undefined | Better HN

0 pointsE63009y ago0 comments

> Under what set of logic does being able to de-reference a pointer confer that it's value is not 0 (which is what the test equates to)?

Simple: undefined behavior makes all physically possible behaviors permissible.

In reality though, such an elimination would only be correct if the compiler was able to prove that the function is ever called with NULL, and if the compiler is smart enough to do that, hopefully the compiler writers are not A-holes and will warn instead of playing silly-buggers.

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vyodaiken9y ago

This is a kind of Department of Motor Vehicles Bureaucrat thinking. For example, there as many thousands of lines of C code that reference *0, which is a perfectly good address in some environments. One should be able to depend on compilers following expressed intentions of the programmer and not making silly deductions based on counter-factual assumptions.

E6300OP9y ago

> This is a kind of Department of Motor Vehicles Bureaucrat thinking.

Sorry, but modern compilers are basically automatic theorem provers. They'll use whatever means necessary to get every last drop of performance. If you play cowboy with them you'll just get hurt.

> For example, there as many thousands of lines of C code that reference *0, which is a perfectly good address in some environments.

It's permissible for a particular platform to define behaviors that the standard has left undefined. If you try to take that code and run it elsewhere, that's your problem.

vyodaiken9y ago

If you are going to do theorem proving, you should try to only prove true theorems.

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lmm9y ago

This battle has been fought and lost. If you require sensible behaviour, just move on and use a language that offers it. C compilers will do what makes them look good on benchmarks, and various "friendly C" efforts have been tried and failed.

vyodaiken9y ago

Au contraire - gcc and clang both appear to do the right thing now.

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mjevans9y ago

You can't though. It's always possible to re-link the objects.

E6300OP9y ago

You can if, for example, the function is static. Also there could be a link-time optimization pass. The linker can see all calls to that function, unless the function is exported.

j / k navigate · click thread line to collapse

0 pointsE63009y ago0 comments

> Under what set of logic does being able to de-reference a pointer confer that it's value is not 0 (which is what the test equates to)?

Simple: undefined behavior makes all physically possible behaviors permissible.

0 comments

vyodaiken9y ago

E6300OP9y ago

> This is a kind of Department of Motor Vehicles Bureaucrat thinking.

Sorry, but modern compilers are basically automatic theorem provers. They'll use whatever means necessary to get every last drop of performance. If you play cowboy with them you'll just get hurt.

> For example, there as many thousands of lines of C code that reference *0, which is a perfectly good address in some environments.

It's permissible for a particular platform to define behaviors that the standard has left undefined. If you try to take that code and run it elsewhere, that's your problem.

vyodaiken9y ago

If you are going to do theorem proving, you should try to only prove true theorems.

1 more reply

lmm9y ago

vyodaiken9y ago

Au contraire - gcc and clang both appear to do the right thing now.

1 more reply

mjevans9y ago

You can't though. It's always possible to re-link the objects.

E6300OP9y ago

You can if, for example, the function is static. Also there could be a link-time optimization pass. The linker can see all calls to that function, unless the function is exported.

j / k navigate · click thread line to collapse