[PATCH] D52835: [Diagnostics] Check integer to floating point number implicit conversions

[Dávid Bolvanský via Phabricator via cfe-commits]

xbolva00 added inline comments.


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Comment at: lib/Sema/SemaChecking.cpp:110
+/// are usually useless
+static unsigned AdjustPrecision(unsigned precision) {
+  return (precision * 59 + 195) / 196;
----------------
rsmith wrote:
> 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++.
You are right. But anyway, it should not block this patch, no? I can put TODO comment there.


https://reviews.llvm.org/D52835