Скачать или смотреть How to Improve Execution Time in C+ + Function Calls

Discover effective methods to enhance the execution time of function calls in C+ + , especially when dealing with frequently used values.
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Optimizing Function Execution Time in C+ +

In C+ + , efficient function execution is crucial, especially when specific values are frequently used. If you find that a particular function, say foo(), is often called with one of 10 specific values, enhancing its performance can be beneficial. This post delves into a common question regarding methods to reduce the execution time of such function calls and evaluates several options to determine the most effective solution.

The Problem at Hand

Let’s examine a C+ + function outlined below:

[[See Video to Reveal this Text or Code Snippet]]

The question arises: if most calls to foo() utilize one of 10 specific values, what method can significantly decrease the function's execution time? The options presented are:

A: Replace * with an if-else block testing for the 10 values, assigning r accordingly.

B: Remove inline.

C: Delete foo() and move the time-consuming operation to its caller.

D: Replace * with code performing a table lookup, with the 10 values and corresponding values of r.

E: Replace * with switch.

Evaluating the Options

Option Analysis

Option A (If-Else Block): Implementing an if-else structure for the 10 specified values may seem efficient but can lead to increased branching. Branch prediction failure can slow down the execution significantly, particularly as the number of conditions grows. However, in a small set like 10 specific values, it could work reasonably well under the right conditions.

Option B (Remove Inline): Removing the inline keyword does not necessarily improve performance. It only eliminates the one-definition rule, while modern compilers are generally proficient in deciding whether to inline a function or not. This makes option B unlikely to yield any performance advantages.

Option C (Delete foo()): Moving the time-consuming logic directly to the caller might seem straightforward, but if the function is optimized for inlining, this relocation might actually worsen performance. Thus, this option generally does not contribute positively to execution speed.

Option D (Table Lookup): Replacing * with a table lookup is a promising approach. This method is branchless, which removes the overhead associated with branching operations, and can lead to faster execution. If the repetitive calls select from a limited and known set of values, this could be the fastest method.

Option E (Switch Statement): Using a switch could potentially perform similarly to the if-else condition, depending on how the compiler optimizes it. However, it still involves branching, similar to option A.

The Compiler’s Role

The performance of any of these options depends heavily on the specific compiler and its optimization capabilities. Here's a general breakdown of findings regarding compiler behavior:

When testing in gcc with the -O3 flag, options D and E can often be optimized to use a lookup table, which can improve performance significantly while A remains a series of conditional jumps.

In contrast, switching to clang with the same optimization level, all options may simplify to an equivalent lookup table mechanism.

Preferred Approach

Assuming no compiler optimizations: Option D is likely the fastest due to its branchless nature.

With optimizations: Both D and E can be equally effective depending on the compiler and version. Option A would typically lag in performance due to additional branching, making D the optimal choice.

Conclusion

In conclusion, to enhance function execution speed in C+ + , especially when dealing with a limited set of input values, using a table lookup (Option D) can lead to remarkable performance improvements. However, always remember that measuring performance through benchmarking in your spec