
+         $display("%d", x);
+      end
+
+      cycle <= cycle + 1;
+      x <= x >> 1;
+
+      if (cycle == 2) begin
+         $write("*-* All Finished *-*\n");
+         $finish;
+      end
    end
 
 endmodule