How to Benchmark Code Execution Times on Intel IA-32 and IA-64 ...

<strong>How</strong> <strong>to</strong> <strong>Benchmark</strong> <strong>Code</strong> <strong>Execution</strong> <strong>Times</strong> on Intel ® IA-32
and IA-64 Instruction Set Architectures
The RDTSCP instruction reads the timestamp register for the second time and
guarantees that the execution of all the code we wanted <strong>to</strong> measure is completed.
The two “mov” instructions coming afterwards s<strong>to</strong>re the edx and eax registers
values in<strong>to</strong> memory. Both instructions are guaranteed <strong>to</strong> be executed after RDTSC
(i.e., they don’t corrupt the measure) since there is a logical dependency between
RDTSCP and the register edx and eax (RDTSCP is writing those registers and the
CPU is obliged <strong>to</strong> wait for RDTSCP <strong>to</strong> be finished before executing the two “mov”).
Finally a CPUID call guarantees that a barrier is implemented again so that it is
impossible that any instruction coming afterwards is executed before CPUID itself
(and logically also before RDTSCP).
With this method we avoid <strong>to</strong> call a CPUID instruction in between the reads of the
real-time registers (avoiding all the problems described in Section 3.1).
3.2.2 Evaluation of the Improved <strong>Benchmark</strong>ing Method
In reference <strong>to</strong> the code presented in Section 3.1, we replace the previous
benchmarking method with the new one, i.e., we replace ln19 <strong>to</strong> ln54 in the
Appendix with the following code:
asm volatile ("CPUID\n\t"
"RDTSC\n\t"
"mov %%edx, %0\n\t"
"mov %%eax, %1\n\t": "=r" (cycles_high), "=r" (cycles_low)::
"%rax", "%rbx", "%rcx", "%rdx");
asm volatile("RDTSCP\n\t"
"mov %%edx, %0\n\t"
"mov %%eax, %1\n\t"
"CPUID\n\t": "=r" (cycles_high1), "=r" (cycles_low1):: "%rax",
"%rbx", "%rcx", "%rdx");
asm volatile ("CPUID\n\t"
"RDTSC\n\t"
"mov %%edx, %0\n\t"
"mov %%eax, %1\n\t": "=r" (cycles_high), "=r" (cycles_low)::
"%rax", "%rbx", "%rcx", "%rdx");
asm volatile("RDTSCP\n\t"
"mov %%edx, %0\n\t"
"mov %%eax, %1\n\t"
"CPUID\n\t": "=r" (cycles_high1), "=r" (cycles_low1):: "%rax",
"%rbx", "%rcx", "%rdx");
for (j=0; j