From 4a057b3c447668b3c8e18b1f304de22e06762cca Mon Sep 17 00:00:00 2001
From: George Rennie <georgerennie@gmail.com>
Date: Thu, 21 Nov 2024 20:51:05 +0100
Subject: [PATCH] read_rtlil: warn on assigns after switches in case rules

---
 frontends/rtlil/rtlil_frontend.cc |  5 +++++
 frontends/rtlil/rtlil_frontend.h  |  1 +
 frontends/rtlil/rtlil_parser.y    | 10 ++++++++++
 3 files changed, 16 insertions(+)

diff --git a/frontends/rtlil/rtlil_frontend.cc b/frontends/rtlil/rtlil_frontend.cc
index 170ed560f..2c1910d13 100644
--- a/frontends/rtlil/rtlil_frontend.cc
+++ b/frontends/rtlil/rtlil_frontend.cc
@@ -31,6 +31,11 @@ void rtlil_frontend_yyerror(char const *s)
 	YOSYS_NAMESPACE_PREFIX log_error("Parser error in line %d: %s\n", rtlil_frontend_yyget_lineno(), s);
 }
 
+void rtlil_frontend_yywarning(char const *s)
+{
+	YOSYS_NAMESPACE_PREFIX log_warning("In line %d: %s\n", rtlil_frontend_yyget_lineno(), s);
+}
+
 YOSYS_NAMESPACE_BEGIN
 
 struct RTLILFrontend : public Frontend {
diff --git a/frontends/rtlil/rtlil_frontend.h b/frontends/rtlil/rtlil_frontend.h
index 189260605..31cfb80b4 100644
--- a/frontends/rtlil/rtlil_frontend.h
+++ b/frontends/rtlil/rtlil_frontend.h
@@ -42,6 +42,7 @@ YOSYS_NAMESPACE_END
 extern int rtlil_frontend_yydebug;
 int rtlil_frontend_yylex(void);
 void rtlil_frontend_yyerror(char const *s);
+void rtlil_frontend_yywarning(char const *s);
 void rtlil_frontend_yyrestart(FILE *f);
 int rtlil_frontend_yyparse(void);
 int rtlil_frontend_yylex_destroy(void);
diff --git a/frontends/rtlil/rtlil_parser.y b/frontends/rtlil/rtlil_parser.y
index deb37d9a6..fc7615364 100644
--- a/frontends/rtlil/rtlil_parser.y
+++ b/frontends/rtlil/rtlil_parser.y
@@ -344,6 +344,16 @@ assign_stmt:
 	TOK_ASSIGN sigspec sigspec EOL {
 		if (attrbuf.size() != 0)
 			rtlil_frontend_yyerror("dangling attribute");
+
+		// See https://github.com/YosysHQ/yosys/pull/4765 for discussion on this
+		// warning
+		if (!switch_stack.back()->empty()) {
+			rtlil_frontend_yywarning(
+				"case rule assign statements after switch statements may cause unexpected behaviour. "
+				"The assign statement is reordered to come before all switch statements."
+			);
+		}
+
 		case_stack.back()->actions.push_back(RTLIL::SigSig(*$2, *$3));
 		delete $2;
 		delete $3;