llvm · michaelrj-google · Oct 2, 2024 · Oct 1, 2024
diff --git a/libc/config/config.json b/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."

diff --git a/libc/docs/configure.rst b/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).

diff --git a/libc/src/__support/FPUtil/dyadic_float.h b/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;
 

diff --git a/libc/src/__support/float_to_string.h b/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

diff --git a/libc/src/stdio/printf_core/CMakeLists.txt b/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()

diff --git a/libc/test/src/stdio/CMakeLists.txt b/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()

diff --git a/libc/test/src/stdlib/CMakeLists.txt b/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(

diff --git a/libc/test/src/stdlib/StrfromTest.h b/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");