fenv_access
The latest version of this topic can be found at fenv_access. Disables (ON) or enables (OFF) optimizations that could change flag tests and mode changes.
Syntax
#pragma fenv_access [ON | OFF]
Remarks
By default, fenv_access is OFF.
For more information on floating-point behavior, see /fp (Specify Floating-Point Behavior).
The kinds of optimizations that are subject to fenv_access are:
Global common subexpression elimination
Code motion
Constant folding
Other floating-point pragmas include:
Example
// pragma_directive_fenv_access_x86.cpp
// compile with: /O2
// processor: x86
#include <stdio.h>
#include <float.h>
#include <errno.h>
#pragma fenv_access (on)
int main() {
double z, b = 0.1, t = 0.1;
unsigned int currentControl;
errno_t err;
err = _controlfp_s(¤tControl, _PC_24, _MCW_PC);
if (err != 0) {
printf_s("The function _controlfp_s failed!\n");
return -1;
}
z = b * t;
printf_s ("out=%.15e\n",z);
}
out=9.999999776482582e-003
Example
The following sample is for compiler producing output files for Itanium processors. /fp:precise keeps the intermediate results in extended precision where values greater than FLT_MAX (3.402823466e+38F) can be calculated and as a result of that sum will have 1.0 result, as it should if manually calculated. /fp:strict keeps intermediate results in their source precision (float) so the first addition produces infinity, which is kept throughout the expression.
// pragma_directive_fenv_access_IPF.cpp
// compile with: /O2 /fp:precise
// processor: IPF
// compiling with /fp:precise prints 1.0F
// compile with /fp:strict to print infinity
#include <stdio.h>
float arr[5] = {3.402823465e+38F,
3.402823462e+38F,
3.402823464e+38F,
3.402823463e+38F,
1.0F};
int main() {
float sum = 0;
sum = arr[0] + arr[1] - arr[2] - arr[3] + arr[4];
printf_s("%f\n", sum);
}
1.000000
Example
When commenting out #pragma fenv_access (on) from the previous sample, note that the output is different because the compiler does compile-time evaluation, which does not use the control mode.
// pragma_directive_fenv_access_2.cpp
// compile with: /O2
#include <stdio.h>
#include <float.h>
int main() {
double z, b = 0.1, t = 0.1;
unsigned int currentControl;
errno_t err;
err = _controlfp_s(¤tControl, _PC_24, _MCW_PC);
if (err != 0) {
printf_s("The function _controlfp_s failed!\n");
return -1;
}
z = b * t;
printf_s ("out=%.15e\n",z);
}
out=1.000000000000000e-002