low_overhead_timers.c

// Some very low-overhead timer/counter interfaces:
//
// rdtsc() returns the number of "nominal" processor cycles since the system booted in a 64-bit unsigned integer.
//       For all recent Intel processors, this counter increments at a fixed rate, independent of the actual
//       core clock speed or the energy-saving mode.
// rdtscp() is the same as rdtsc except that it is partially ordered -- it will not execute until all prior
//       instructions in program order have executed.  (See also full_rdtscp)
// full_rdtscp() returns the number of "nominal" processor cycles in a 64-bit unsigned integer and also 
//       modifies its two integer arguments to show the processor socket and processor core that were in use
//       when the call was made.  (Note: the various cores in a chip usually have very similar values for 
//       the TSC, but they are allowed to vary by processor.  This function guarantees that you know exactly
//       which processor the TSC reading came from.)
// get_core_number() uses the RDTSCP instruction, but returns only the core number in an integer variable.
// get_socket_number() uses the RDTSCP instruction, but returns only the socket number in an integer variable.
// rdpmc_instructions() uses a "fixed-function" performance counter to return the count of retired instructions on
//       the current core in the low-order 48 bits of an unsigned 64-bit integer.
// rdpmc_actual_cycles() uses a "fixed-function" performance counter to return the count of actual CPU core cycles
//       executed by the current core.  Core cycles are not accumulated while the processor is in the "HALT" state,
//       which is used when the operating system has no task(s) to run on a processor core.
// rdpmc_reference_cycles() uses a "fixed-function" performance counter to return the count of "reference" (or "nominal")
//       CPU core cycles executed by the current core.  This counts at the same rate as the TSC, but does not count
//       when the core is in the "HALT" state.  If a timed section of code shows a larger change in TSC than in
//       rdpmc_reference_cycles, the processor probably spent some time in a HALT state.
// rdpmc() reads the programmable core performance counter number specified in the input argument.
//		 No error or bounds checking is performed.
//
// get_TSC_frequency() parses the Brand Identification string from the CPUID instruction to get the "nominal"
//       frequency of the processor, which is also the invariant TSC frequency, and returned as a float value in Hz.
//       This can then be used to convert TSC cycles to seconds.
//

extern inline __attribute__((always_inline)) unsigned long rdtsc()
{
   unsigned long a, d;

   __asm__ volatile("rdtsc" : "=a" (a), "=d" (d));

   return (a | (d << 32));
}


extern inline __attribute__((always_inline)) unsigned long rdtscp()
{
   unsigned long a, d, c;

   __asm__ volatile("rdtscp" : "=a" (a), "=d" (d), "=c" (c));

   return (a | (d << 32));
}

extern inline __attribute__((always_inline)) unsigned long full_rdtscp(int *chip, int *core)
{
   unsigned long a, d, c;

   __asm__ volatile("rdtscp" : "=a" (a), "=d" (d), "=c" (c));
	*chip = (c & 0xFFF000UL)>>12;
	*core = c & 0xFFFUL;

   return (a | (d << 32));
}


extern inline __attribute__((always_inline)) int get_core_number()
{
   unsigned long a, d, c;

   __asm__ volatile("rdtscp" : "=a" (a), "=d" (d), "=c" (c));

   return ( c & 0xFFFUL );
}

extern inline __attribute__((always_inline)) int get_socket_number()
{
   unsigned long a, d, c;

   __asm__ volatile("rdtscp" : "=a" (a), "=d" (d), "=c" (c));

   return ( (c & 0xF000UL)>>12 );
}


extern inline __attribute__((always_inline)) unsigned long rdpmc_instructions()
{
   unsigned long a, d, c;

   c = (1UL<<30);
   __asm__ volatile("rdpmc" : "=a" (a), "=d" (d) : "c" (c));

   return (a | (d << 32));
}

extern inline __attribute__((always_inline)) unsigned long rdpmc_actual_cycles()
{
   unsigned long a, d, c;

   c = (1UL<<30)+1;
   __asm__ volatile("rdpmc" : "=a" (a), "=d" (d) : "c" (c));

   return (a | (d << 32));
}

extern inline __attribute__((always_inline)) unsigned long rdpmc_reference_cycles()
{
   unsigned long a, d, c;

   c = (1UL<<30)+2;
   __asm__ volatile("rdpmc" : "=a" (a), "=d" (d) : "c" (c));

   return (a | (d << 32));
}

extern inline __attribute__((always_inline)) unsigned long rdpmc(int c)
{
        unsigned long a, d;

        __asm__ volatile("rdpmc" : "=a" (a), "=d" (d) : "c" (c));

        return (a | (d << 32));
}



#include <stdio.h>
#include <stdlib.h>

// core performance counter width varies by processor
// the width is contained in bits 23:16 of the EAX register
// after executing the CPUID instruction with an initial EAX
// argument of 0x0a (subleaf 0x0 in ECX).
int get_core_counter_width()
{
	unsigned int eax, ebx, ecx, edx;
	unsigned int leaf, subleaf;
	int width;

	leaf = 0x0000000a;
	subleaf = 0x0;
	__asm__ __volatile__ ("cpuid" : \
	  "=a" (eax), "=b" (ebx), "=c" (ecx), "=d" (edx) : "a" (leaf), "c" (subleaf));

	return((eax & 0x00ff0000) >> 16);
}

// Utility routine to compute counter differences taking into account rollover
// when the performance counter width is not known at compile time.  
// Use the "get_counter_width()" function to get the counter width on the
// current system, then use that as the third argument to this function.
// 64-bit counters don't generally roll over, but I added a special case
// for this 
unsigned long corrected_pmc_delta(unsigned long end, unsigned long start, int pmc_width)
{
	unsigned long error_return=0xffffffffffffffff;
	unsigned long result;
	// sanity checks
	if ((pmc_width <= 0) || (pmc_width > 64)) {
		fprintf(stderr,"ERROR: corrected_pmc_delta() called with illegal performance counter width %d\n",pmc_width);
		return(error_return);
	}
	// Due to the specifics of unsigned arithmetic, for pmc_width == sizeof(unsigned long),
	// the simple calculation (end-start) gives the correct delta even if the counter has
	// rolled (leaving end < start).
	if (pmc_width == 64) {
		return (end - start);
	} else {
		// for pmc_width < sizeof(unsigned long), rollover must be detected and corrected explicitly
		if (end >= start) {
			result = end - start;
		} else {
			// I think this works independent of ordering, but this makes the most intuitive sense
			result = (end + 1UL<<pmc_width) - start;
		}
		return (result);
	}
}

// Ugly, ugly, ugly hack to get nominal frequency from CPUID Brand String
// on Intel processors.
// Converted from C++ to C.
// Only works for products that use "GHz" as the frequency designator,
// not "MHz" or "THz".  So far this works on all processors tested.
// Return value is frequency in Hz, so user will need to divide by 1e9
// if GHz is desired....
float get_TSC_frequency()
{
	unsigned int eax, ebx, ecx, edx;
	unsigned int leaf, subleaf;
	unsigned int  intbuf[12];
	char *buffer;
	int i,j,k,base,start,stop,length;
	float freq_GHz;
	float frequency;

	subleaf=0;

	base = 0;
	for (leaf=0x80000002; leaf<0x80000005; leaf++) {
		// printf("DEBUG: leaf = %x\n",leaf);
		__asm__ __volatile__ ("cpuid" : \
		  "=a" (eax), "=b" (ebx), "=c" (ecx), "=d" (edx) : "a" (leaf), "c" (subleaf));

		// printf("leaf = %x, eax = %8.8x, ebx = %8.8x, ecx = %8.8x, edx = %8.8x\n",leaf, eax, ebx, ecx, edx);
		intbuf[base] = eax;
		intbuf[base+1] = ebx;
		intbuf[base+2] = ecx;
		intbuf[base+3] = edx;
		base += 4;
		// printf("  DEBUG: %.8s %.8s %.8s %.8s\n",eax,ebx,ecx,edx);
	}
	// for (base=0; base<12; base++) {
	// 	printf("base[%d] = %8.8x\n",base,intbuf[base]);
	// }
	// printf("444444443333333333222222222211111111110000000000\n");
	// printf("765432109876543210987654321098765432109876543210\n");
	// printf("%48.48s\n",(char *)&intbuf[0]);
	buffer = (char *) &intbuf[0];
	// for (base=0; base<48; base++) {
	// 	printf("%c",buffer[base]);
	// }
	// printf("\n");
	// printf("000000000011111111112222222222333333333344444444\n");
	// printf("012345678901234567890123456789012345678901234567\n");
	// printf("\n");
	// printf("\n");
	// printf("Scanning backwards to try to find the frequency digits....\n");
	for (base=47; base>0; base--){
		if (buffer[base] == 0x7a) {
			// printf("Found z at location %d\n",base);
			if (buffer[base-1] == 0x48) {
				// printf("Found H at location %d\n",base-1);
				if (buffer[base-2] == 0x47) {
					// printf("Found G at location %d\n",base-2);
					// printf(" -- need to extract string now\n");
					i = base-3;
					stop = base-3;
					// printf("begin reverse search at stop character location %d\n",i);
					while(buffer[i] != 0x20) {
						// printf("found a non-blank character %c (%x) at location %d\n",buffer[i],buffer[i],i);
						i--;
					}
					start = i+1;
					length = stop - start + 1;
					k = length+1;
					// for (j=stop; j<start; j--) {
						// printf("DEBUG: buffer[%d] = %c\n",j,buffer[j]);
						// k--;
					// }
					// printf("DEBUG: starting position of frequency string is %d\n",start);
					//
					// note that sscanf will automatically stop when the string changes from digits
					// to non-digits, so I don't need to NULL-terminate the string in the buffer.
					//
					sscanf((char *)&buffer[start],"%f",&freq_GHz);
					// printf("Frequency is %f GHz\n",freq_GHz);
					frequency = 1.0e9*freq_GHz;
					return (frequency);
				}
			}
		}
	}
	return(-1.0);
}