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Signed right shift (-logical-signed-right-shift)

Specify how to treat the sign bit for logical right shifts on signed variables

Description

Choose between arithmetic and logical shift for right shift operations on negative values.

This option does not modify compile-time expressions. For more details, see Limitation.

Set Option

User interface (desktop products only): In your project configuration, the option is on the Target & Compiler node.

User interface (Polyspace Platform, desktop products only): In your project configuration, the option is on the Build tab on the Target & Compiler node. This option is renamed to Shift right on signed integers as arithmetic shift in Polyspace platform user interface. See Shift right on signed integers as arithmetic shift

Command line and options file: Use the option -logical-signed-right-shift. See Command-Line Information.

Why Use This Option

The C99 Standard (sec 6.5.7) states that for a right-shift operation x1>>x2, if x1 is signed and has negative values, the behavior is implementation-defined. Different compilers choose between arithmetic and logical shift. Use this option to emulate your compiler.

Settings

Default: Arithmetical

Arithmetical

The sign bit remains:

(-4) >> 1 = -2
(-7) >> 1 = -4
  7  >> 1 = 3
Logical

0 replaces the sign bit:

(-4) >> 1 = (-4U) >> 1 = 2147483646
(-7) >> 1 = (-7U) >> 1 = 2147483644
  7  >> 1 = 3

Limitation

In compile-time expressions, this Polyspace® option does not change the standard behavior for right shifts.

For example, consider this right shift expression:

int arr[ ((-4) >> 20) ];
The compiler computes array sizes, so the expression (-4) >> 20 is evaluated at compilation time. Logically, this expression is equivalent to 4095. However, arithmetically, the result is -1. This statement causes a compilation error (arrays cannot have negative size) because the standard right-shift behavior for signed integers is arithmetic.

Command-Line Information

When using the command line, arithmetic is the default computation mode. When this option is set, logical computation is performed.

Parameter: -logical-signed-right-shift
Default: Arithmetic signed right shifts
Example (Bug Finder): polyspace-bug-finder -logical-signed-right-shift
Example (Code Prover): polyspace-code-prover -logical-signed-right-shift
Example (Bug Finder Server): polyspace-bug-finder-server -logical-signed-right-shift
Example (Code Prover Server): polyspace-code-prover-server -logical-signed-right-shift

See Also

Topics