From 6ab19ed5bc7c1c6f52d5108bc5de0f3f8246d44d Mon Sep 17 00:00:00 2001
From: Vas Crabb <vas@vastheman.com>
Date: Tue, 24 Oct 2017 17:41:38 +1100
Subject: [PATCH] Added 32- and 64-bit population count utilities.  Only used
 in ARM7 core for now.  Requires -msse4.2 or -mpopcnt to use CPU feature on
 x86, requires -mpopcntb to use CPU feature on POWER.

---
 src/devices/cpu/arm7/arm7ops.cpp | 10 +----
 src/osd/eminline.h               | 63 ++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+), 9 deletions(-)

diff --git a/src/devices/cpu/arm7/arm7ops.cpp b/src/devices/cpu/arm7/arm7ops.cpp
index 8e52635ecea..9f6c998abed 100644
--- a/src/devices/cpu/arm7/arm7ops.cpp
+++ b/src/devices/cpu/arm7/arm7ops.cpp
@@ -263,15 +263,7 @@ int arm7_cpu_device::storeInc(uint32_t pat, uint32_t rbv, int mode)
 int arm7_cpu_device::storeDec(uint32_t pat, uint32_t rbv, int mode)
 {
 	// pre-count the # of registers being stored
-	// TODO[RH]: This is just a popcnt. Consider eminline intrinsic.
-	int result = 0;
-	for (int i = 15; i >= 0; i--)
-	{
-		if ((pat >> i) & 1)
-		{
-			result++;
-		}
-	}
+	int const result = population_count_32(pat & 0x0000ffff);
 
 	// adjust starting address
 	rbv -= (result << 2);
diff --git a/src/osd/eminline.h b/src/osd/eminline.h
index 43316491a08..a42dd24d58a 100644
--- a/src/osd/eminline.h
+++ b/src/osd/eminline.h
@@ -295,6 +295,69 @@ inline uint8_t count_leading_ones(uint32_t val)
 #endif
 
 
+/*-------------------------------------------------
+    population_count_32 - return the number of
+    one bits in a 32-bit value
+-------------------------------------------------*/
+
+#ifndef population_count_32
+inline unsigned population_count_32(uint32_t val)
+{
+#if defined(__GNUC__)
+	// uses CPU feature if available, otherwise falls back to implementation similar to what follows
+	static_assert(sizeof(val) == sizeof(unsigned));
+	return unsigned(__builtin_popcount(static_cast<unsigned>(val)));
+#else
+	// optimal Hamming weight assuing fast 32*32->32
+	constexpr uint32_t m1(0x55555555);
+	constexpr uint32_t m2(0x33333333);
+	constexpr uint32_t m4(0x0f0f0f0f);
+	constexpr uint32_t h01(0x01010101);
+	val -= (val >> 1) & m1;
+	val = (val & m2) + ((val >> 2) & m2);
+	val = (val + (val >> 4)) & m4;
+	return unsigned((val * h01) >> 24);
+#endif
+}
+#endif
+
+
+/*-------------------------------------------------
+    population_count_64 - return the number of
+    one bits in a 64-bit value
+-------------------------------------------------*/
+
+#ifndef population_count_64
+inline unsigned population_count_64(uint64_t val)
+{
+#if defined(__GNUC__)
+	// uses CPU feature if available, otherwise falls back to implementation similar to what follows
+	static_assert(sizeof(val) == sizeof(unsigned long long));
+	return unsigned(__builtin_popcountll(static_cast<unsigned long long>(val)));
+#else
+	// guess that architectures with 64-bit pointers have 64-bit multiplier
+	if (sizeof(void *) >= sizeof(uint64_t))
+	{
+		// optimal Hamming weight assuming fast 64*64->64
+		constexpr uint64_t m1(0x5555555555555555);
+		constexpr uint64_t m2(0x3333333333333333);
+		constexpr uint64_t m4(0x0f0f0f0f0f0f0f0f);
+		constexpr uint64_t h01(0x0101010101010101);
+		val -= (val >> 1) & m1;
+		val = (val & m2) + ((val >> 2) & m2);
+		val = (val + (val >> 4)) & m4;
+		return unsigned((val * h01) >> 56);
+	}
+	else
+	{
+		// fall back to two 32-bit operations to avoid slow multiply
+		return population_count_32(uint32_t(val)) + population_count_32(uint32_t(val >> 32));
+	}
+#endif
+}
+#endif
+
+
 /***************************************************************************
     INLINE TIMING FUNCTIONS
 ***************************************************************************/