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freeworld

What are unions good for?

28 posts in this topic

On x86 processors, that kind of type-punning often has to write the floating-point value to memory, read it into an integer register, perform the integer operation, write it to memory again, and read it back into a floating-point register so it may not be as fast as you think it is. If you're determined to use that kind of approximation, though, there are some slightly [url="http://ilab.usc.edu/wiki/index.php/Fast_Square_Root"]better versions[/url]. They're faster than the normal sqrt() but less accurate. Setting the floating-point model to "Fast" in Visual C++ allows it to use a native SQRT or SQRTSS instruction and avoid the normal library function call.

[url="http://en.wikipedia.org/wiki/Fast_inverse_square_root"]Fast reciprocal square root[/url] may still have a use in performance-critical code, but even that it less useful than it once. If you're using SSE, you may be better off using RSQRTSS or the _mm_rsqrt_ss() intrinsic. (With fast floating point and SSE, the VC++ optimizer might be smart enough to replace 1 / sqrt with a reciprocal square root instruction. I wouldn't count on it, though...)
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[quote]That is anything but clear. Can you justify that statement?
After fixing the above, I see the following code generated (ICC 12.1):
000000013F7B1058 mov edx,dword ptr [dest]
000000013F7B105B shr edx,1
000000013F7B105D mov dword ptr [dest],edx
[/quote]

VS2010 wouldn't optimise out the std::copy calls, but after switching to memcpy, it's now producing fully optimised source. (identical to using a straight up cast to int*)

[quote]Nope. Please see ANSI C99 6.7.3#15.[/quote]

What does the standard say on this? And do you know where I can find a copy of the standard for download?
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[quote]On x86 processors, that kind of type-punning often has to write the floating-point value to memory, read it into an integer register, perform the integer operation, write it to memory again, and read it back into a floating-point register so it may not be as fast as you think it is. If you're determined to use that kind of approximation, though, there are some slightly better versions. They're faster than the normal sqrt() but less accurate. Setting the floating-point model to "Fast" in Visual C++ allows it to use a native SQRT or SQRTSS instruction and avoid the normal library function call.[/quote]

Yeah it's not worth the trouble in this day and age when you can simply enable SSE2 and FP:fast. I should have mentioned a more relevant application of type punning. For example, generating a hash values out of non-integer types.

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[quote]What does the standard say on this?[/quote]
Reading single paragraphs is not the path to wisdom :)

[quote]And do you know where I can find a copy of the standard for download?[/quote]
[url="http://lmgtfy.com/?q=buy+ansi+%22programming+languages%22+%2Bc+standard"]http://lmgtfy.com/?q...2+%2Bc+standard[/url]
I don't think you'll find anything but the draft C++0x standard for free legal download, but $30 should be affordable.


[quote]For example, generating a hash values out of non-integer types.[/quote]
Ah, that's a good example.
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