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Red Ant

[C++] Are compilers allowed to optimize ...

12 posts in this topic

the following function away?

[code]
template < typename FloatType >
inline const bool is_NaN( FloatType number )
{
// A NaN doesn't compare equal to anything, not even itself.
return number != number;
}
[/code]

Obviously, I intend to use this to test a floating point variable for NaN (not a number) ... can't use std::isnan() because our compiler doesn't support C++11. My worry is that the compiler might optimize the whole call away, on account of comparing a number to itself being so unbelievably stupid. <_<
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I wouldn't be surprised if it optimized it away for integer-type variables. For floating point, I'm not sure though; seeing as it is a pretty standard test for NaN, I would expect compiler writers to be aware of this and not optimize it away.

[edit]

If you're using MSVC++, you may be able to [url="http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.15"]use _isnan()[/url]. If you're not using MSVC++ and your compiler still doesn't support C's isnan(), I pity your soul.
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That doesn't make any sense. If the compiler can optimize it out of a function it can optimize it out of a define.
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[quote name='alvaro' timestamp='1324399839' post='4895774']
If a compiler optimizes that away, it has a bug. It cannot be optimized away precisely because of the behavior of NaNs. I think you are safe.
[/quote]

I suspected as much, but it's good to have it confirmed by you guys. Thanks. ;)
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You might find this article interesting: [url=http://msdn.microsoft.com/en-us/library/aa289157(v=vs.71).aspx]Microsoft Visual C++ Floating-Point Optimization[/url]
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[quote name='Red Ant' timestamp='1324398984' post='4895766']
the following function away?

[code]
template < typename FloatType >
inline const bool is_NaN( FloatType number )
{
// A NaN doesn't compare equal to anything, not even itself.
return number != number;
}
[/code]

Obviously, I intend to use this to test a floating point variable for NaN (not a number) ... can't use std::isnan() because our compiler doesn't support C++11. My worry is that the compiler might optimize the whole call away, on account of comparing a number to itself being so unbelievably stupid. <_<
[/quote]

One note on style, regarding the const-qualified bool return-type (note: bool is a built-in type):
"[font="Georgia"]For built-in types, it doesn’t matter whether you return by value as a [b]const[/b], so you should avoid confusing the client programmer and leave off the [b]const[/b] when returning a built-in type by value."
// [url="http://linuxtopia.org/online_books/programming_books/thinking_in_c++/Chapter08_014.html"]http://linuxtopia.or...pter08_014.html[/url]
[/font]
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[quote name='Matt-D' timestamp='1324416244' post='4895879']
One note on style, regarding the const-qualified bool return-type (note: bool is a built-in type):
"[font="Georgia"]For built-in types, it doesn’t matter whether you return by value as a [b]const[/b], so you should avoid confusing the client programmer and leave off the [b]const[/b] when returning a built-in type by value."
// [url="http://linuxtopia.org/online_books/programming_books/thinking_in_c++/Chapter08_014.html"]http://linuxtopia.or...pter08_014.html[/url]
[/font]
[/quote]

I'm aware of that, but I've made a habit of doing it just because it makes it impossible to write silly stuff like

is_NaN( 6.3433 ) = false;


P.S. Never mind, apparently for built-in return types the line above is always impossible to compile. So yeah, basically you're right ... I've been doing it wrong all these years. :)

.
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That NaN check might not work if you enable fp-fast or otherwise allow deviance from the floating point spec.

Just so you know.
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[quote name='Red Ant' timestamp='1324419662' post='4895906']
I'm aware of that, but I've made a habit of doing it just because it makes it impossible to write silly stuff like

is_NaN( 6.3433 ) = false;
[/quote]
That assignment is always impossible, no matter if you return a bool or a const bool. Again, the const is simply ignored here, it doesn't make any difference whatsoever. The language simply does not allow assigning to scalar rvalues.
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[quote name='DevFred' timestamp='1324900005' post='4897425']
That assignment is always impossible, no matter if you return a bool or a const bool. Again, the const is simply ignored here, it doesn't make any difference whatsoever. The language simply does not allow assigning to scalar rvalues.
[/quote]

Yes, if you read my last post you'll see that I've already conceded that point. ;)
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