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Comment by gumby | Hacker News Reader

I used to be a compiler developer and this was no big deal (except when it was an actual code generator bug!). The compiler switches tell the compiler to do different things, so of course some of them change the output semantics. It’s up to the user to decide what she wants — it’s not like the differences are obscure or surprising.

Then I went on to other things and realized that developers are experts in their domains, and those “obvious” things were only clear to someone steeped in the code — not obvious at all. In fact some of them are no longer obvious to me any more after decades doing other things. Hopefully I’ve become at least slightly less arrogant and thoughtless by now.

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I used to be a compiler maintainer because I was the maintainer for a large scientific suite -- i.e. I was on both sides of the fence -- and I haven't really changed my opinion. The arrogance I've seen has mainly been from scientific users who lecture you about the code you've worked on and the language standard, neither of which they know. A classic case is ignoring the Fortran storage association rules (that happen to have some significance in GCC history).

Are you talking about things like very small differences in the results of floating point calculations?

Very small errors in floating point calculations can be made arbitrarily large.

There are domains where floating point problems being ill-conditioned is the norm rather than an exception. Try running `1e100+1e20-1e100` in python, js, etc....

Some optimization switches are off by default as they can subtly change the semantics of your code.

Yes, this can trip up even quite knowledgeable people as with the fp post from Lemire the other day — he thought gcc was making a mistake, but it was doing the correct thing.

But there are plenty of others, for example telling the compiler not to worry about aliasing. I might use this on certain modules or individual files which were carefully written for this.

Or asking for more aggressive loop unrolling Which could increase code size but improve performance (or not!).

99.9% the defaults are fine.

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