Fix t_dist_cppstyle Perl performance issue (verilator#4085).

Changes

@@ -24,8 +24,9 @@ Verilator 5.015 devel
 * Support 'let'.
 * Optimize Verilator executable size by refactoring error reporting routines (#4446). [Anthony Donlon]
 * Optimize Verilation runtime pointers and graphs (#4396) (#4397) (#4398). [Krzysztof Bieganski, Antmicro Ltd]
-* Optimize preparations towards multithreading Verilation (#4463) (#4476) (#4477) (#4479).  [Kamil Rakoczy, Antmicro Ltd]
+* Optimize preparations towards multithreaded Verilation (#4291) (#4463) (#4476) (#4477) (#4479).  [Kamil Rakoczy, Antmicro Ltd]
 * Fix Windows filename format, etc (#3873) (#4421). [Anthony Donlon].
+* Fix t_dist_cppstyle Perl performance issue (#4085). [Srinivasan Venkataramanan]
 * Fix using type in parameterized classes without #() (#4281) (#4440). [Anthony Donlon]
 * Fix false INFINITELOOP on forever..mailbox.get() (#4323). [Srinivasan Venkataramanan]
 * Fix data type of condition operation on class objects (#4345) (#4352). [Ryszard Rozak, Antmicro Ltd]

test_regress/t/t_dist_cppstyle.pl

@@ -63,18 +63,19 @@ sub checkPattern {
     my $pattern = shift;
     my $message = shift;
 
-    my $offset = 0;
-    my $buffer = $contents;
-    while ($buffer =~ s/.*?^($pattern)//sm) {
-      my $lineno = offset_to_lineno($contents, $offset + $-[-1]);
-      $offset += $+[1];
-      error("$filename:$lineno: $message");
+    my $lineno = 0;
+    my $buffer;
+    foreach my $line (split(/\n/, $contents . "\n\n")) {
+        ++$lineno;
+        if ($line ne "") {
+            # Don't do whole file at once - see issue #4085
+            # Build a buffer until a newline so we check a block at a time.
+            $buffer .= $line . "\n";
+            next;
+        }
+        if ($buffer =~ s/.*?^($pattern)//sm) {
+            error("$filename:$lineno: $message");
+        }
+        $buffer = "";
     }
 }
-
-sub offset_to_lineno {
-    my $contents = shift;
-    my $offset = shift;
-    my $count = (substr $contents, 0, $offset) =~ tr/\n//;
-    return $count + 1;
-}