rsmith added inline comments.
================
Comment at: lib/Sema/SemaChecking.cpp:110
+/// are usually useless
+static unsigned AdjustPrecision(unsigned precision) {
+ return (precision * 59 + 195) / 196;
----------------
xbolva00 wrote:
> xbolva00 wrote:
> > xbolva00 wrote:
> > > craig.topper wrote:
> > > > scanon wrote:
> > > > > erichkeane wrote:
> > > > > > Hmm.... I don't terribly understand how this function works. Also,
> > > > > > comment above needs to end in a period.
> > > > > >
> > > > > > Can you elaborate further as to how this works? Those are 3 pretty
> > > > > > suspect looking magic numbers...
> > > > > It's attempting to compute the number of good base-10 digits (59/196
> > > > > ~= log2(10)). We should really just make APFloat print the shortest
> > > > > round-trippable digit sequence instead. Yes, this is tricky to
> > > > > implement, but we don't need to implement it. There are two recent
> > > > > high-quality implementations available, which are both significantly
> > > > > faster than previous algorithms: Ryu and Swift's
> > > > > (https://github.com/apple/swift/blob/master/stdlib/public/runtime/SwiftDtoa.cpp).
> > > > > Swift's has the virtue of already being used in LLVM-family
> > > > > languages and having a tidy single-file implementation, but either
> > > > > would be perfectly usable, I think.
> > > > >
> > > > > Neither supports float128 yet, but we could simply drop them in for
> > > > > float, double, and float80.
> > > > Function names should start with lower case I think?
> > > float80 has precision = 64 so we can put MAX value of unsigned long long
> > > into it with no issues, or?
> > Other static functions here use upper case, e.g. "PrettyPrintInRange", so
> > I follow it :)
> > float80 has precision = 64 so we can put MAX value of unsigned long long
> > into it with no issues, or?
>
> ah, int128_t
LLVM will need to carry an implementation of the "shortest round-trippable
digit sequence" soon regardless of what we do here, because it's [part of the
C++17 standard library](https://en.cppreference.com/w/cpp/utility/to_chars). We
should aim for an implementation that can be shared by `APFloat` and libc++.
https://reviews.llvm.org/D52835
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