X86PageEntry microbenchmark

pagingbench.cpp

#include <cstdint>
#include <chrono>
#include <iostream>
#include <vector>

#ifdef _MSC_VER
#pragma pack (1)
#endif
struct X86_PageEntryBlock{
#ifdef WORDS_BIGENDIAN
	uint32_t		base:20;
	uint32_t		avl:3;
	uint32_t		g:1;
	uint32_t		pat:1;
	uint32_t		d:1;
	uint32_t		a:1;
	uint32_t		pcd:1;
	uint32_t		pwt:1;
	uint32_t		us:1;
	uint32_t		wr:1;
	uint32_t		p:1;
#else
	uint32_t		p:1;
	uint32_t		wr:1;
	uint32_t		us:1;
	uint32_t		pwt:1;
	uint32_t		pcd:1;
	uint32_t		a:1;
	uint32_t		d:1;
	uint32_t		pat:1;
	uint32_t		g:1;
	uint32_t		avl:3;
	uint32_t		base:20;
#endif
} __attribute__((packed));
#ifdef _MSC_VER
#pragma pack ()
#endif

union X86PageEntry {
	uint32_t load = 0;
	X86_PageEntryBlock block;
};

struct X86PageEntryNew {
#ifdef WORDS_BIGENDIAN
	uint32_t    base:20;
	uint32_t    avl:3;
	uint32_t    g:1;
	uint32_t    pat:1;
	uint32_t    d:1;
	uint32_t    a:1;
	uint32_t    pcd:1;
	uint32_t    pwt:1;
	uint32_t    us:1;
	uint32_t    wr:1;
	uint32_t    p:1;
#else
	uint32_t    p:1;
	uint32_t    wr:1;
	uint32_t    us:1;
	uint32_t    pwt:1;
	uint32_t    pcd:1;
	uint32_t    a:1;
	uint32_t    d:1;
	uint32_t    pat:1;
	uint32_t    g:1;
	uint32_t    avl:3;
	uint32_t    base:20;
#endif
	
	constexpr void set(const uint32_t value) {
#ifdef WORDS_BIGENDIAN
		base = (value >> 12) & 0xFFFFF;
		avl = (value >> 9) & 0x7;
		g = (value >> 8) & 0x1;
		pat = (value >> 7) & 0x1;
		d = (value >> 6) & 0x1;
		a = (value >> 5) & 0x1;
		pcd = (value >> 4) & 0x1;
		pwt = (value >> 3) & 0x1;
		us = (value >> 2) & 0x1;
		wr = (value >> 1) & 0x1;
		p = value & 0x1;
#else
		p = value & 0x1;
		wr = (value >> 1) & 0x1;
		us = (value >> 2) & 0x1;
		pwt = (value >> 3) & 0x1;
		pcd = (value >> 4) & 0x1;
		a = (value >> 5) & 0x1;
		d = (value >> 6) & 0x1;
		pat = (value >> 7) & 0x1;
		g = (value >> 8) & 0x1;
		avl = (value >> 9) & 0x7;
		base = (value >> 12) & 0xFFFFF;
#endif
	}
	
	constexpr uint32_t get() const {
		uint32_t value = 0;
#ifdef WORDS_BIGENDIAN
		value |= (base << 12);
		value |= (avl << 9);
		value |= (g << 8);
		value |= (pat << 7);
		value |= (d << 6);
		value |= (a << 5);
		value |= (pcd << 4);
		value |= (pwt << 3);
		value |= (us << 2);
		value |= (wr << 1);
		value |= p;
#else
		value |= p;
		value |= (wr << 1);
		value |= (us << 2);
		value |= (pwt << 3);
		value |= (pcd << 4);
		value |= (a << 5);
		value |= (d << 6);
		value |= (pat << 7);
		value |= (g << 8);
		value |= (avl << 9);
		value |= (base << 12);
#endif
		return value;
	}
};


int main() {
	const std::size_t numIterations = 100000000;
	std::vector<uint32_t> values(numIterations);
	for (std::size_t i = 0; i < numIterations; ++i) {
		values[i] = static_cast<uint32_t>(i);
	}
	
	auto start = std::chrono::high_resolution_clock::now();
	
	// Test old union/struct
	X86PageEntry oldEntry;
	for (std::size_t i = 0; i < numIterations; ++i) {
		oldEntry.load = values[i];
		volatile uint32_t tmp = oldEntry.load;  // Prevent optimization
	}
	
	auto end = std::chrono::high_resolution_clock::now();
	auto durationOld = std::chrono::duration_cast<std::chrono::microseconds>(end - start).count();
	std::cout << "Old struct/union duration: " << durationOld << " microseconds\n";
	
	start = std::chrono::high_resolution_clock::now();
	
	// Test new struct
	X86PageEntryNew newEntry;
	for (std::size_t i = 0; i < numIterations; ++i) {
		newEntry.set(values[i]);
		volatile uint32_t tmp = newEntry.get();  // Prevent optimization
	}
	
	end = std::chrono::high_resolution_clock::now();
	auto durationNew = std::chrono::duration_cast<std::chrono::microseconds>(end - start).count();
	std::cout << "New struct duration: " << durationNew << " microseconds\n";
	
	return 0;
}