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The Java language allows platforms to use available floating-point hardware that can provide extended floating-point support with exponents that contain more bits than the standard Java primitive type double (in the absence of the strictfp modifier). Consequently, these platforms can represent a superset of the values that can be represented by the standard floating-point types. Floating-point computations on such platforms can produce different results than would be obtained if the floating-point computations were restricted to the standard representations of float and double. According to the JLS, §15.4, "FP-strict Expressions" [JLS 2005]:

The net effect [of non-fp-strict evaluation], roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.

Programs that require Sometimes it is desired to obtain consistent results from floating-point operations , across different JVMs and platforms must use the strictfp modifier. This modifier requires the JVM and the platform to behave as though all floating-point computations were performed using values limited to those that can be represented by a standard Java float or double, guaranteeing that the result of the computations will match exactly across all JVMs and platforms.

Using the strictfp modifier leaves execution unchanged on platforms that lack platform-specific, extended floating-point support. It can have substantial impact, however, on both the efficiency and the resulting values of floating-point computations when executing on platforms that provide extended floating-point support. On these platforms, using the strictfp modifier increases the likelihood that intermediate operations will overflow or underflow because it restricts the range of intermediate values that can be represented; it can also reduce computational efficiency. These issues are unavoidable when portability is the main concern.

. This guarantee is imposed by the strictfp modifier which ensures that intermediate operations do not result in arithmetic underflow or overflow commonly encountered while dealing with float and double types. The strictfp modifier can be used with a class, method, or interface. :

An expression is FP-strict if when any of the contained containing classes, methods and , or interfaces is defined declared to be a strictfp. Constant expressions containing floating-point operations are also evaluated strictly. All compile-time constant expressions are by default , strictfpFP-strict.

Notably, the strict behavior cannot be Strict behavior is not inherited by a subclass that extends a strictfp superclassFP-strict superclass. An overriding method can independently choose to be FP-strict when the overridden method is not, or vice versa.

Noncompliant Code Example

This noncompliant code example does not enforce the strictfp constraintsmandate FP-strict computation. Double.MAX_VALUE is being multiplied by 1.1 and reduced back by dividing by 1.1, according to the evaluation order. JVM implementations are not required to report an overflow resulting from the initial multiplication, although they can chose . If Double.MAX_VALUE is the maximum value permissible by the platform, the calculation will yield the result infinity.

However, if the platform provides extended floating-point support, this program might print a numeric result roughly equivalent to Double.MAX_VALUE.

The JVM may choose to treat this case as strictfp. The ability to use extended exponent ranges to represent intermediate values is thus implementation definedFP-strict; if it does so, overflow occurs. Because the expression is not FP-strict, an implementation may use an extended exponent range to represent intermediate results.

class StrictfpExample {
  public static void main(String[] args) {
    double d = Double.MAX_VALUE;
    System.out.println("This value \"" + ((d * 1.1) / 1.1) + "\" cannot be represented as double.");
  }
}

Compliant Solution

To be compliantFor maximum portability, use the strictfp modifier within an expression (class, method, or interface) to guarantee that intermediate results do not vary due to because of implementation-defined compiler optimizations or by design. This code snippet is guaranteed to return positive INFINITY due to behavior. The calculation in this compliant solution is guaranteed to produce infinity because of the intermediate overflow condition, regardless of what floating-point support is provided by the platform.

strictfp class StrictfpExample {
  public static void main(String[] args) {
    double d = Double.MAX_VALUE;
    System.out.println("This value \"" + ((d * 1.1) / 1.1) + "\" cannot be represented as double.");
  }
}

Noncompliant Code Example

Native floating-point hardware provides greater range than double. On these platforms, the JIT is permitted to use floating-point registers to hold values of type float or type double (in the absence of the strictfp modifier), even though the registers support values with greater exponent range than that of the primitive types. Consequently, conversion from float to double can cause an effective loss of magnitude.

class Example {
  double d = 0.0;

  public void example() {
    float f = Float.MAX_VALUE;
    float g = Float.MAX_VALUE;
    this.d = f * g;
    System.out.println("d (" + this.d + ") might not be equal to " +
                       (f * g));
  }

  public static void main(String[] args) {
    Example ex = new Example();
    ex.example();
  }
}

Magnitude loss would also occur if the value were stored to memory – for example, to a field of type float.

Compliant Solution

This compliant solution uses the strictfp keyword to require exact conformance with standard Java floating-point. Consequently, the intermediate value of both computations of f * g is identical to the value stored in this.d, even on platforms that support extended range exponents.

strictfp class Example {
  double d = 0.0;

  public void example() {
    float f = Float.MAX_VALUE;
    float g = Float.MAX_VALUE;
    this.d = f * g;
    System.out.println("d (" + this.d + ") might not be equal to " +
                       (f * g));
  }

  public static void main(String[] args) {
    Example ex = new Example();
    ex.example();
  }
}

Exceptions

NUM06-J-EX0: This rule applies only to calculations that require consistent floating-point results on all platforms. Applications that lack this requirement need not comply.

Risk Assessment

Not using Failure to use the strictfp modifier can result in platform nonportable, implementation-defined behavior with respect to the accuracy behavior of floating-point operations.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[JLS 05|AA. Java References#JLS 05]\] 15.4 FP-strict Expressions
\[[JPL 05|AA. Java References#JPL 05]\] 9.1.3. Strict and Non-Strict Floating-Point Arithmetic
\[[McCluskey 01|AA. Java References#McCluskey 01]\] Making Deep Copies of Objects, Using strictfp, and Optimizing String Performance
\[[Darwin 04|AA. Java References#Darwin 04]\] Ensuring the Accuracy of Floating-Point Numbers

Related Guidelines

Bibliography

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Image Added      Image Added      Image AddedFLP02-J. Do not attempt comparisons with NaN expressions      05. Floating Point (FLP)      05. Floating Point (FLP)