On Sun, Sep 3, 2023 at 1:10 PM Kuan-Wei Chiu [off-list ref] wrote:

On Sun, Sep 03, 2023 at 06:30:58PM +0500, Roman Mamedov wrote:

quoted

On Sun,  3 Sep 2023 17:50:59 +0800
Kuan-Wei Chiu [off-list ref] wrote:

quoted

Replace the conditional statements in the cmp_stripe() function with a
branchless version to improve code readability and potentially enhance
performance.

The new code will always do two comparisons and a subtraction (3
instructions in total), whereas the old version could return after just 1
comparison, or after 2 comparisons. So depending on the data values it is 3x
to 1.5x as much operations performed than before, there unlikely to be any
enhancement of performance.

Also IMO the previous version is more easily readable.

The reason behind my proposed changes was to eliminate conditional
branches in the code. While the original code could occasionally achieve
early returns, many compilers, such as x86-64 gcc 13.2 compiling with
-O2 flag, still generate branch instructions. Processors typically have
deep pipelines, and a branch prediction miss can result in a high
penalty. Therefore, even though early return might not be possible, the
new branchless version of code could still offer efficiency
improvements.

We need more information to support the efficiency improvement here.
In this case, I would like to see some benchmark results (micro
benchmark is fine).

If we cannot show the difference in performance, I would rather keep
current code.

Thanks,
Song

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