[libc] Make printf decimal long doubles use hex

[michaelrj]

Summary

Decimal long doubles are not commonly used, and aren't currently
supported by the algorithm used for decimal float conversions. To avoid
giving incorrect answers, this patch adds a temporary exception to print
long doubles in hexadecimal even when decimal is requested.

[sivachandra]

libc/src/stdio/printf_core/float_hex_converter.h

32–34

I think, whats going on here is that you are converting the other conversion names, e, f and g to a and E, F and G to A. If this is correct, can you add a comment?

[michaelrj]

add comment to clarify the variable named "a"

libc/src/stdio/printf_core/converter.cpp

 
 namespace __llvm_libc {
 namespace printf_core {
 
 int convert(Writer *writer, const FormatSection &to_conv) {
   if (!to_conv.has_conv)
     return writer->write(to_conv.raw_string);
 
+#ifndef LIBC_COPT_PRINTF_DISABLE_FLOAT
+  // TODO(michaelrj): Undo this once decimal long double support is done.
+  if (to_conv.length_modifier == LengthModifier::L) {
+    switch (to_conv.conv_name) {
+    case 'f':
+    case 'F':
+    case 'e':
+    case 'E':
+    case 'g':
+    case 'G':
+      return convert_float_hex_exp(writer, to_conv);
+    default:
+      break;
+    }
+  }
+#endif // LIBC_COPT_PRINTF_DISABLE_FLOAT
+
   switch (to_conv.conv_name) {
   case '%':
     return writer->write("%");
   case 'c':
     return convert_char(writer, to_conv);
   case 's':
     return convert_string(writer, to_conv);
   case 'd':

libc/src/stdio/printf_core/float_hex_converter.h

 namespace __llvm_libc {
 namespace printf_core {
 
 using MantissaInt = fputil::FPBits<long double>::UIntType;
 
 LIBC_INLINE int convert_float_hex_exp(Writer *writer,
                                       const FormatSection &to_conv) {
   // All of the letters will be defined relative to variable a, which will be
-  // the appropriate case based on the name of the conversion.
-  // Since the name of the conversion is also 'a', we can just use it directly.
-  const char a = to_conv.conv_name;
+  // the appropriate case based on the name of the conversion. This converts any
+  // conversion name into the letter 'a' with the appropriate case.
+  const char a = (to_conv.conv_name & 32) | 'A';
 
   bool is_negative;
   int exponent;
   MantissaInt mantissa;
   bool is_inf_or_nan;
   uint32_t mantissa_width;
   int exponent_bias;
   if (to_conv.length_modifier == LengthModifier::L) {