People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed

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People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed, but my compiler can't even replace i/=2 with a shift.

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A: Was i signed or unsigned? If it was signed, a shift is not equivalent (hint: think about the result if i is negative and odd), so the compiler was correct not to use it.

2015-02-06, 1729👍, 0💬

People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed
People claim that optimizing compilers are good and that we no longer have to write things in assembler for speed, but my compiler can't even replace i/=2 with a shift. A: Was i signed or unsigned? If it was signed, a shift is not equivalent (hint: think about the result if i is negative and odd), s... 2015-02-06, 1730👍, 0💬

Are pointers really faster than arrays?
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Is there a way to have non-constant case labels (i.e. ranges or arbitrary expressions)? No. The switch statement was originally designed to be quite simple for the compiler to translate, therefore case labels are limited to single, constant, integral expressions. You can attach several case labels t... 2015-02-02, 1607👍, 0💬

Is there a way to switch on strings?
Is there a way to switch on strings? Not directly. Sometimes, it's appropriate to use a separate function to map strings to integer codes, and then switch on those: #define CODE_APPLE 1 #define CODE_ORANGE 2 #define CODE_NONE 0 switch(classifyfunc(string)) { case CODE_APPLE: ... case CODE_ORANGE: ..... 2015-02-04, 1584👍, 0💬

Are the outer parentheses in return statements really optional?
Are the outer parentheses in return statements really optional? Yes. Long ago, in the early days of C, they were required, and just enough people learned C then, and wrote code which is still in circulation, that the notion that they might still be required is widespread. (As it happens, parentheses... 2015-02-02, 1579👍, 0💬

Can I use base-2 constants (something like 0b101010)? Is there a printf format for binary?
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I have been replacing multiplications and divisions with shift operators, because shifting is more efficient.
I have been replacing multiplications and divisions with shift operators, because shifting is more efficient. This is an excellent example of a potentially risky and usually unnecessary optimization. Any compiler worthy of the name can replace a constant, power-of-two multiplication with a left shif... 2015-02-09, 1544👍, 0💬

How can I swap two values without using a temporary?
How can I swap two values without using a temporary? The standard hoary old assembly language programmer's trick is: a ^= b; b ^= a; a ^= b; But this sort of code has little place in modern, HLL programming. Temporary variables are essentially free, and the idiomatic code using three assignments, na... 2015-02-06, 1493👍, 0💬

Which is more efficient, a switch statement or an if else chain?
Which is more efficient, a switch statement or an if else chain? The differences, if any, are likely to be slight. The switch statement was designed to be efficiently implementable, though the compiler may choose to use the equivalent of an if/else chain (as opposed to a compact jump table) if the c... 2015-02-04, 1489👍, 0💬

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