Why do I see people use for(;;) more than while(1) for infinite loops?

Question

I see for(;;) used much more frequently than while(1) in the wild, and I am curious if there is any meaningful reason for this. Compiling using GCC with -O0 optimization, both statements yield a single jump instruction. Is it more than just a coding style? Are there any coding standards that state that for(;;) is preferred over while(1)? Is there some efficiency reason that I'm missing? I realize that the goal is usually to make code the most readable that it can be, but I don't see much of a difference between the two styles as far as readability to warrant such a stark difference in usage.

maybe because while(1) involve a value (costant 1) whereas for(;;) Is pure statement — frhack, Nov 23 '20 at 23:46
Seems like some compilers generate different code https://godbolt.org/z/9Kqqex — Tony Tannous, Nov 23 '20 at 23:49

Tony Tannous · Accepted Answer · 2020-11-23T23:59:24.500

2

while(1) involves a constant which might result in an overhead when compiling without optimization such as pushing 1 and checking if the expression is z,n or positive.

MSVC demo with while(1) https://godbolt.org/z/MMMT9n

$LN2@main:
    mov eax, 1
    test eax, eax
    je SHORT $LN1@main
    jmp SHORT $LN2@main
$LN1@main:

With for(;;)

$LN4@main:
    jmp SHORT $LN4@main

edited Nov 23 '20 at 23:59

answered Nov 23 '20 at 23:53

Tony Tannous

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Very neat website. It seems that the switch from GCC to MSVC causes that extra inefficiency to show. – Kevin Montambault Nov 24 '20 at 00:04
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@KevinMontambault - is there any reason to talk about efficiency of not optimized code? With even default optimization, there will only be one `jmp`... – Vlad Feinstein Nov 24 '20 at 00:44
Who does actually care about the efficiency of the -O0 generated code? – 0___________ Nov 24 '20 at 02:09
@P__JsupportswomeninPoland no one should. But OP asked if there's any difference between them and I showed an example in which the generated code differs for `-O0` – Tony Tannous Nov 24 '20 at 10:24

score 2 · Answer 2 · answered Nov 24 '20 at 01:11

With some older compilers, for example, Microsoft Visual Studio 2015 before update 3, the while(1) construct will trigger a warning about conditional expression being constant, see Remarks section for warning C4127.

A related question was asked on Stack Overflow over ten years ago about this, see Why MSVC generates warning C4127 when constant is used in “while”.

I remember stumbling upon this, back in the days when we were using Visual Studio 2012. Nowadays, this seems to be sorted out, but still, the for(;;) form seems more idiomatic to me.

Why do I see people use for(;;) more than while(1) for infinite loops?

2 Answers2