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Handling of integer with values > 2^32 on Sparc 32 bits

I coded a little program that measures the time spent into a loop (via an inline Sparc assembly code snippet).

Everything is right until I set number of iterations above roughly 4.0+9 (above 2^32).

Here's the code snippet :

#include <stdio.h>
#include <sys/time.h>
#include <unistd.h>
#include <math.h>
#include <stdint.h>

int main (int argc, char *argv[])
{
  // For indices
  int i;
  // Set the number of executions
  int nRunning = atoi(argv[1]);
  // Set the sums
  double avgSum = 0.0;
  double stdSum = 0.0;
  // Average of execution time
  double averageRuntime = 0.0;
  // Standard deviation of execution time
  double deviationRuntime = 0.0;

  // Init sum
  unsigned long long int sum = 0;
  // Number of iterations
  unsigned long long int nLoop = 4000000000ULL;
  //uint64_t nLoop = 4000000000;
    
  // DEBUG
  printf("sizeof(unsigned long long int) = %zu\n",sizeof(unsigned long long int));
  printf("sizeof(unsigned long int) = %zu\n",sizeof(unsigned long int));

  // Time intervals
  struct timeval tv1, tv2;
  double diff;

  // Loop for multiple executions
  for (i=0; i<nRunning; i++)
  {
   // Start time
   gettimeofday (&tv1, NULL);

   // Loop with Sparc assembly into C source
   asm volatile ("clr %%g1\n\t" 
                 "clr %%g2\n\t" 
                 "mov %1, %%g1\n" // %1 = input parameter
                 "loop:\n\t" 
                 "add %%g2, 1, %%g2\n\t" 
                 "subcc %%g1, 1, %%g1\n\t" 
                 "bne loop\n\t" 
                 "nop\n\t" 
                 "mov %%g2, %0\n" // %0 = output parameter
                 : "=r" (sum)     // output
                 : "r" (nLoop)    // input
                 : "g1", "g2");   // clobbers

   // End time
   gettimeofday (&tv2, NULL);

   // Compute runtime for loop
   diff = (tv2.tv_sec - tv1.tv_sec) * 1000000ULL + (tv2.tv_usec - tv1.tv_usec);
   
   // Summing diff time
   avgSum += diff;
   stdSum += (diff*diff);

   // DEBUG
   printf("diff = %e\n", diff);
   printf("avgSum = %e\n", avgSum);

  }
  // Compute final averageRuntime   
  averageRuntime = avgSum/nRunning;

  // Compute standard deviation
  deviationRuntime = sqrt(stdSum/nRunning-averageRuntime*averageRuntime);

  // Print results
  printf("(Average Elapsed time, Standard deviation) = %e usec  %e usec\n", averageRuntime, deviationRuntime);
  // Print sum from assembly loop
  printf("Sum = %llu\n", sum);

For example, with nLoop < 2^32, I get correct values for diff, avgSum and stdSum. Indeed, the printf ,with nLoop = 4.0e+9, gives :

sizeof(unsigned long long int) = 8
sizeof(unsigned long int) = 4
diff = 9.617167e+06
avgSum = 9.617167e+06
diff = 9.499878e+06
avgSum = 1.911704e+07
(Average Elapsed time, Standard deviation) = 9.558522e+06 usec  5.864450e+04 usec
Sum = 4000000000

The code is compiled on Debian Sparc 32 bits Etch with gcc 4.1.2.

Unfortunately, if I take for example nLoop = 5.0e+9, I get small and incorrect values for measured times; here's the printf output in this case :

sizeof(unsigned long long int) = 8
sizeof(unsigned long int) = 4
diff = 5.800000e+01
avgSum = 5.800000e+01
diff = 4.000000e+00
avgSum = 6.200000e+01
(Average Elapsed time, Standard deviation) = 3.100000e+01 usec  2.700000e+01 usec
Sum = 5000000000

I don't know where the issue could come from, I have done other tests using uint64_t but without success.

Maybe the problem is that I handle large integers (> 2^32) with 32 bits OS or it may be the assembly inline code which doesn't support 8 bytes integer.

Update 1

Following the advice of @AndrewHenle, I took the same code but instead of inline Sparc Assembly snippet, I have just put a simple loop.

Here's the program with the simple loop which has got nLoop = 5.0e+9 (see the line "unsigned long long int nLoop = 5000000000ULL;", so above the limit 2^32-1 :

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <unistd.h>
#include <math.h>
#include <stdint.h>

int main (int argc, char *argv[])
{
  // For indices of nRunning
  int i;
  // For indices of nRunning
  unsigned long long int j;
  // Set the number of executions
  int nRunning = atoi(argv[1]);
  // Set the sums
  unsigned long long int avgSum = 0;
  unsigned long long int stdSum = 0;
  // Average of execution time
  double averageRuntime = 0.0;
  // Standard deviation of execution time
  double deviationRuntime = 0.0;

  // Init sum
  unsigned long long int sum;
  // Number of iterations
  unsigned long long int nLoop = 5000000000ULL;
    
  // DEBUG
  printf("sizeof(unsigned long long int) = %zu\n",sizeof(unsigned long long int));
  printf("sizeof(unsigned long int) = %zu\n",sizeof(unsigned long int));

  // Time intervals
  struct timeval tv1, tv2;
  unsigned long long int diff;

  // Loop for multiple executions
  for (i=0; i<nRunning; i++)
  {
   // Reset sum
   sum = 0;

   // Start time
   gettimeofday (&tv1, NULL);

   // Loop with Sparc assembly into C source
   /* asm volatile ("clr %%g1\n\t" 
                 "clr %%g2\n\t" 
                 "mov %1, %%g1\n" // %1 = input parameter
         "loop:\n\t" 
         "add %%g2, 1, %%g2\n\t" 
         "subcc %%g1, 1, %%g1\n\t" 
         "bne loop\n\t" 
         "nop\n\t" 
         "mov %%g2, %0\n" // %0 = output parameter
         : "=r" (sum)     // output
         : "r" (nLoop)    // input
         : "g1", "g2");   // clobbers
   */

   // Classic loop
   for (j=0; j<nLoop; j++)
      sum ++;

   // End time
   gettimeofday (&tv2, NULL);

   // Compute runtime for loop
   diff = (unsigned long long int) ((tv2.tv_sec - tv1.tv_sec) * 1000000 + (tv2.tv_usec - tv1.tv_usec));
   
   // Summing diff time
   avgSum += diff;
   stdSum += (diff*diff);

   // DEBUG
   printf("diff = %llu\n", diff);
   printf("avgSum = %llu\n", avgSum);
   printf("stdSum = %llu\n", stdSum);
   // Print sum from assembly loop
   printf("Sum = %llu\n", sum);

  }
  // Compute final averageRuntime   
  averageRuntime = avgSum/nRunning;

  // Compute standard deviation
  deviationRuntime = sqrt(stdSum/nRunning-averageRuntime*averageRuntime);

  // Print results
  printf("(Average Elapsed time, Standard deviation) = %e usec  %e usec\n", averageRuntime, deviationRuntime);

  return 0;

}

This code snippet is working fine, i.e the variable sum is printed as (see "printf("Sum = %llu\n", sum)") :

Sum = 5000000000

So the problem comes from the version with Sparc Assembly block.

I suspect, in this assembly code, the line "mov %1, %%g1\n" // %1 = input parameter to badly store nLoop into %g1 register (I think that %g1 is a 32 bits register, so can't store values above 2^32-1).

However, the output parameter (variable sum) at the line :

"mov %%g2, %0\n" // %0 = output parameter

is above the limit since it is equal to 5000000000.

I attach the vimdiff between the version with Assembly loop and without it :

figure

On the left, program With Assembly, on the right, Without Assembly (just a simple loop instead

I remind you my issue is that, for nLoop > 2^32-1 and with Assembly loop, I get a valid sum parameter at the end of execution but not valid (too short) average and standard deviation times (spent into loop); here's an example of output with nLoop = 5000000000ULL :

sizeof(unsigned long long int) = 8
sizeof(unsigned long int) = 4
diff = 17
avgSum = 17
stdSum = 289
Sum = 5000000000
diff = 4
avgSum = 21
stdSum = 305
Sum = 5000000000
(Average Elapsed time, Standard deviation) = 1.000000e+01 usec  7.211103e+00 usec

With taking nLoop = 4.0e+9, i.e nLoop = 4000000000ULL, there is no problem, the times values are valid.

Update 2

I am searching more deeply by generating Assembly code. The version with nLoop = 4000000000 (4.0e+9) is below :

    .file   "loop-WITH-asm-inline-4-Billions.c"
    .section    ".rodata"
    .align 8
.LLC1:
    .asciz  "sizeof(unsigned long long int) = %zu\n"
    .align 8
.LLC2:
    .asciz  "sizeof(unsigned long int) = %zu\n"
    .align 8
.LLC3:
    .asciz  "diff = %llu\n"
    .align 8
.LLC4:
    .asciz  "avgSum = %llu\n"
    .align 8
.LLC5:
    .asciz  "stdSum = %llu\n"
    .align 8
.LLC6:
    .asciz  "Sum = %llu\n"
    .global __udivdi3
    .global __cmpdi2
    .global __floatdidf
    .align 8
.LLC7:
    .asciz  "(Average Elapsed time, Standard deviation) = %e usec  %e usec\n"
    .align 8
.LLC0:
    .long   0
    .long   0
    .section    ".text"
    .align 4
    .global main
    .type   main, #function
    .proc   04
main:
    save    %sp, -248, %sp
    st  %i0, [%fp+68]
    st  %i1, [%fp+72]
    ld  [%fp+72], %g1
    add %g1, 4, %g1
    ld  [%g1], %g1
    mov %g1, %o0
    call    atoi, 0
     nop
    mov %o0, %g1
    st  %g1, [%fp-68]
    st  %g0, [%fp-64]
    st  %g0, [%fp-60]
    st  %g0, [%fp-56]
    st  %g0, [%fp-52]
    sethi   %hi(.LLC0), %g1
    or  %g1, %lo(.LLC0), %g1
    ldd [%g1], %f8
    std %f8, [%fp-48]
    sethi   %hi(.LLC0), %g1
    or  %g1, %lo(.LLC0), %g1
    ldd [%g1], %f8
    std %f8, [%fp-40]
    mov 0, %g2
    sethi   %hi(4000000000), %g3
    std %g2, [%fp-24]
    sethi   %hi(.LLC1), %g1
    or  %g1, %lo(.LLC1), %o0
    mov 8, %o1
    call    printf, 0
     nop
    sethi   %hi(.LLC2), %g1
    or  %g1, %lo(.LLC2), %o0
    mov 4, %o1
    call    printf, 0
     nop
    st  %g0, [%fp-84]
    b   .LL2
     nop
.LL3:
    st  %g0, [%fp-32]
    st  %g0, [%fp-28]
    add %fp, -92, %g1
    mov %g1, %o0
    mov 0, %o1
    call    gettimeofday, 0
     nop
    ldd [%fp-24], %o4
    clr %g1
    clr %g2
    mov %o4, %g1
loop:
    add %g2, 1, %g2
    subcc %g1, 1, %g1
    bne loop
    nop
    mov %g2, %o4

    std %o4, [%fp-32]
    add %fp, -100, %g1
    mov %g1, %o0
    mov 0, %o1
    call    gettimeofday, 0
     nop
    ld  [%fp-100], %g2
    ld  [%fp-92], %g1
    sub %g2, %g1, %g2
    sethi   %hi(999424), %g1
    or  %g1, 576, %g1
    smul    %g2, %g1, %g3
    ld  [%fp-96], %g2
    ld  [%fp-88], %g1
    sub %g2, %g1, %g1
    add %g3, %g1, %g1
    st  %g1, [%fp-12]
    sra %g1, 31, %g1
    st  %g1, [%fp-16]
    ldd [%fp-64], %o4
    ldd [%fp-16], %g2
    addcc   %o5, %g3, %g3
    addx    %o4, %g2, %g2
    std %g2, [%fp-64]
    ld  [%fp-16], %g2
    ld  [%fp-12], %g1
    smul    %g2, %g1, %g4
    ld  [%fp-16], %g2
    ld  [%fp-12], %g1
    smul    %g2, %g1, %g1
    add %g4, %g1, %g4
    ld  [%fp-12], %g2
    ld  [%fp-12], %g1
    umul    %g2, %g1, %g3
    rd  %y, %g2
    add %g4, %g2, %g4
    mov %g4, %g2
    ldd [%fp-56], %o4
    addcc   %o5, %g3, %g3
    addx    %o4, %g2, %g2
    std %g2, [%fp-56]
    sethi   %hi(.LLC3), %g1
    or  %g1, %lo(.LLC3), %o0
    ld  [%fp-16], %o1
    ld  [%fp-12], %o2
    call    printf, 0
     nop
    sethi   %hi(.LLC4), %g1
    or  %g1, %lo(.LLC4), %o0
    ld  [%fp-64], %o1
    ld  [%fp-60], %o2
    call    printf, 0
     nop
    sethi   %hi(.LLC5), %g1
    or  %g1, %lo(.LLC5), %o0
    ld  [%fp-56], %o1
    ld  [%fp-52], %o2
    call    printf, 0
     nop
    sethi   %hi(.LLC6), %g1
    or  %g1, %lo(.LLC6), %o0
    ld  [%fp-32], %o1
    ld  [%fp-28], %o2
    call    printf, 0
     nop
    ld  [%fp-84], %g1
    add %g1, 1, %g1
    st  %g1, [%fp-84]
.LL2:
    ld  [%fp-84], %g2
    ld  [%fp-68], %g1
    cmp %g2, %g1
    bl  .LL3
     nop
    ld  [%fp-68], %g1
    sra %g1, 31, %g1
    ld  [%fp-68], %g3
    mov %g1, %g2
    ldd [%fp-64], %o0
    mov %g2, %o2
    mov %g3, %o3
    call    __udivdi3, 0
     nop
    mov %o0, %g2
    mov %o1, %g3
    std %g2, [%fp-136]
    ldd [%fp-136], %o0
    mov 0, %o2
    mov 0, %o3
    call    __cmpdi2, 0
     nop
    mov %o0, %g1
    cmp %g1, 1
    bl  .LL6
     nop
    ldd [%fp-136], %o0
    call    __floatdidf, 0
     nop
    std %f0, [%fp-144]
    b   .LL5
     nop
.LL6:
    ldd [%fp-136], %o4
    and %o4, 0, %g2
    and %o5, 1, %g3
    ld  [%fp-136], %o5
    sll %o5, 31, %g1
    ld  [%fp-132], %g4
    srl %g4, 1, %o5
    or  %o5, %g1, %o5
    ld  [%fp-136], %g1
    srl %g1, 1, %o4
    or  %g2, %o4, %g2
    or  %g3, %o5, %g3
    mov %g2, %o0
    mov %g3, %o1
    call    __floatdidf, 0
     nop
    std %f0, [%fp-144]
    ldd [%fp-144], %f8
    ldd [%fp-144], %f10
    faddd   %f8, %f10, %f8
    std %f8, [%fp-144]
.LL5:
    ldd [%fp-144], %f8
    std %f8, [%fp-48]
    ld  [%fp-68], %g1
    sra %g1, 31, %g1
    ld  [%fp-68], %g3
    mov %g1, %g2
    ldd [%fp-56], %o0
    mov %g2, %o2
    mov %g3, %o3
    call    __udivdi3, 0
     nop
    mov %o0, %g2
    mov %o1, %g3
    std %g2, [%fp-128]
    ldd [%fp-128], %o0
    mov 0, %o2
    mov 0, %o3
    call    __cmpdi2, 0
     nop
    mov %o0, %g1
    cmp %g1, 1
    bl  .LL8
     nop
    ldd [%fp-128], %o0
    call    __floatdidf, 0
     nop
    std %f0, [%fp-120]
    b   .LL7
     nop
.LL8:
    ldd [%fp-128], %o4
    and %o4, 0, %g2
    and %o5, 1, %g3
    ld  [%fp-128], %o5
    sll %o5, 31, %g1
    ld  [%fp-124], %g4
    srl %g4, 1, %o5
    or  %o5, %g1, %o5
    ld  [%fp-128], %g1
    srl %g1, 1, %o4
    or  %g2, %o4, %g2
    or  %g3, %o5, %g3
    mov %g2, %o0
    mov %g3, %o1
    call    __floatdidf, 0
     nop
    std %f0, [%fp-120]
    ldd [%fp-120], %f8
    ldd [%fp-120], %f10
    faddd   %f8, %f10, %f8
    std %f8, [%fp-120]
.LL7:
    ldd [%fp-48], %f8
    ldd [%fp-48], %f10
    fmuld   %f8, %f10, %f8
    ldd [%fp-120], %f10
    fsubd   %f10, %f8, %f8
    std %f8, [%fp-112]
    ldd [%fp-112], %f8
    fsqrtd  %f8, %f8
    std %f8, [%fp-152]
    ldd [%fp-152], %f10
    ldd [%fp-152], %f8
    fcmpd   %f10, %f8
    nop
    fbe .LL9
     nop
    ldd [%fp-112], %o0
    call    sqrt, 0
     nop
    std %f0, [%fp-152]
.LL9:
    ldd [%fp-152], %f8
    std %f8, [%fp-40]
    sethi   %hi(.LLC7), %g1
    or  %g1, %lo(.LLC7), %o0
    ld  [%fp-48], %o1
    ld  [%fp-44], %o2
    ld  [%fp-40], %o3
    ld  [%fp-36], %o4
    call    printf, 0
     nop
    mov 0, %g1
    mov %g1, %i0
    restore
    jmp %o7+8
     nop
    .size   main, .-main
    .ident  "GCC: (GNU) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)"
    .section    ".note.GNU-stack"

When I generate the Assembly code version with nLoop = 5000000000 (5.0e+9), the differences are illustrated on the following figure (with vimdiff) :

vimdiff differences

the block of "4 Billions" version :

mov     0, %g2                                                                                                                           
sethi   %hi(4000000000), %g3

is replaced in "5 Billions" version by :

 mov     1, %g2
 sethi   %hi(705032192), %g3                                                   
 or      %g3, 512, %g3

I can see that 5.0+e9 can't be coded on 32 bits, since the instruction

sethi   %hi(705032192), %g3

Paradoxically, when I compile the version "5 Billions" Assembly code, the ouput parameter sum is computed well, i.e is equal to 5 Billions, and I can't explain it.

Solution

  • A lot depends on what version of sparc and what ABI your are using. If you are using sparc v8 or earlier, you have 32 bit mode with only 32 bit registers. In which case when you try to load 5000000000 into a 32-bit register it fails and loads 5000000000 mod 232 instead (which is 705032704). This is what appears to be happening.

    If on the other hand you have a 64-bit sparc processor running in 32 bit mode (what is usually called v8plus), then you can use 64 bit registers, so this would work.