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eccerr0r
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Joined: 01 Jul 2004
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Location: almost Mile High in the USA
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 Posted: Mon Mar 10, 2025 4:04 pm Post subject: |
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On another note, ultimately since we're doing popcount on constants, I'm doubly surprised gcc didn't optimize all these away to another constant since f(constant) = another constant (at least standard arithmetic operators it's smart enough to do)...
granted fibonacci(999999) or factorial(999999) do take a lot of iterations to run (if not using their closed form solutions) and the compiler would still need to do it, so there's probably some stuff best left for runtime ... else it'd need to know a lot of solutions to iterative functions.
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John R. Graham
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| Posted: Mon Mar 10, 2025 8:14 pm Post subject: |
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| Hu wrote: | | You may like the related trick that value & (value - 1) is false when value is a power of 2, and true otherwise. |
Yeah, that would follow, wouldn't it, because all powers of two only have a single one bit.
- John
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John R. Graham
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| Posted: Mon Mar 10, 2025 8:18 pm Post subject: |
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| eccerr0r wrote: | | On another note, ultimately since we're doing popcount on constants, I'm doubly surprised gcc didn't optimize all these away to another constant since f(constant) = another constant (at least standard arithmetic operators it's smart enough to do)... |
At LTO time, you mean? I've seen a rule that says you can't optimize out of existence a function with global visibility. Certainly the compiler doesn't know how it might be used elsewhere.
- John
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