From 5fc4e57db717902739207986ce972fcc298035f1 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 9 Nov 2019 16:03:00 -0500
Subject: [PATCH] Eliminates non-portable use of `fls`.

---
 Components/68901/MFP68901.cpp |  7 ++++---
 Components/68901/MFP68901.hpp | 12 ++++++++++++
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/Components/68901/MFP68901.cpp b/Components/68901/MFP68901.cpp
index 3d41cb7e9..2765f5406 100644
--- a/Components/68901/MFP68901.cpp
+++ b/Components/68901/MFP68901.cpp
@@ -301,7 +301,7 @@ void MFP68901::update_interrupts() {
 		if(interrupt_vector_ & 0x8) {
 			// Software interrupt mode: permit only if neither this interrupt
 			// nor a higher interrupt is currently in service.
-			const int highest_bit = 1 << (fls(firing_interrupts) - 1);
+			const int highest_bit = msb16(firing_interrupts);
 			interrupt_line_ = !(interrupt_in_service_ & ~(highest_bit - 1));
 		} else {
 			// Auto-interrupt mode; just signal.
@@ -326,8 +326,7 @@ int MFP68901::acknowledge_interrupt() {
 		return NoAcknowledgement;
 	}
 
-	const int selected = fls(interrupt_pending_ & interrupt_mask_) - 1;
-	const int mask = 1 << selected;
+	const int mask = msb16(interrupt_pending_ & interrupt_mask_);
 
 	// Clear the pending bit regardless.
 	interrupt_pending_ &= ~mask;
@@ -339,6 +338,8 @@ int MFP68901::acknowledge_interrupt() {
 
 	update_interrupts();
 
+	int selected = 0;
+	while((1 << selected) != mask) ++selected;
 	LOG("Interrupt acknowledged: " << selected);
 	return (interrupt_vector_ & 0xf0) | uint8_t(selected);
 }
diff --git a/Components/68901/MFP68901.hpp b/Components/68901/MFP68901.hpp
index 450c97ed3..2933a8eaf 100644
--- a/Components/68901/MFP68901.hpp
+++ b/Components/68901/MFP68901.hpp
@@ -137,6 +137,18 @@ class MFP68901: public ClockingHint::Source {
 		void begin_interrupts(int interrupt);
 		void end_interrupts(int interrupt);
 		void update_interrupts();
+
+		/// @returns the most significant bit set in v, assuming it is one of the least significant 16.
+		inline static int msb16(int v) {
+			// Saturate all bits below the MSB.
+			v |= v >> 1;
+			v |= v >> 2;
+			v |= v >> 4;
+			v |= v >> 8;
+
+			// Throw away lesser bits.
+			return (v+1) >> 1;
+		}
 };
 
 }