[PATCH] D54737: [clang-tidy] Add the abseil-duration-comparison check

[Sam McCall via Phabricator via cfe-commits]

sammccall added inline comments.


================
Comment at: clang-tidy/abseil/DurationComparisonCheck.cpp:25
+static llvm::Optional<DurationScale> getScaleForInverse(llvm::StringRef Name) {
+  static const std::unordered_map<std::string, DurationScale> ScaleMap(
+      {{"ToDoubleHours", DurationScale::Hours},
----------------
aaron.ballman wrote:
> sammccall wrote:
> > hwright wrote:
> > > JonasToth wrote:
> > > > hwright wrote:
> > > > > JonasToth wrote:
> > > > > > I think this could be made a `DenseMap` with just the right amount of dense entries. 12 elements seems not too much to me.
> > > > > > Does the key-type need to be `std::string`, or could it be `StringRef`(or `StringLiteral` making everything `constexpr` if possible)?
> > > > > > Is there some strange stuff with dangling pointers or other issues going on?
> > > > > Conceptually, this could easily be `constexpr`, but my compiler doesn't seem to want to build something which is called `static constexpr llvm::DenseMap<llvm::StringRef, DurationScale>`.  It's chief complaint is that such a type has a non-trivial destructor. Am I using this correctly?
> > > > I honestly never tried to make a `constexpr DenseMap<>` but it makes sense it is not possible, as `DenseMap` is involved with dynamic memory after all, which is not possible with `constexpr` (yet). So you were my test-bunny ;)
> > > > 
> > > > I did reread the Data-structures section in the LLVM manual, i totally forgot about `StringMap` that maps strings to values.
> > > > `DenseMap` is good when mapping small values to each other, as we do here (`const char* -> int (whatever the enum deduces too)`), which would fit.
> > > > `StringMap` does allocations for the strings, which we don't need.
> > > > 
> > > > `constexpr` aside my bet is that `DenseMap` fits this case the better, because we can lay every thing out and never touch it again. Maybe someone else has better arguments for the decision :)
> > > > 
> > > > Soooooo: could you please try `static const llvm::DenseMap<llvm::StringRef, DurationScale>`? :)
> > > > (should work in theory: https://godbolt.org/z/Qo7Nv4)
> > > `static const llvm::DenseMap<llvm::StringRef, DurationScale>` works here. :)
> > Sorry for the drive-by...
> > This has a non-trivial destructor, so violates https://llvm.org/docs/CodingStandards.html#do-not-use-static-constructors at least in spirit.
> > 
> > Best to leak it (`auto &ScaleMap = *new const llvm::DenseMap<...>(...)`) to avoid the destructor issues. The constructor issues are already handled by making it function-local.
> I do not think this violates the coding standard -- that's talking mostly about global objects, not local objects, and is concerned about startup performance and initialization orders. I don't see any destructor ordering issues with this, so I do not think any of that applies here and leaking would be less than ideal.
There are three main issues with global objects that the coding standard mentions:
 - performance when unused. Not relevant to function-local statics, only constructed when used
 - static initialization order fiasco (constructors). Not relevant to function-local statics, constructed in well-defined order
 - static initialization order fiasco (destructors). Just as relevant to function-local statics as to any other object!

That's why I say it violates the rule in spirit: the destructor is global. https://isocpp.org/wiki/faq/ctors#construct-on-first-use-v2

> I don't see any destructor ordering issues with this
The reason these constructs are disallowed in coding guidelines is that humans can't see the problems, and they manifest in non-local ways :-(

> leaking would be less than ideal
Why? The current code will (usually) destroy the object when the program exits. This is a waste of CPU, the OS will free the memory.


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  https://reviews.llvm.org/D54737/new/

https://reviews.llvm.org/D54737