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Comment by throwaway9870 | Hacker News Reader

I still don't get it. You need to put an if statement in that function to raise the exception. So I still don't see where 50% comes from.

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At the very least you are redundantly executing logic without the exception. The check for eof has to be done implicitly anyway inside read because it has to fill the bit buffer with data from the byte buffer or the byte buffer with data from the file. And if both fail, then we already know the result of eof, so no need to duplicate checking for eof in the outer read calling loop.

Here is the full commit with ad-hoc benchmark results in the commit message:

https://github.com/mxmlnkn/pragzip/commit/0b1af498377838c30f...

and here the benchmarks I ran at that time:

https://github.com/mxmlnkn/pragzip/blob/0b1af498377838c30fea...

In this commit, I even replace std::optional with an exception. But, I didn't document benchmark results and the commit message begins with "try" so they probably weren't significantly better but also not worse or else I wouldn't commit it:

https://github.com/mxmlnkn/pragzip/commit/0f1babaf3b9cbe2912...

At the end of that file are infrequently updated results of the benchmarks.

As you can see, it's part of my random-seekable multi-threaded gzip and bzip2 parallel decompression libraries.

Wihtout seeing the assembly it is difficult to say but for such a large difference, the culprit often seems to the loop iop cache. If you check the alignment of the top of the loop instructions (and it inlined away the calls) sometimes you can see an alignment change for some trivial bullshit that has vasat performance impacts because it causes a series of other missed optmizations.

That damn loop cache is great for performance when you hit it,but sucks horrible when you don't even think out it and spend days trying to figure out some performance quandry.

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