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What every compiler writer should know about programmers (2015) [pdf]

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Re: What every compiler writer should know about programmers (2015) [pdf]

#3

Post by zephen »

Here's a cogent argument that any decision by compiler writers that they can do whatever they wish whenever they encounter an "undefined behavior" construct is rubbish:

https://www.yodaiken.com/2021/05/19/undefined-behavior-in-c-...

And here's a cautionary tale of how a compiler writer doing whatever they wish once they encounter undefined behavior makes debugging intractable:

https://www.quora.com/What-is-the-most-subtle-bug-you-have-h...

Re: What every compiler writer should know about programmers (2015) [pdf]

#4

Post by Joker_vD »

zephen wrote: post #3

Here's a cogent argument that any decision by compiler writers that they can do whatever they wish whenever they encounter an "undefined behavior" construct is rubbish: https://www.yodaiken.com/2021/05/19/undefined-behavior-in-c-... And here's a cautionary tale of how a compiler writer doing whatever they wish once they encounter undefined behavior makes debugging intractable: https://www.quora.com/What-is-the-most-s…

Wow, that's a very torturous reading of a specific line in a standard. And it doesn't really matter what Yodaiken thinks this line means because standard is written by C implementers for (mostly) C implementers. So if C compile writers think this line means they can use UB for optimizing purposes, then that's what it means.

Yeah, I know it breaks the common illusion among the C programmers that they're "close to the bare metal", but illusions should be dispersed, not indulged. The C programmers program for the abstract C machine which is then mediated by the C compilers into machine code the way the implementers of C compilers publicly documented.

Re: What every compiler writer should know about programmers (2015) [pdf]

#5

Post by Joker_vD »

In the end it all boils to a very simple argument. The C programmers want the C compilers to behave one way, the C implementers want the C compilers to behave the other way. Since the power structure is what it is — the C implementers are the ones who write the C standard and are the ones who actually get to implement the C compilers — the C compilers do, and will, behave the way the C implementers want them to.

In this situation the C programmers can either a) accept that they're programming in a language that exists as it exists, not as they'd like it to exist; b) angrily deny a); or c) switch to some other system-level language with defined semantics.

Re: What every compiler writer should know about programmers (2015) [pdf]

#6

Post by seg_lol »

Joker_vD wrote: post #5

In the end it all boils to a very simple argument. The C programmers want the C compilers to behave one way, the C implementers want the C compilers to behave the other way. Since the power structure is what it is — the C implementers are the ones who write the C standard and are the ones who actually get to implement the C compilers — the C compilers do, and will, behave the way the C implementers want them to. In t…

How about we agree on the ABI and everyone can have their own C compiler. Everyone C's the world through their own lenses.

Re: What every compiler writer should know about programmers (2015) [pdf]

#8

Post by WalterBright »

Joker_vD wrote: post #5

In the end it all boils to a very simple argument. The C programmers want the C compilers to behave one way, the C implementers want the C compilers to behave the other way. Since the power structure is what it is — the C implementers are the ones who write the C standard and are the ones who actually get to implement the C compilers — the C compilers do, and will, behave the way the C implementers want them to. In t…

> behave the way the C implementers want them to

If you don't please your users, you won't have any users.

Re: What every compiler writer should know about programmers (2015) [pdf]

#10

Post by chc4 »

Joker_vD wrote: post #4
zephen wrote: post #3

Here's a cogent argument that any decision by compiler writers that they can do whatever they wish whenever they encounter an "undefined behavior" construct is rubbish: https://www.yodaiken.com/2021/05/19/undefined-behavior-in-c-... And here's a cautionary tale of how a compiler writer doing whatever they wish once they encounter undefined behavior makes debugging intractable: https://www.quora.com/What-is-the-most-s…

Wow, that's a very torturous reading of a specific line in a standard. And it doesn't really matter what Yodaiken thinks this line means because standard is written by C implementers for (mostly) C implementers. So if C compile writers think this line means they can use UB for optimizing purposes, then that's what it means. Yeah, I know it breaks the common illusion among the C programmers that they're "close to the…

Yeah, this is basically Sovereign Citizen-tier argumentation: through some magic of definitions and historical readings and arguing about commas, I prove that actually everyone is incorrect. That's not how programming languages work! If everyone for 10+ years has been developing compilers with some definition of undefined behavior, and all modern compilers use undefined behavior in order to drive optimization passes which depend on those invariants, there is no possible way to argue that they're wrong and you know the One True C Programming Language interpretation instead.

Moreover, compiler authors don't just go out maliciously trying to ruin programs through finding more and more torturous undefined behavior for fun: the vast majority of undefined behavior in C are things that if a compiler wasn't able to assume were upheld by the programmer would inhibit trivial optimizations that the programmer also expects the compiler to be able to do.

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