On Thu, 4 Jan 2007, Segher Boessenkool wrote:
>
> > (in which case, nearly all real-world code is broken)
>
> Not "nearly all" -- but lots of code, yes.
I wouldn't say "lots of code". I would say "all real projects".
NOBODY will guarantee you that they follow all standards to the letter.
Some use compiler extensions knowingly, but pretty much _everybody_ ends
up depending on subtle issues without even realizing it. It's almost
impossible to write a real program that has no bugs, and if they don't
show up in testing (because the compiler didn't generate buggy assembly
code from source code that had the _potential_ for bugs), they often won't
get fixed.
The kernel does things like compare pointers across objects, and the
kernel EXPECTS it to work. I seriously doubt that the kernel is even
unusual in this. The common way to avoid AB-BA deadlocks in any threaded
code (whether kernel or user space) is to just take two locks in a
specific order, and the common way to do that for locks of the same type
is simply to compare the addresses).
The fact that this is "undefined" behaviour matters not a _whit_. Not for
the kernel, and I bet not for a lot of other applications either.
So "nearly all" is probably _understating_ things rather than overstating
it as you claim. Anybody who thinks that they have proven the correctness
of their program is likely lying. It's a good thing if they have _tested_
all the code-paths, but they've invariably been tested with a compiler
that doesn't go out of its way to try to generate "legal but idiotic"
code. So the testing won't generally find cases where the compiler may
have been _allowed_ to do something else.
The end result: any nontrivial project always has dodgy code. Because
people simply don't write perfect code.
Compiler people who don't realize this aren't compiler people. They're
academics involved with mental masturbation.
Linus
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