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Compiler Warning (Level 3) C4738

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The latest version of this topic can be found at Compiler Warning (Level 3) C4738.

storing 32-bit float result in memory, possible loss of performance

C4738 warns that the result of an assignment, cast, passed argument, or other operation may need to be rounded or that the operation ran out of registers and needed to use memory (spilling). This can result in performance loss.

To resolve this warning and avoid rounding, compile with /fp:fast or use doubles instead of floats.

To resolve this warning and avoid running out of registers, change the order of computation and modify your use of inlining

This warning is off by default. For more information, see Compiler Warnings That Are Off by Default.

Example

The following sample generates C4738:

// C4738.cpp  
// compile with: /c /fp:precise /O2 /W3  
// processor: x86  
#include <stdio.h>  
  
#pragma warning(default : 4738)  
  
float func(float f)  
{  
    return f;  
}  
  
int main()  
{  
    extern float f, f1, f2;  
    double d = 0.0;  
  
    f1 = func(d);  
    f2 = (float) d;  
    f = f1 + f2;   // C4738  
    printf_s("%f\n", f);  
}