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.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D54737/new/
https://reviews.llvm.org/D54737
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