topic Hi Johannes, in Intel® oneAPI Math Kernel Library

Underflow exception when using vmsExp

Johannes_B_1 — Wed, 11 May 2016 08:01:36 GMT

The following code generates an exception, although I set the error mode to VML_ERRMODE_IGNORE. Is there a way to use high accuracy (VML_HA) computations without raising underflow exceptions when computing exp (-88.) ?

float testData[1] = {-88.77909088};
vmsExp (1, testData, testData, VML_HA | VML_FTZDAZ_ON | VML_ERRMODE_IGNORE);

I am using the Intel MKL version 11.3.2 with Visual Studio 2013, statically linking mkl_intel_c.lib, mkl_core.lib and mkl_sequential.lib into my project.

Hi Johannes,

Jingwei_Z_Intel — Thu, 12 May 2016 16:46:31 GMT

I think the issue is related to the control mode set in the MXCSR register at runtime. By default, it's set to 0x9fc0 (i.e. FTZ and DAZ on) on Windows, that may have caused the hardware instructions used by vmsExp to set the underflow exceptions unexpectedly. If you change MXCSR value to 0x1f80, that should solve your problem. Here is my test program and results:

#include <mkl.h>
#include <stdio.h>
#include <errno.h>
#include <immintrin.h>

int main() {
    float testData[] = {-88.77909088}; 
    printf("input  = %f = 0x%08x\n", testData[0], ((int*)testData)[0]);
    errno = 0;
    printf("errno  = %d\n", errno);
    //_mm_setcsr(0x1f80);
    printf("mxcsr  = %x\n", _mm_getcsr());
    vmsExp (1, testData, testData, VML_HA | VML_FTZDAZ_ON | VML_ERRMODE_IGNORE);
    printf("result = %f = 0x%08x\n", testData[0], ((int*)testData)[0]);
    printf("errno  = %d\n", errno);
    printf("mxcsr  = %x\n", _mm_getcsr());
    return 0;
}

Default result:

input  = -88.779091 = 0xc2b18ee5
errno  = 0
mxcsr  = 9fc0
result = 0.000000 = 0x00000000
errno  = 0
mxcsr  = 9ff1

If $MXCSR is set to 0x1f80:

input  = -88.779091 = 0xc2b18ee5
errno  = 0
mxcsr  = 1f80
result = 0.000000 = 0x00000000
errno  = 0
mxcsr  = 1f80

Please let me know if this helps.

Thank you very much! The

Johannes_B_1 — Fri, 13 May 2016 09:25:00 GMT

Thank you very much!

The solution works, although I figured out after a while, that the FTZ and DAZ flags do not seem to have something to do with it. I rather have to set the "invalid operation mask" to get the code working, using the following instructions:

_mm_setcsr (_mm_getcsr() | _MM_MASK_INVALID);

Also, in my case, the default value of the MXCSR register seems to be 0x1920 instead of 0x9FC0 and may have something to do with the compiler optimization options.