From: whitequark <whitequark@whitequark.org>
Date: Thu, 16 Apr 2020 16:30:43 +0000 (+0000)
Subject: cxxrtl: fix misleading example, caution about race conditions.
X-Git-Tag: working-ls180~627^2~1
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=9043632dcc9b4ab198e03ffbdb8ba232b047ef28;p=yosys.git

cxxrtl: fix misleading example, caution about race conditions.

Fixes #1944.
---

diff --git a/backends/cxxrtl/cxxrtl.cc b/backends/cxxrtl/cxxrtl.cc
index d6b901aa0..8f7f9d7a3 100644
--- a/backends/cxxrtl/cxxrtl.cc
+++ b/backends/cxxrtl/cxxrtl.cc
@@ -1640,21 +1640,30 @@ struct CxxrtlBackend : public Backend {
 		log("\n");
 		log("    write_cxxrtl [options] [filename]\n");
 		log("\n");
-		log("Write C++ code for simulating the design. The generated code requires a driver;\n");
-		log("the following simple driver is provided as an example:\n");
+		log("Write C++ code for simulating the design. The generated code requires a driver\n");
+		log("that instantiates the design, toggles its clock, and interacts with its ports.\n");
+		log("\n");
+		log("The following driver may be used as an example for a design with a single clock\n");
+		log("driving rising edge triggered flip-flops:\n");
 		log("\n");
 		log("    #include \"top.cc\"\n");
 		log("\n");
 		log("    int main() {\n");
 		log("      cxxrtl_design::p_top top;\n");
+		log("      top.step();\n");
 		log("      while (1) {\n");
-		log("        top.p_clk.next = value<1> {1u};\n");
-		log("        top.step();\n");
+		log("        /* user logic */\n");
 		log("        top.p_clk.next = value<1> {0u};\n");
 		log("        top.step();\n");
+		log("        top.p_clk.next = value<1> {1u};\n");
+		log("        top.step();\n");
 		log("      }\n");
 		log("    }\n");
 		log("\n");
+		log("Note that CXXRTL simulations, just like the hardware they are simulating, are\n");
+		log("subject to race conditions. If, in then example above, the user logic would run\n");
+		log("simultaneously with the rising edge of the clock, the design would malfunction.\n");
+		log("\n");
 		log("The following options are supported by this backend:\n");
 		log("\n");
 		log("    -header\n");