C++ deprecates some operations on volatile objects
Programming do-gooders sometimes fall into the trap of thinking that banning the use of a problematic language construct removes the possibility of the problems associated with that construct’s usage construct from occurring. The do-gooders overlook the fact that developers use language constructs because they solve a coding need, and that banning usage does not make the coding need go away. If a particular usage is banned, then developers have to come up with an alternative to handle their coding need. The alternative selected may have just as many, or more, problems associated with its use as the original usage.
The C++ committee has fallen into this do-gooder trap by deprecating the use of some unary operators (i.e., ++
and --
) and compound assignment operators (e.g., +=
and &=
) on objects declared with the volatile
type-specifier. The new wording appears in the 2020 version of the C++ Standard; see sections 7.6.1.5, 7.6.2.2, 7.6.19, and 9.6.
Listing a construct as being deprecated gives notice that it might be removed in a future revision of the standard (languages committees tend to accumulate deprecated constructs and rarely actually remove a construct; breaking existing code is very unpopular).
What might be problematic about objects declared with the volatile
type-specifier?
By declaring an object with the volatile
type-specifier a developer is giving notice that its value can change through unknown mechanisms at any time. For instance, an array may be mapped to the memory location where the incoming bytes from a communications port are stored, or the members of a struct
may represent the various status and data information relating to some connected hardware device.
The presence of volatile in an object’s declaration requires that the compiler not optimise away assignments or accesses to said object (because such assignments or accesses can have effects unknown to the compiler).
volatile int k = 0; int i = k, // value of k not guaranteed to be 0 j = k; // value of k may have changed from that assigned to i if (i != j) printf("The value of k changed from %d to %d\n", i, j); |
volatile int k = 0; int i = k, // value of k not guaranteed to be 0 j = k; // value of k may have changed from that assigned to i if (i != j) printf("The value of k changed from %d to %d\n", i, j);
If, at some point in the future, developers cannot rely on code such as k+=3;
being supported by the compiler, what are they to do?
Both the C and C++ Standards state:
“The behavior of an expression of the form E1 op = E2
is equivalent to E1 = E1 op E2
except that E1
is evaluated only once.”
So the code k=k+3;
cannot be relied upon to have the same effect as k+=3;
.
One solution, which does not make use of any deprecated language constructs, is:
volatile int k; int temp; /* ... */ temp=k; temp+=3; k=temp; |
volatile int k; int temp; /* ... */ temp=k; temp+=3; k=temp;
In what world is the above code less problematic than writing k+=3;
?
I understand that in the C++ world there are templates, operator overloading, and various other constructs that can make it difficult to predict how many times an object might be accessed. The solution is to specify the appropriate behavior for volatile objects in these situations. Simply deprecating them for some operators is all cost for no benefit.
We can all agree that the use of volatile
has costs and benefits. What is WG21’s (the ISO C++ Committee) cost/benefit analysis for deprecating this usage?
The WG21 proposal P1152, “Deprecating volatile”, claims that it “… preserves the useful parts of volatile, and removes the dubious / already broken ones.”
The proposal is essentially a hatchet job, with initial sections written in the style of the heroic fantasy novel The Name of the Wind, where “…kinds of magic are taught in the university as academic disciplines and have daily-life applications…”; cut-and-pasting of text from WG14 (ISO C committee) documents and C++17 adds bulk. Various issues unrelated to the deprecated constructs are discussed, and it looks like more thought is needed in some of these areas.
Section 3.3, “When is volatile useful?”, sets the tone. The first four paragraphs enumerate what volatile
is not useful, before the fifth paragraph admits that “volatile
is nonetheless a useful concept to have …” (without listing any reasons for this claim).
How did this deprecation get accepted into the 2020 C++ Standard?
The proposal appeared in October 2018, rather late in the development timeline of a standard published in 2020; were committee members punch drunk by this stage, and willing to wave through what appears to be a minor issue? The document contains 1,662 pages of close text, and deprecation is only giving notice of something that might happen in the future.
Soon after the 2020 Standard was published, the pushback started. Proposal P2327, “De-deprecating volatile compound operations”, noted: “deprecation was not received too well in the embedded community as volatile is commonly used”. However, the authors don’t think that ditching the entire proposal is the solution, instead they propose to just de-deprecate the bitwise compound assignments (i.e., |=
, &=
, and ^=
).
The P2327 proposal contains some construct usage numbers, obtained by grep’ing the headers of three embedded SDKs. Unsurprisingly, there were lots of bitwise compound assignments (all in macros setting various flags).
I used Coccinelle to detect actual operations on volatile objects in the Silabs Gecko SDK C source (one of the SDKs measured in the proposal; semgrep handles C and C++, but does not yet fully handle volatile
). The following table shows the number of occurrences of each kind of language construct on a volatile object (code and data):
Construct Occurrences V++ 83 V-- 5 ++V 9 --V 2 bit assign 174 arith assign 27 |
Construct Occurrences V++ 83 V-- 5 ++V 9 --V 2 bit assign 174 arith assign 27
Will the deprecated volatile usage appear in C++23? Probably, purely because the deadline for change has passed. Given WG21’s stated objective of a 3-year iteration, the debate will have to wait for work on to start on C++26.
The increment and decrement deprecation remains. In response to National Body comments on balloting of the draft C++23 document, it was “resolved to un-deprecate all volatile compound assignments.” Thanks to Jonathan Wakely for the correction.
volatile handling sometimes overly volatile
The contents of some storage locations, used by a program, might be modified outside of the control of that program, e.g., a real-time clock or the input port of a communications device. In some cases writing to particular storage locations has an external effect, e.g., a sequence of bits is sent down a communications channel. This kind of behavior commonly occurs in embedded systems
C and C++ support the existence of variables that have been mapped to such storage locations through the use of the volatile type qualifier.
volatile long flag; volatile time_t timer; struct { int f1 : 2; volatile int f2 : 2; int f3 : 3; } x; |
volatile long flag; volatile time_t timer; struct { int f1 : 2; volatile int f2 : 2; int f3 : 3; } x;
When a variable is declared using volatile compilers must assume that its value can change in ways unknown to the compiler and that storing values into such a variable can have external effects. Consequently almost all optimizations involving such variables are off limits. Idioms such as timer = timer;
are used to reset or refresh timers and are not dead code.
Volatile is a bit of an inconvenience for writers of code optimizers, requiring them to add checks to make sure the expression or statement they want to attempt to optimize does not contain a volatile qualified variable. In many environments the semantics of volatile are not applicable, which means that bugs in optimizers have a much smaller chance of being detected than faults in other language constructs.
There have been a number of research projects investigating the use of the const qualifier, but as far as I know only one that has investigated volatile. The ‘volatile’ project found that all of the compilers tested generated incorrect code for some of the tests. License agreements prevented the researchers giving details for some compilers. One interesting observation for gcc was that the number of volatile related faults increased with successive compiler releases; it looks like additional optimizations were being implemented and these were not checking for variables being volatile qualified.
One practical output from the project was a compiler stress tester targeting volatile qualified variables. The code generated by stress testers often causes compilers to crash and hang and the researchers reported the same experience with this tool.
There is one sentence in the C Standard whose overly broad wording is sometimes a source of uncertainty: What constitutes an access to an object that has volatile-qualified type is implementation-defined. This sentence applies in two cases: datatypes containing lots of bytes and bit-fields.
To save space/money/time/power some processors access storage a byte, or half-word, at a time. This means that, for instance, an access to a 32-bit storage location may occur as two separate 16-bit operations; from the volatile perspective does that constitute two accesses?
Most processors do not contain instructions capable of loading/storing a specified sequence of bits from a byte. This means that accesses to bit-fields involve one or more bytes being loaded and the appropriate bits extracted. In the definition of x
above an access to field f1
is likely to result in field f2
(and f3
) being accessed; from the volatile perspective does that constitute two accesses?
Unfortunately it is very difficult to obtain large quantities of source code for programs aimed at the embedded systems market. This means that it is not possible to obtain reliable information on common usage patterns of volatile variables. If anybody knows of any such code base please let me know.
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