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

[Steve Canon via Phabricator via cfe-commits]

scanon added inline comments.


================
Comment at: lib/Sema/SemaChecking.cpp:110
+/// are usually useless
+static unsigned AdjustPrecision(unsigned precision) {
+  return (precision * 59 + 195) / 196;
----------------
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.


https://reviews.llvm.org/D52835