[libc] Enable dyadic float for float printf (#110765)

Dyadic floats were an existing option for float to string conversion,
but it had become stale. This patch fixes it up as well as adding proper
config options and test support. Due to the test changes this is a
followup to #110759

Author: michaelrj-google

libc/config/config.json

@@ -22,6 +22,14 @@
       "value": false,
       "doc": "Use large table for better printf long double performance."
     },
+    "LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_DYADIC_FLOAT": {
+      "value": false,
+      "doc": "Use dyadic float for faster and smaller but less accurate printf doubles."
+    },
+    "LIBC_CONF_PRINTF_FLOAT_TO_STR_NO_SPECIALIZE_LD": {
+      "value": false,
+      "doc": "Use the same mode for double and long double in printf."
+    },
     "LIBC_CONF_PRINTF_DISABLE_FIXED_POINT": {
       "value": false,
       "doc": "Disable printing fixed point values in printf and friends."

libc/docs/configure.rst

@@ -40,6 +40,8 @@ to learn about the defaults for your platform and target.
     - ``LIBC_CONF_PRINTF_DISABLE_INDEX_MODE``: Disable index mode in the printf format string.
     - ``LIBC_CONF_PRINTF_DISABLE_STRERROR``: Disable handling of %m to print strerror in printf and friends.
     - ``LIBC_CONF_PRINTF_DISABLE_WRITE_INT``: Disable handling of %n in printf format string.
+    - ``LIBC_CONF_PRINTF_FLOAT_TO_STR_NO_SPECIALIZE_LD``: Use the same mode for double and long double in printf.
+    - ``LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_DYADIC_FLOAT``: Use dyadic float for faster and smaller but less accurate printf doubles.
     - ``LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE``: Use large table for better printf long double performance.
 * **"pthread" options**
     - ``LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT``: Default number of spins before blocking if a mutex is in contention (default to 100).

libc/src/__support/FPUtil/dyadic_float.h

@@ -357,7 +357,7 @@ template <size_t Bits> struct DyadicFloat {
     return as<T, /*ShouldSignalExceptions=*/false>();
   }
 
-  LIBC_INLINE explicit constexpr operator MantissaType() const {
+  LIBC_INLINE constexpr MantissaType as_mantissa_type() const {
     if (mantissa.is_zero())
       return 0;
 

libc/src/__support/float_to_string.h

@@ -20,6 +20,7 @@
 #include "src/__support/libc_assert.h"
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/config.h"
+#include "src/__support/sign.h"
 
 // This file has 5 compile-time flags to allow the user to configure the float
 // to string behavior. These were used to explore tradeoffs during the design
@@ -232,7 +233,7 @@ LIBC_INLINE UInt<MID_INT_SIZE> get_table_positive_df(int exponent, size_t i) {
   if (shift_amount < 0) {
     return 1;
   }
-  fputil::DyadicFloat<INT_SIZE> num(false, 0, 1);
+  fputil::DyadicFloat<INT_SIZE> num(Sign::POS, 0, 1);
   constexpr UInt<INT_SIZE> MOD_SIZE =
       (UInt<INT_SIZE>(EXP10_9)
        << (CALC_SHIFT_CONST + (IDX_SIZE > 1 ? IDX_SIZE : 0)));
@@ -242,16 +243,17 @@ LIBC_INLINE UInt<MID_INT_SIZE> get_table_positive_df(int exponent, size_t i) {
        0x89705f4136b4a597}};
 
   static const fputil::DyadicFloat<INT_SIZE> FIVE_EXP_MINUS_NINE(
-      false, -276, FIVE_EXP_MINUS_NINE_MANT);
+      Sign::POS, -276, FIVE_EXP_MINUS_NINE_MANT);
 
   if (i > 0) {
-    fputil::DyadicFloat<INT_SIZE> fives = fputil::pow_n(FIVE_EXP_MINUS_NINE, i);
+    fputil::DyadicFloat<INT_SIZE> fives =
+        fputil::pow_n(FIVE_EXP_MINUS_NINE, static_cast<uint32_t>(i));
     num = fives;
   }
   num = mul_pow_2(num, shift_amount);
 
   // Adding one is part of the formula.
-  UInt<INT_SIZE> int_num = static_cast<UInt<INT_SIZE>>(num) + 1;
+  UInt<INT_SIZE> int_num = num.as_mantissa_type() + 1;
   if (int_num > MOD_SIZE) {
     auto rem =
         int_num
@@ -339,23 +341,24 @@ LIBC_INLINE UInt<MID_INT_SIZE> get_table_negative_df(int exponent, size_t i) {
 
   int shift_amount = CALC_SHIFT_CONST - exponent;
 
-  fputil::DyadicFloat<INT_SIZE> num(false, 0, 1);
+  fputil::DyadicFloat<INT_SIZE> num(Sign::POS, 0, 1);
   constexpr UInt<INT_SIZE> MOD_SIZE =
       (UInt<INT_SIZE>(EXP10_9)
        << (CALC_SHIFT_CONST + (IDX_SIZE > 1 ? IDX_SIZE : 0)));
 
   constexpr UInt<INT_SIZE> TEN_EXP_NINE_MANT(EXP10_9);
 
-  static const fputil::DyadicFloat<INT_SIZE> TEN_EXP_NINE(false, 0,
+  static const fputil::DyadicFloat<INT_SIZE> TEN_EXP_NINE(Sign::POS, 0,
                                                           TEN_EXP_NINE_MANT);
 
   if (i > 0) {
-    fputil::DyadicFloat<INT_SIZE> tens = fputil::pow_n(TEN_EXP_NINE, i);
+    fputil::DyadicFloat<INT_SIZE> tens =
+        fputil::pow_n(TEN_EXP_NINE, static_cast<uint32_t>(i));
     num = tens;
   }
   num = mul_pow_2(num, shift_amount);
 
-  UInt<INT_SIZE> int_num = static_cast<UInt<INT_SIZE>>(num);
+  UInt<INT_SIZE> int_num = num.as_mantissa_type();
   if (int_num > MOD_SIZE) {
     auto rem =
         int_num

libc/src/stdio/printf_core/CMakeLists.txt

@@ -10,6 +10,12 @@ endif()
 if(LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE)
   list(APPEND printf_config_copts "-DLIBC_COPT_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE")
 endif()
+if(LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_DYADIC_FLOAT)
+  list(APPEND printf_config_copts "-DLIBC_COPT_FLOAT_TO_STR_USE_DYADIC_FLOAT")
+endif()
+if(LIBC_CONF_PRINTF_FLOAT_TO_STR_NO_SPECIALIZE_LD)
+  list(APPEND printf_config_copts "-DLIBC_COPT_FLOAT_TO_STR_NO_SPECIALIZE_LD")
+endif()
 if(LIBC_CONF_PRINTF_DISABLE_FIXED_POINT)
   list(APPEND printf_config_copts "-DLIBC_COPT_PRINTF_DISABLE_FIXED_POINT")
 endif()

libc/test/src/stdio/CMakeLists.txt

@@ -116,6 +116,9 @@ add_libc_test(
 if(LIBC_CONF_PRINTF_DISABLE_FLOAT)
   list(APPEND sprintf_test_copts "-DLIBC_COPT_PRINTF_DISABLE_FLOAT")
 endif()
+if(LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_DYADIC_FLOAT)
+  list(APPEND sprintf_test_copts "-DLIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION")
+endif()
 if(LIBC_CONF_PRINTF_DISABLE_INDEX_MODE)
   list(APPEND sprintf_test_copts "-DLIBC_COPT_PRINTF_DISABLE_INDEX_MODE")
 endif()

libc/test/src/stdlib/CMakeLists.txt

@@ -168,13 +168,17 @@ add_libc_test(
     .strtol_test_support
 )
 
+if(LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_DYADIC_FLOAT)
+  list(APPEND strfrom_test_copts "-DLIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION")
+endif()
+
 add_header_library(
   strfrom_test_support
   HDRS
-  StrfromTest.h
+    StrfromTest.h
   DEPENDS
-  libc.src.__support.CPP.type_traits
-  libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.CPP.type_traits
+    libc.src.__support.FPUtil.fp_bits
 )
 
 add_libc_test(
@@ -186,6 +190,8 @@ add_libc_test(
   DEPENDS
     .strfrom_test_support
     libc.src.stdlib.strfromf
+  COMPILE_OPTIONS
+    ${strfrom_test_copts}
 )
 
 add_libc_test(
@@ -197,6 +203,8 @@ add_libc_test(
   DEPENDS
     .strfrom_test_support
     libc.src.stdlib.strfromd
+  COMPILE_OPTIONS
+    ${strfrom_test_copts}
 )
 
 add_libc_test(
@@ -208,6 +216,8 @@ add_libc_test(
   DEPENDS
     .strfrom_test_support
     libc.src.stdlib.strfroml
+  COMPILE_OPTIONS
+    ${strfrom_test_copts}
 )
 
 add_libc_test(

libc/test/src/stdlib/StrfromTest.h

@@ -156,6 +156,9 @@ class StrfromTest : public LIBC_NAMESPACE::testing::Test {
     written = func(buff, 99, "%f", 1.5);
     ASSERT_STREQ_LEN(written, buff, "1.500000");
 
+// Dyadic float is only accurate to ~50 digits, so skip this 300 digit test.
+// TODO: Create way to test just the first ~50 digits of a number.
+#ifndef LIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION
     written = func(buff, 499, "%f", 1e300);
     ASSERT_STREQ_LEN(written, buff,
                      "100000000000000005250476025520442024870446858110815915491"
@@ -167,6 +170,7 @@ class StrfromTest : public LIBC_NAMESPACE::testing::Test {
                      "111903896764088007465274278014249457925878882005684283811"
                      "566947219638686"
                      "5459400540160.000000");
+#endif // DLIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION
 
     written = func(buff, 99, "%f", 0.1);
     ASSERT_STREQ_LEN(written, buff, "0.100000");