Re: Pre-main construction order in modules

On 01.04.2025 10:26, Muttley@DastardlyHQ.org wrote:

On Mon, 31 Mar 2025 19:42:48 -0400
James Kuyper <jameskuyper@alumni.caltech.edu> wibbled:

On 31.03.2025 17:01, Muttley@DastardlyHQ.org wrote:

On Mon, 31 Mar 2025 15:39:53 +0300
Paavo Helde <eesnimi@osa.pri.ee> wibbled:

On 31.03.2025 12:09, Muttley@DastardlyHQ.org wrote:
"If [initialization] is deferred, it strongly happens before any
non-initialization odr-use of any non-inline function or non-inline
variable defined in the same translation unit as the variable to be
initialized."

>
Who writes this stuff? Its borderline gibberish.

>
Keep in mind that the key thing that makes this seem like gibberish is
the use of precisely defined technical jargon, which is used for the
same reason that jargon is used in many other contexts: it has a more
precisely specified meaning than more informal wording would have had.
>
Key pieces of jargon: "strongly happens before", "non-initialization",

 
"strongly happens before" is genuine gibberish. Either something happens before
or it doesn't, the adverb is entirely superfluous.

Terms are explicitly defined by the C++ standard precisely because their
meaning cannot be inferred by using ordinary English to interpret the
words that make them up. Both "happens before" and "strongly happens
before" are separate pieces of C++ jargon, and the requirements for
qualifying as "strongly happens before" are in fact stronger than those
for "happens before".

"An evaluation A happens before an evaluation B (or, equivalently, B
happens after A) if:
(10.1)— A is sequenced before B, or
(10.2)— A inter-thread happens before B.
The implementation shall ensure that no program execution demonstrates a
cycle in the “happens before” relation. [Note: This cycle would
otherwise be possible only through the use of consume operations. — end
note]" (6.9.2.1p10).

the first occurrence of "happens before" in that clause is in italics,
an ISO convention indicating that it is a piece of specialized jargon
whose definition is provided by the sentence in which it occurs. Note
that "sequenced before" and "inter-thread happens before" are two other
pieces of standard-defined jargon.

"An evaluation A strongly happens before an evaluation D if, either
(12.1)— A is sequenced before D, or
(12.2)— A synchronizes with D, and both A and D are sequentially
consistent atomic operations (31.4), or
(12.3)— there are evaluations B and C such that A is sequenced before B,
B simply happens before C, and C is sequenced before D, or
(12.4)— there is an evaluation B such that A strongly happens before B,
and B strongly happens before D.
[Note: Informally, if A strongly happens before B, then A appears to be
evaluated before B in all contexts. Strongly happens before excludes
consume operations. — end note]"

The first occurrence of "strongly happens before" in that clause is also
italicized.

Note that "A is sequenced before D" would be sufficient to ensure that
both "A happens before D" and "A strongly happens before D" are true.
The difference between the two terms only shows up if A is not sequenced
before D. In that case, at a minimum you must have "A inter-thread
happens before D", but is not synchronized with D.

For instance A inter-thread happens before D if it is dependency-ordered
before D. (6.9.2.1p9). This could happen if A performs a release
operation on an atomic object M , and, in another thread, D performs a
consume operation on M and reads the value written by A (6.9.2.1p8).

In that case, the requirements associated with "A happens before D" must
be met, but those associated with "A strongly happens before D" need not be.

Do you need an explanation of the parts of the above explanation that
use the terms "release" and "consume"? I have little experience with
multi-threaded code - someone else might be able to explain those better
than I can.