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Use POPCNT, if available, to count the number of set bits 

Replace the current implementation of bitcount() with a one that uses the
POPCNT instruction available in most newer CPUs. Also, replace the basic
implementation with a one based on a lookup table, which has much better
performance than the old one.
This commit is contained in:
Daniel Kamil Kozar 2016-10-31 00:13:19 +01:00
parent b266d6c2d5
commit bc101f4151
1 changed files with 63 additions and 6 deletions
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69
src/util/util.c
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@ -195,15 +195,72 @@ int bitscanforward(int source)
#endif
}
#if defined(__GNUC__) && (defined(__x86_64__) || defined(__i386__))
#include <cpuid.h>
#define OpenRCT2_POPCNT_GNUC
#elif defined(_MSC_VER) && (_MSC_VER >= 1500) && (defined(_M_X64) || defined(_M_IX86)) // VS2008
#include <intrin.h>
#define OpenRCT2_POPCNT_MSVC
#endif
static int bitcount_available(void)
{
// POPCNT support is declared as the 23rd bit of ECX with CPUID(EAX = 1).
#if defined(OpenRCT2_POPCNT_GNUC)
// we could use __builtin_cpu_supports, but it requires runtime support from
// the compiler's library, which clang doesn't have yet.
unsigned int eax, ebx, ecx = 0, edx; // avoid "maybe uninitialized"
__get_cpuid(1, &eax, &ebx, &ecx, &edx);
return (ecx & (1 << 23));
#elif defined(OpenRCT2_POPCNT_MSVC)
int regs[4];
__cpuid(regs, 1);
return (regs[2] & (1 << 23));
#else
return 0;
#endif
}
static int bitcount_popcnt(int source)
{
#if defined(OpenRCT2_POPCNT_GNUC)
// use asm directly in order to actually emit the instruction : using
// __builtin_popcount results in an extra call to a library function.
int rv;
asm volatile ("popcnt %1,%0" : "=r"(rv) : "rm"(source) : "cc");
return rv;
#elif defined(OpenRCT2_POPCNT_MSVC)
return __popcnt(source);
#else
assert(false && "bitcount_popcnt() called, without support compiled in");
return INT_MAX;
#endif
}
static int bitcount_lut(int source)
{
// https://graphics.stanford.edu/~seander/bithacks.html
static const unsigned char BitsSetTable256[256] =
{
#define B2(n) n, n+1, n+1, n+2
#define B4(n) B2(n), B2(n+1), B2(n+1), B2(n+2)
#define B6(n) B4(n), B4(n+1), B4(n+1), B4(n+2)
B6(0), B6(1), B6(1), B6(2)
};
return BitsSetTable256[source & 0xff] +
BitsSetTable256[(source >> 8) & 0xff] +
BitsSetTable256[(source >> 16) & 0xff] +
BitsSetTable256[source >> 24];
}
int bitcount(int source)
{
int result = 0;
for (int i = 0; i < 32; i++) {
if (source & (1u << i)) {
result++;
}
static int(*bitcount_fn)(int);
if(bitcount_fn == 0)
{
bitcount_fn = bitcount_available() ? bitcount_popcnt : bitcount_lut;
}
return result;
return bitcount_fn(source);
}
bool strequals(const char *a, const char *b, int length, bool caseInsensitive)