Does using ?: to do an assignment produce faster assembly than using if()?

Question

I'd like to know which one of the following two forms of lazy instantiation generates faster assembly code. Here's the code:

1st:

if (!_separatorTopBorderColor) _separatorTopBorderColor = UIColorFromHex(0x393A3B);
return _separatorTopBorderColor;

2nd:

_separatorTopBorderColor = _separatorTopBorderColor ?: UIColorFromHex(0x393A3B);
return _separatorTopBorderColor;

If there's any performance difference, your compiler is Doing It Wrong. I can't say that no hideously wrong/evil compiler exists in the world, but certainly none that you ought to be using. — Charles Duffy, May 21 '12 at 19:15
Note that `?:` (with no expression in the middle) is not valid C. — R.. GitHub STOP HELPING ICE, May 21 '12 at 19:42

score 1 · Answer 1 · answered May 21 '12 at 19:11

1

This is really a question of ternary operators vs regular if-statements. Neither will be faster, so it's really a matter of aesthetics/preference.

answered May 21 '12 at 19:11

Dima

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score 1 · Answer 2 · answered May 21 '12 at 19:15

1

No. Simple as that. And why should it.

answered May 21 '12 at 19:15

Jens

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score 1 · Accepted Answer · answered May 21 '12 at 19:18

1

It might be an issue 10 years ago, but nowadays, compilers literally sees any difference with ternary operators and if-else statements. My advise is that you should concentrate on keeping your code more readable, maintainable, and efficient. Don't care about resource or memory usage in this case.

answered May 21 '12 at 19:18

Vishal

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I doubt it was an issue even for the very first C compiler. The abstract syntax tree would very likely be **identical** to that of an if/else. – Jens May 23 '12 at 14:58
Thank you! I just wanted to be sure. – Rudolf Adamkovič May 25 '12 at 23:22

score 0 · Answer 4 · answered May 21 '12 at 19:17

Well, choose whichever is more readable.

Now, from a compiler optimization perspective, most optimizing compilers contain an optimizer pass called "if-conversion" or something like that, which is a pass which can convert a SIMPLE branch into a conditional move instruction. In your case you have a function call right there in one of the branches, so this doesn't matter. Secondly, on a superscalar OoO processor with decent branch prediction, conditional moves are in most cases a pessimization compared to a simple branch, so in fact compilers targeting such CPU's will most likely not do the if-conversion.

Does using ?: to do an assignment produce faster assembly than using if()?

4 Answers4