## +24 ## A New Dragon in the Den: Fast Conversion From Floating-Point Numbers ## CASSIO NERI ## 2024 September 15 - 20 WARNING: This PDF poorly reproduces the slides shown at the talk. You are strongly It's not a seven-headed beast. It's not rocket science. ## A New Dragon in the Den ## Fast conversion from floating-point numbers ## Cassio Neri (Independent Researcher) CppCon 2024 - Aurora # Statistical

0 ); } } // namespace N1 int main() { N1::g(); // Ill-formed, no conversion available N1::h(); // Ill-formed, no conversion available } ## (Un-)Qualified Name Lookup void f( double ); // (1) of candidates that match the number of given arguments and that could be called (with or without conversion). 2. Find the best match from the list of viable candidates: Determine a single function that arguments void f( int, double ); // More parameters than arguments void f( std::string ); // No conversion available void f( floating_point auto ); // Violated constraints int main() { f( 42 ); // Call

implicit conversion ## Overload Resolution ## • Type Conversions example of an implicit conversion char str[] = "ABC"; int data = str[0]; // data will equal 65 ## o explicit conversion ■ static_cast another type of explicit conversion called an functional cast if (std::string("root") == current_directory) { // do something } ## Overload Resolution ## • Standard Conversion Categories ## ☐ exact match (1) ■ no conversion is required o lvalue transformations (2) ☑ lvalue to rvalue conversion (based on value categories) ☑ array to pointer conversion ■ function to pointer conversion qualification

Calling f(int) requires no conversion at all. • It’s an exact match. - Calling f(long int) requires converting int into "long int". • This is an integral conversion. • Calling f(char *) requires requires converting int into “pointer to char”. • This is a pointer conversion. ## Exact Matches Always Win ■ The compiler prefers “cheap” conversions. ■ Exact matches are the cheapest of all. Thus, choice:ConversionRankno conversionExact Match “cheap”array-to-pointer conversionqualification conversionetc

Fast linear median-of-seven filter • Fast small-kernel convolution • Faster (?) UTF-8 to UTF-32 conversion (with AVX2) ## • No heavy code, but lots of pictures • Thinking "vertically" ## Small-Kernel jpg) ## UTF-8 to UTF-32 Conversion Algorithm ## The UTF-8 Decoding DFA  ## The Basic Conversion Algorithm (UTF-8 to UTF-32)  ## The Basic Conversion Algorithm (UTF-8 to UTF-32) KEWB_ALIGN_FN std::ptrdiff_t UtfUtils::BasicConvert(char8_t const*

System Information Functions ..... 202 9.20. System Administration Functions ..... 209 Type Conversion ..... 215 10.1. Overview ..... 215 10.2. Operators ..... 216 10.3. Functions ..... 219 Character Sets .....358 21.2.2. Setting the Character Set .....361 21.2.3. Automatic Character Set Conversion Between Server and Client .....362 21.2.4. Further Reading .....364 22. Routine Database Maintenance Reference.....793 I. SQL Commands.....795 ABORT.....796 ALTER AGGREGATE.....798 ALTER CONVERSION.....800 ALTER DATABASE.....802 ALTER DOMAIN.....804 ALTER FUNCTION.....807 ALTER GROUP

System Information Functions ..... 202 9.20. System Administration Functions ..... 209 Type Conversion ..... 215 10.1. Overview ..... 215 10.2. Operators ..... 216 10.3. Functions ..... 219 Character Sets .....358 21.2.2. Setting the Character Set .....361 21.2.3. Automatic Character Set Conversion Between Server and Client .....362 21.2.4. Further Reading .....364 22. Routine Database Maintenance Reference.....793 I. SQL Commands.....795 ABORT.....796 ALTER AGGREGATE.....798 ALTER CONVERSION.....800 ALTER DATABASE.....802 ALTER DOMAIN.....804 ALTER FUNCTION.....807 ALTER GROUP

System Information Functions.....171 9.20. System Administration Functions.....176 10. Type Conversion.....179 10.1. Overview.....179 10.2. Operators.....180 10.3. Functions.....183 10.4. Value Sets ..... 292 20.2.2. Setting the Character Set ..... 293 20.2.3. Automatic Character Set Conversion Between Server and Client ..... 294 20.2.4. Further Reading ..... 296 21. Routine Database Reference ..... 632 I. SQL Commands ..... 634 ABORT ..... 635 ALTER AGGREGATE ..... 637 ALTER CONVERSION ..... 639 ALTER DATABASE ..... 640 ALTER DOMAIN ..... 642 ALTER FUNCTION ..... 644 ALTER

System Information Functions ..... 181 9.20. System Administration Functions ..... 186 10. Type Conversion ..... 189 10.1. Overview ..... 189 10.2. Operators ..... 190 10.3. Functions ..... 193 Character Sets .....309 20.2.2. Setting the Character Set .....310 20.2.3. Automatic Character Set Conversion Between Server and Client .....311 20.2.4. Further Reading .....314 21. Routine Database Maintenance Reference ..... 669 I. SQL Commands ..... 671 ABORT ..... 672 ALTER AGGREGATE ..... 674 ALTER CONVERSION ..... 676 ALTER DATABASE ..... 678 ALTER DOMAIN ..... 680 ALTER FUNCTION ..... 683 ALTER