avoid dangling procedural else

* Remove unneeded begin/end when the loop is a do/while
* Dangling else check passes through statement attributes (bug fix)
* Dangling else check passes through most control flow structures to reduce unneeded begin/end
* Add test case and update changelog

---------

Co-authored-by: qcorradi <q.corradi22@imperial.ac.uk>
Co-authored-by: Zachary Snow <zach@zachjs.com>

CHANGELOG.md

@@ -13,6 +13,8 @@
 * Fixed unneeded scoping of constant function calls used in type lookups
 * `/*/` is no longer interpreted as a self-closing block comment, e.g.,
   `$display("a"/*/,"b"/* */);` previously printed "ab", but now prints "a"
+* Fixed missing `begin`/`end` when disambiguating procedural branches tagged
+  with an attribute
 * Fixed keywords included in the "1364-2001" and "1364-2001-noconfig"
   `begin_keywords` version specifiers
 

src/Language/SystemVerilog/AST/Stmt.hs

@@ -87,8 +87,9 @@ instance Show Stmt where
         where aStr = if a == Args [] [] then "" else show a
     show (Asgn  o t v e) = printf "%s %s %s%s;" (show v) (show o) tStr (show e)
         where tStr = maybe "" showPad t
-    show (If u c s Null) = printf "%sif (%s)%s"         (showPad u) (show c) (showBranch s)
-    show (If u c s1 s2 ) = printf "%sif (%s)%s\nelse%s" (showPad u) (show c) (showBlockedBranch s1) (showElseBranch s2)
+    show (If u c s1 s2) =
+        -- print the then branch inside a block to avoid dangling else issues
+        printf "%sif (%s)%s%s" (showPad u) (show c) (showBlockedBranch s1) (showElseBranch s2)
     show (While     e s) = printf  "while (%s) %s" (show e) (show s)
     show (RepeatL   e s) = printf "repeat (%s) %s" (show e) (show s)
     show (DoWhile   e s) = printf "do %s while (%s);" (show s) (show e)
@@ -119,28 +120,32 @@ showBranch (Block Seq "" [] stmts@[CommentStmt{}, _]) =
 showBranch block@Block{} = ' ' : show block
 showBranch stmt = '\n' : (indent $ show stmt)
 
+-- add a block around the true branch of an if statement when a dangling else is
+-- possible to avoid any potential ambiguity downstream
 showBlockedBranch :: Stmt -> String
 showBlockedBranch stmt =
     showBranch $
-    if isControl stmt
+    if danglingElse stmt
         then Block Seq "" [] [stmt]
         else stmt
-    where
-        isControl s = case s of
+
+danglingElse :: Stmt -> Bool
+danglingElse s = case s of
     If{} -> True
-            For{} -> True
-            While{} -> True
-            RepeatL{} -> True
-            DoWhile{} -> True
-            Forever{} -> True
-            Foreach{} -> True
-            Timing _ subStmt -> isControl subStmt
-            Block Seq "" [] [CommentStmt{}, subStmt] -> isControl subStmt
+    For   _ _ _ subStmt -> danglingElse subStmt
+    While   _   subStmt -> danglingElse subStmt
+    RepeatL _   subStmt -> danglingElse subStmt
+    Forever     subStmt -> danglingElse subStmt
+    Foreach _ _ subStmt -> danglingElse subStmt
+    Timing  _   subStmt -> danglingElse subStmt
+    StmtAttr  _ subStmt -> danglingElse subStmt
+    Block Seq "" [] [CommentStmt{}, subStmt] -> danglingElse subStmt
     _ -> False
 
 showElseBranch :: Stmt -> String
-showElseBranch stmt@If{} = ' ' : show stmt
-showElseBranch stmt = showBranch stmt
+showElseBranch Null = ""
+showElseBranch stmt@If{} = "\nelse " ++ show stmt
+showElseBranch stmt = "\nelse" ++ showBranch stmt
 
 showShortBranch :: Stmt -> String
 showShortBranch stmt@Asgn{} = ' ' : show stmt

test/core/dangling_else.sv

@@ -0,0 +1,52 @@
+`define TEST(id, ctrl) \
+    always @* #(2 * id) \
+        if (a) \
+            ctrl if (b) \
+                $display("%0d A 1", id); \
+            else \
+                $display("%0d A 2", id); \
+    always @* #(2 * id + 1) \
+        if (a) begin \
+            ctrl if (b) \
+                $display("%0d B 1", id); \
+        end else \
+            $display("%0d B 2", id);
+
+`define NOTHING
+`define ATTRIBUTE (* test *)
+`define FOR for (i = 0; i < 1; i = i + 1)
+`define WHILE while (i < 1)
+`define REPEAT repeat (b)
+`define FOREVER forever #10
+`define TIMING #10
+
+// The other tested constructs are supported in Verilog-2005.
+`ifdef REF
+    `define FOREACH repeat ($bits(i))
+`else
+    `define FOREACH foreach (i[j])
+`endif
+
+module top;
+    reg a, b;
+    integer i;
+
+    `TEST(0, `NOTHING)
+    `TEST(1, `ATTRIBUTE)
+    `TEST(2, `FOR)
+    `TEST(3, `WHILE)
+    `TEST(4, `REPEAT)
+    `TEST(5, `FOREACH)
+    `TEST(6, `FOREVER)
+    `TEST(7, `TIMING)
+
+    initial begin
+        repeat (2) begin
+            #50 a = 0; b = 0;
+            #50 a = 0; b = 1;
+            #50 a = 1; b = 0;
+            #50 a = 1; b = 1;
+        end
+        $finish(0);
+    end
+endmodule

test/core/dangling_else.v

@@ -0,0 +1,2 @@
+`define REF
+`include "dangling_else.sv"