From: anton@mips.complang.tuwien.ac.at
Niklas Holsti writes:
>On 2025-11-05 7:17, Anton Ertl wrote:
>> Stallman obviously knew what to say about their semantics when he
>> added labels-as-values to GNU C with gcc 2.0.
>
>
>I don't know what Stallman said, or would have said if asked, but I
>guess something like "the semantics is a jump to the (address of the)
>label to which the value refers", which is machine-level semantics and
>not semantics in the abstract C machine.
You can look at his specification in the documentation of, say, 7th
edition Unix (where Ritchie apparently took the effort to document
semantics), and see how he specified that. I doubt he specified
"semantics in the abstract C machine", but I expect that he specified
semantics at the C level.
Concerning how Stallman documented it, you can look at the gcc
documentation from 2.0 until Stallman passed maintainership on
(gcc-2.7?).
If you look at the curent documentation
, it talks
about the "address of a label" and "jump to one", which you might
consider to be a machine-level description. You can also describe
this at a C source level or "C abstract machine" level, but I don't
expect the description to become any clearer.
>The problem in the abstract C machine is a "goto label-value" statement
>where the label-value refers to a label in a different function. Does
>gcc prevent that at compile time? If not, I would expect the semantics
>to be Undefined Behavior, the usual cop-out when nothing useful can be said.
The gcc documentation says:
|You may not use this mechanism to jump to code in a different
|function. If you do that, totally unpredictable things happen.
- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
Mitch Alsup,
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
