Functions & Macro-Wrapper using RDTSC instruction

[SergeyKostrov]

Hi everybody,

Here are a couple of functions &macro-wrapper to measure time intervals with as better as possible accuracyusing RDTSC instruction:

//*** Version 1 - Win32 ***//

template < class T > inline _RTdeclspec_naked RTuint64 HrtClock( RTvoid )
{
//printf( "[ HrtClock ]\\n" );

_asm
{
rdtsc
ret
}
};

//*** Version 2 - Win32 ***//

_RTdeclspec_naked RTuint64 HrtClock( void )
{
//printf( "[ HrtClock ]\\n" );

_asm
{
rdtsc
ret
}
}

//*** Version 3 - Win32 ***//

void HrtClock( RTuint64 *puiClock )
{
//printf( "[ HrtClock ]\\n" );

RTuint64 v;
_asm
{
push eax
push edx
_emit 0x0f
_emit 0x31
mov dword ptr v, eax
mov dword ptr v+4, edx
pop edx
pop eax
}
*puiClock = v;
}

//*** Version 4 - Linux32 ( with GCC or MinGW ) ***//

RTclock_t HrtClock( RTvoid )
{
//printf( "[ HrtClock ]\\n" );

RTclock_t ctValue;
__asm__
(
"rdtsc;"
"mov %%edx, %%ecx;" : "=a" ( ctValue )
);
return ( RTclock_t )ctValue;
};

//*** Version 5 - Linux32 ( with GCC or MinGW ) ***//

#define HrtClock( Value ) \\
(\\
{\\
__asm__ __volatile__\\
(\\
".byte 0x0f; .byte 0x31" \\
: "=A" ( Value ) \\
);\\
}\\
)\\

[TimP]

For several years, Microsoft and Intel compilers have a built-in macro __rdtsc which handles all the platforms.

[SergeyKostrov]

It is declared in 'intrin.h' header file and actually this is an intrinsic function ( not a macro ):

...
unsigned __int64 __rdtsc( void );
...

[SergeyKostrov]

This is a follow up because I've detectedsomeissues with Version 4 example for Linux32 with GCC or
MinGW C/C++ compilers

This is what Intel's documentation says about RDTSC instruction in an 'Instruction Set Reference' Volume 2B:

...
Loads the current value of the processor's time-stamp counter ( a 64-bit MSR ) into the EDX:EAX
registers.
...

Found issues are as follows:

1. On a 32-bit platforms 'clock_t' type is declared as a32-bit type 'long' ( 4 bytes ) and it is verified with
all 32-bitC/C++ compilers I use:

...
typedef long clock_t;
...

2. "=A" has to be used instead of "=a"

3. A pair of registers 'edx' and 'ecx'is used instead of 'edx' and 'eax', and if you change:

...
"mov %%edx, %%ecx;" : "=A" ( ctValue )
...
to
...
"mov %%edx, %%eax;" : "=A" ( ctValue )
...

it doesn't work! So, theregisters 'edx' and 'ecx'have to beused anyway

4. Finally, a working C/C++ code looks like this:

inline RTuint64 HrtClock( RTvoid )
{
RTuint64 uiValue;
__asm__
(
"rdtsc;"
"mov %%edx, %%ecx;" : "=A" ( uiValue )
);
return ( RTuint64 )uiValue;
}

[SergeyKostrov]

Here are some assembler codes generated by a 'g++' C/C++ compiler.

The left example doesn't work. The right example works.

... ...
.stabn 68,0,284,LM5832-__Z8HrtClockv .stabn 68,0,284,LM5832-__Z8HrtClockv
LM5832: LM5832:
/APP /APP
rdtsc;mov %edx, %eax;rdtsc;mov %edx, %ecx;
/NO_APP /NO_APP
movl%eax, -8(%ebp) movl%eax, -8(%ebp)
movl%edx, -4(%ebp) movl%edx, -4(%ebp)
.stabn 68,0,287,LM5833-__Z8HrtClockv .stabn 68,0,287,LM5833-__Z8HrtClockv
LM5833: LM5833:
movl-8(%ebp), %eax movl-8(%ebp), %eax
movl-4(%ebp), %edx movl-4(%ebp), %edx
LBE895: LBE895:
LBE894: LBE894:
.stabn 68,0,288,LM5834-__Z8HrtClockv .stabn 68,0,288,LM5834-__Z8HrtClockv
LM5834: LM5834:
leave leave
ret ret
... ...

It is not clear why the left example doesn't work and it always returns 0.

[A_T_Intel]

It is worth mentioning the problem with RDTSC and out of order execution:

http://en.wikipedia.org/wiki/Time_Stamp_Counter

Starting with the Pentium Pro, Intel processors have supported out-of-order execution, where instructions are not necessarily performed in the order they appear in the executable. This can cause RDTSC to be executed later than expected, producing a misleading cycle count.^[3] This problem can be solved by executing a serializing instruction, such as CPUID, to force every preceding instruction to complete before allowing the program to continue, or by using the RDTSCP instruction, which is a serializing variant of the RDTSC instruction (starting from Core i7^[4] and starting from AMD Athlon 64 X2 CPUs with AM2 Socket (Windsor & Brisbane)).

[SergeyKostrov]

Thank you, Perry.

Unfortunately, I can'tuse RDTSCP instruction.

I had a problem withincorrect values returned from 'HrtClock' function that uses RTDSC instruction.It
was related to a declaration of return value as'clock_t' ( long \ 32-bit )instead of 'uint64' ( 64-bit ).

I wanted to inform that possiblythere is a bug in the MinGW C/C++ compiler related to '%%edx, %%ecx' declaration.

Best regards,
Sergey

[TimP]

I've encountered no difficulty with rdtsc under mingw:

[bash]unsigned long long int rdtsc( )
{
[/bash]

....

[bash]#elif defined(__GNUC__)
#if defined i386
   long long a;
   asm volatile("rdtsc":"=A" (a));
   return a;
#elif defined __x86_64
   unsigned int _hi,_lo;
   asm volatile("rdtsc":"=a"(_lo),"=d"(_hi));
   return ((unsigned long long int)_hi << 32) | _lo;

.... 
[/bash]

Evidently, tick count differences should be taken in 64-bit unsigned arithmetic.
The left shift operator displays correctly if you go to edit mode.

[SergeyKostrov]

Thank you, Tim. I'll try your example of using RDTSC instruction.

>>...Evidently, tick count differences should be taken in 64-bit unsigned arithmetic...

Always! When I tried to use 'clock_t', type'long' 32-bit on a32-bit platform, I had sometimes absolutely
incorrect values returned from my'HrtClock' function.

Best regards,
Sergey

[TimP]

Yes, 32-bit differences (taken from the low order 32 bits) will work until the counter has a carry into the high order bits, which takes very few seconds.

[SergeyKostrov]

I decided to use

__asm__ volatile( "rdtsc;" : "=A" ( uiValue ) ); ( version B )

instead of

__asm__( "rdtsc;"
"mov %%edx, %%ecx;" : "=A" ( uiValue ) );( version A )

because 'version B' doesn't have 'mov %%edx, %%ecx'.

In terms of assembler codes compiled by a MinGW C/C++ compiler I haven't found any problems
with theversion B:

...
.stabn 68,0,244,LM2990-__Z8HrtClockv
LM2990:
pushl %ebp
movl %esp, %ebp
subl $8, %esp
LBB548:
LBB549:
.stabn 68,0,258,LM2991-__Z8HrtClockv
LM2991:
/APP
rdtsc;
/NO_APP
movl %eax, -8(%ebp)
movl %edx, -4(%ebp)
.stabn 68,0,260,LM2992-__Z8HrtClockv
LM2992:
movl -8(%ebp), %eax
movl -4(%ebp), %edx
LBE549:
LBE548:
.stabn 68,0,261,LM2993-__Z8HrtClockv
LM2993:
leave
ret
.stabs "uiValue:(93,22)",128,0,249,-8
...

and it works. No runtimeissues or problems detected so far.

Best regards,
Sergey

[SergeyKostrov]

This is a follow up. All three widely used Visual Studios 2005, 2008 and 2010 have a declaration of '__rdtsc' intrinsic function
in 'intrin.h' header file:

...
__MACHINEI( unsigned __int64 __rdtsc( void ) )
...