[libc] Fix printf %f rounding condition

[michaelrj]

Summary

When running the tbin2dec tests I found a rounding error in my code.
Upon inspection I realized it was due to a lack of parenthesis when
calculating the number of trailing digits. This patch fixes that mistake
and adds unit tests to catch regressions in future.

libc/src/stdio/printf_core/float_dec_converter.h

int write_zeroes(uint32_t num_zeroes) {
     return 0;
   }
 
   int right_pad() {
     return padding_writer.write_right_padding(writer, total_digits);
   }
 };
 
+// This implementation is based on the Ryu Printf algorithm by Ulf Adams:
+// Ulf Adams. 2019. Ryū revisited: printf floating point conversion.
+// Proc. ACM Program. Lang. 3, OOPSLA, Article 169 (October 2019), 23 pages.
+// https://doi.org/10.1145/3360595
 template <typename T, cpp::enable_if_t<cpp::is_floating_point_v<T>, int> = 0>
 LIBC_INLINE int convert_float_decimal_typed(Writer *writer,
                                             const FormatSection &to_conv,
                                             fputil::FPBits<T> float_bits) {
   // signed because later we use -MANT_WIDTH
   constexpr int32_t MANT_WIDTH = fputil::MantissaWidth<T>::VALUE;
   bool is_negative = float_bits.get_sign();
   int exponent = float_bits.get_exponent();
for (; i < blocks; ++i) {
         uint32_t last_digit = 0;
         for (uint32_t k = 0; k < BLOCK_SIZE - maximum; ++k) {
           last_digit = digits % 10;
           digits /= 10;
         }
         RoundDirection round;
         // Is m * 10^(additionalDigits + 1) / 2^(-exponent) integer?
         const int32_t requiredTwos =
-            -exponent - MANT_WIDTH - static_cast<int32_t>(precision) - 1;
+            -(exponent - MANT_WIDTH) - static_cast<int32_t>(precision) - 1;
         const bool trailingZeros =
             requiredTwos <= 0 ||
             (requiredTwos < 60 &&
              multiple_of_power_of_2(float_bits.get_explicit_mantissa(),
                                     static_cast<uint32_t>(requiredTwos)));
         switch (fputil::get_round()) {
         case FE_TONEAREST:
           // Round to nearest, if it's exactly halfway then round to even.
LIBC_INLINE int convert_float_dec_exp_typed(Writer *writer,
   if (maximum == last_block_size) {
     BlockInt extra_block = float_converter.get_block(cur_block - 1);
     last_digit = extra_block / ((MAX_BLOCK / 10) + 1);
   }
 
   RoundDirection round;
   // Is m * 10^(additionalDigits + 1) / 2^(-exponent) integer?
   const int32_t requiredTwos =
-      -exponent - MANT_WIDTH - static_cast<int32_t>(precision) - 1;
+      -(exponent - MANT_WIDTH) - static_cast<int32_t>(precision) - 1;
   const bool trailingZeros =
       requiredTwos <= 0 ||
       (requiredTwos < 60 &&
        multiple_of_power_of_2(float_bits.get_explicit_mantissa(),
                               static_cast<uint32_t>(requiredTwos)));
   switch (fputil::get_round()) {
   case FE_TONEAREST:
     // Round to nearest, if it's exactly halfway then round to even.
if (maximum == last_block_size) {
     BlockInt extra_block = float_converter.get_block(cur_block - 1);
     last_digit = extra_block / ((MAX_BLOCK / 10) + 1);
   }
 
   // TODO: unify this code across the three float conversions.
   RoundDirection round;
   // Is m * 10^(additionalDigits + 1) / 2^(-exponent) integer?
   const int32_t requiredTwos =
-      -exponent - MANT_WIDTH - static_cast<int32_t>(exp_precision) - 1;
+      -(exponent - MANT_WIDTH) - static_cast<int32_t>(exp_precision) - 1;
   // TODO: rename this variable to remove confusion with trailing_zeroes
   const bool trailingZeros =
       requiredTwos <= 0 ||
       (requiredTwos < 60 &&
        multiple_of_power_of_2(float_bits.get_explicit_mantissa(),
                               static_cast<uint32_t>(requiredTwos)));
   switch (fputil::get_round()) {
   case FE_TONEAREST:

libc/test/src/stdio/sprintf_test.cpp

*/
   ASSERT_STREQ_LEN(written, buff, "0.00000");
 
   written = __llvm_libc::sprintf(buff, "%.5f", 1.008);
   ASSERT_STREQ_LEN(written, buff, "1.00800");
 
   written = __llvm_libc::sprintf(buff, "%.5f", 1.008e3);
   ASSERT_STREQ_LEN(written, buff, "1008.00000");
 
+  // Found with the help of Fred Tydeman's tbin2dec test.
+  written = __llvm_libc::sprintf(buff, "%.1f", 0x1.1000000000006p+3);
+  ASSERT_STREQ_LEN(written, buff, "8.5");
+
+  written = __llvm_libc::sprintf(buff, "%.0f", 0x1.1000000000006p+3);
+  ASSERT_STREQ_LEN(written, buff, "9");
+
   // Subnormal Precision Tests
 
   written = __llvm_libc::sprintf(buff, "%.310f", 0x1.0p-1022);
   ASSERT_STREQ_LEN(
       written, buff,
       "0."
       "000000000000000000000000000000000000000000000000000000000000000000000000"
       "000000000000000000000000000000000000000000000000000000000000000000000000"