Use rdtsc function in Assembly

Question

I am trying to profile an x86 Assembly program using Ubuntu 12.04. I'd like to use the rdtsc function. The problem is, according to a comment, that I should get the number of cycles in rdx but with the following code I get a too high number:

SECTION .bss

SECTION .dat

SECTION .text


global main         

main:           
nop

cpuid
rdtsc
shl rdx, 32
or rdx, rax
mov r8, rdx

xor esi,esi
mov esi,19        ; instructions to be monitored


cpuid
rdtsc
shl rdx, 32
or rdx, rax
sub rdx, r8

Running it in a debugger I get the following results on registers after the sub instruction:

rax     0xd88102bc
rbx     0x0
rcx     0xf0
rdx     0x44f3914a0
rsi     0x13
rdi     0x1
rbp     0x0
rsp     0x7fffffffdf38
r8      0x11828947ee1c

I can't figure out why the number of cycles in rdx is so high for so simple instructions. Is the right number in rcx? Isn't it too high too?

Thanks in advance

Answer 1

I'm not sure what's happening, but when you're calling C functions from assembler you should usually prefix them with a leading underscore, for example call _clock . This is because the C compiler will add this prefix to all functions it generates.

Additionally as you're on a 64-bit architecture the 64-bit result should end up in rax , you should ensure you're looking at that, not eax and ebx .

Finally I'd suggest rather than using clock you should use the assembler instruction rdtsc . This will return a 64-bit result in edx:eax . It's relative rather than absolute and is measured in cycles rather than some fractions of seconds, but it should be exactly what you need for profiling.

Example:

cpuid
rdtsc
shl rdx, 32
or rdx, rax
mov r8, rdx
<expensive assembler code>
cpuid
rdtsc
shl rdx, 32
or rdx, rax
sub rdx, r8

This will leave the number of ticks that elapsed in rdx . The cpuid instructions are to prevent the processor from reordering instructions around the profiling points.