From 889575736485854fa87fb247da83a3bc12e9dd41 Mon Sep 17 00:00:00 2001
From: Robert O'Callahan <rocallahan@google.com>
Date: Mon, 13 Oct 2025 00:44:15 +0000
Subject: [PATCH] Ensure that `new_id(_suffix)()` cannot create collisions with
 existing `IdString`s.

---
 kernel/rtlil.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index 2f7319d4a..fb0849902 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -23,6 +23,7 @@
 #include "kernel/yosys_common.h"
 #include "kernel/yosys.h"
 
+#include <charconv>
 #include <string_view>
 #include <unordered_map>
 
@@ -200,6 +201,16 @@ struct RTLIL::IdString
 			if ((unsigned)ch <= (unsigned)' ')
 				log_error("Found control character or space (0x%02x) in string '%s' which is not allowed in RTLIL identifiers\n", ch, std::string(p).c_str());
 
+		if (p.substr(0, 6) == "$auto$") {
+			// Ensure new_id(_suffix) will not create collisions.
+			size_t autoidx_pos = p.find_last_of('$');
+			int p_autoidx;
+			std::string_view v = p.substr(autoidx_pos + 1);
+			if (std::from_chars(v.begin(), v.end(), p_autoidx).ec == std::errc()) {
+				autoidx = std::max(autoidx, p_autoidx + 1);
+			}
+		}
+
 	#ifndef YOSYS_NO_IDS_REFCNT
 		if (global_free_idx_list_.empty()) {
 			log_assert(global_id_storage_.size() < 0x40000000);