[astrange]

> The compiler should optimize without changing the semantic of the code, even if it contains undefined/unspecified behavior.

That is what it does. "Undefined behavior" is a lack of semantics, so it is preserving semantics when it leaves those paths out. You can make a "defined C" with `-fsanitize-trap=undefined`, but C was never a high level assembler, and performance is critical for C users too.

[Asooka]

C was always a high-level assembler. UB was meant to be "this cannot be defined portably, refer to your CPU's documentation". What do you gain by making C unusable by default? Just make it 5% slower by default, but possible to reason about and give people the option to shoot themselves in the foot. I don't know what more you need than the creator of the language telling you "this is not possible to implement sanely".

[tsukikage]

> UB was meant to be "this cannot be defined portably, refer to your CPU's documentation"

You're probably thinking of implementation-defined behaviour, which is the term for behaviour that, although not fully specified by the standard, is a valid behaviour for the program.

Undefined behaviour is different. From the article: "Undefined behavior gives the implementor license not to catch certain program errors that are difficult to diagnose."

Many people don't notice the distinction, and their intuition about what compilers should do with their code matches implementation-defined behaviour; and therefore they complain when, for UB, it does not behave this way.

There is a very strong argument for reclassifying many specific UBs as implementation-defined behaviours, but that's a rather different conversation from "the compiler should always do something sensible when encountering UB".

[pcwalton]

Even assuming the 5% number were correct (depending on how expansive your definition of UB is, it may not be), asking everyone who doesn't adjust their compiler flags to accept a 5% slowdown for some theoretical benefits is at odds with economic reality.

[vyodaiken]

Maybe you should tell the Linux developers that they are making a mistake.

[Asooka]

Even assuming the 5% number were correct (depending on how expansive your optimisations with UB assumptions are, it may not be), asking everyone who doesn't adjust their compiler flags to accept their programs being silently miscompiled for some theoretical benefits is at odds with economic reality.

[pcwalton]

Times like this I wish I were allowed to say exactly how much money a 1% fleet-wide loss in performance costs a big tech company.

[vyodaiken]

A) there is no evidence such phenomena - especially in net- are due to unsafe optimizations, b) those companies dont need to shift their costs to other users

[naasking]

> "Undefined behavior" is a lack of semantics, so it is preserving semantics when it leaves those paths out.

It's not preserving semantics, it's inferring a valid program from an invalid one, but since the programmer entered an invalid program, unilaterally deciding that the valid program was the intended program seems dubious at best. These should really be errors, or warnings at the very least.