From 56509d7938d2451f1503c266c2f18a2d0cafe608 Mon Sep 17 00:00:00 2001
From: George Rennie <georgerennie@gmail.com>
Date: Thu, 28 Nov 2024 23:55:52 +0100
Subject: [PATCH] proc_dff: refactor shrinking logic in optimizers

---
 passes/proc/proc_dff.cc | 47 +++++++++++++++++++----------------------
 1 file changed, 22 insertions(+), 25 deletions(-)

diff --git a/passes/proc/proc_dff.cc b/passes/proc/proc_dff.cc
index 0a6ae1140f4..91a13a2189f 100644
--- a/passes/proc/proc_dff.cc
+++ b/passes/proc/proc_dff.cc
@@ -25,6 +25,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <algorithm>
+#include <type_traits>
 
 USING_YOSYS_NAMESPACE
 PRIVATE_NAMESPACE_BEGIN
@@ -163,13 +164,7 @@ class Dff {
 				return async_rules[i].value[bit] == async_rules[i + 1].value[bit];
 			};
 
-			const bool lsb_optimizable = bit_optimizable(0);
-
-			size_t new_size;
-			for (new_size = 1; new_size < size(); new_size++)
-				if (bit_optimizable(new_size) != lsb_optimizable)
-					break;
-			resize(new_size);
+			const bool lsb_optimizable = shrink_while_matching_values(bit_optimizable);
 
 			if (!lsb_optimizable) {
 				i++;
@@ -212,16 +207,7 @@ class Dff {
 			return foldable;
 		};
 
-		// Work out how many bits from the lsb can be folded into the same
-		// number of rules
-		const size_t lsb_foldable_rules = foldable_rules(0);
-
-		size_t new_size;
-		for (new_size = 1; new_size < size(); new_size++)
-			if (foldable_rules(new_size) != lsb_foldable_rules)
-				break;
-
-		resize(new_size);
+		const size_t lsb_foldable_rules = shrink_while_matching_values(foldable_rules);
 
 		if (lsb_foldable_rules == 0)
 			return;
@@ -253,14 +239,8 @@ class Dff {
 		const auto& [val, trigger] = async_rules.front();
 		log_assert(GetSize(val) > 0);
 
-		const bool lsb_wire = val[0].is_wire();
-
-		size_t new_size;
-		for (new_size = 1; new_size < size(); new_size++)
-			if (val[new_size].is_wire() != lsb_wire)
-				break;
-
-		resize(new_size);
+		const auto bit_is_wire = [&](const size_t i) { return val[i].is_wire(); };
+		shrink_while_matching_values(bit_is_wire);
 	}
 
 	void generate() {
@@ -432,6 +412,23 @@ class Dff {
 			value = value.extract(0, new_size);
 	}
 
+	// Given some function that maps from an index to a value, this resizes
+	// the dff to a range starting at the LSB that all return the same value
+	// from the function as the LSB. This function also returns the value
+	// calculated for the LSB.
+	template <typename F>
+	typename std::result_of<F(size_t)>::type shrink_while_matching_values(F f) {
+		const auto base_val = f(0);
+
+		size_t new_size;
+		for (new_size = 1; new_size < size(); new_size++)
+			if (f(new_size) != base_val)
+				break;
+
+		resize(new_size);
+		return base_val;
+	}
+
 	RTLIL::Process& proc;
 	RTLIL::Module& mod;