From fdcca7c1650f277791a0c183f11642acef5c26d6 Mon Sep 17 00:00:00 2001
From: "Emil J. Tywoniak" <emil@tywoniak.eu>
Date: Tue, 1 Oct 2024 16:02:41 +0200
Subject: [PATCH] driver: add --hash-seed

---
 kernel/driver.cc | 7 +++++++
 kernel/hashlib.h | 9 ++++++++-
 kernel/yosys.cc  | 1 +
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/driver.cc b/kernel/driver.cc
index 65f090993..c5e0e48b8 100644
--- a/kernel/driver.cc
+++ b/kernel/driver.cc
@@ -18,6 +18,7 @@
  */
 
 #include "kernel/yosys.h"
+#include "kernel/hashlib.h"
 #include "libs/sha1/sha1.h"
 #include "libs/cxxopts/include/cxxopts.hpp"
 #include <iostream>
@@ -276,6 +277,8 @@ int main(int argc, char **argv)
 	options.add_options("developer")
 		("X,trace", "enable tracing of core data structure changes. for debugging")
 		("M,randomize-pointers", "will slightly randomize allocated pointer addresses. for debugging")
+		("hash-seed", "mix up hashing values with <seed>, for extreme optimization and testing",
+			cxxopts::value<uint64_t>(), "<seed>")
 		("A,abort", "will call abort() at the end of the script. for debugging")
 		("x,experimental", "do not print warnings for the experimental <feature>",
 			cxxopts::value<std::vector<std::string>>(), "<feature>")
@@ -427,6 +430,10 @@ int main(int argc, char **argv)
 		if (result.count("infile")) {
 			frontend_files = result["infile"].as<std::vector<std::string>>();
 		}
+		if (result.count("hash-seed")) {
+			int seed = result["hash-seed"].as<uint64_t>();
+			Hasher::set_fudge((Hasher::hash_t)seed);
+		}
 
 		if (log_errfile == NULL) {
 			log_files.push_back(stdout);
diff --git a/kernel/hashlib.h b/kernel/hashlib.h
index 21e4e155b..15c2c1afb 100644
--- a/kernel/hashlib.h
+++ b/kernel/hashlib.h
@@ -81,13 +81,20 @@ class Hasher {
 		// traditionally 5381 is used as starting value for the djb2 hash
 		state = 5381;
 	}
+	static void set_fudge(uint32_t f) {
+		fudge = f;
+	}
 
 	private:
 	uint32_t state;
+	static uint32_t fudge;
 	// The XOR version of DJB2
 	[[nodiscard]]
 	static uint32_t mkhash(uint32_t a, uint32_t b) {
-		return ((a << 5) + a) ^ b;
+		uint32_t hash = ((a << 5) + a) ^ b;
+		if (fudge)
+			hash = fudge ^ mkhash_xorshift(hash);
+		return hash;
 	}
 	public:
 	void hash32(uint32_t i) {
diff --git a/kernel/yosys.cc b/kernel/yosys.cc
index 774c7f37d..9f44c829c 100644
--- a/kernel/yosys.cc
+++ b/kernel/yosys.cc
@@ -92,6 +92,7 @@ std::set<std::string> yosys_input_files, yosys_output_files;
 bool memhasher_active = false;
 uint32_t memhasher_rng = 123456;
 std::vector<void*> memhasher_store;
+uint32_t Hasher::fudge = 0;
 
 std::string yosys_share_dirname;
 std::string yosys_abc_executable;