[libc][math] Add option to set a specific exponent for frexp with Inf/NaN inputs.

libc/cmake/modules/LLVMLibCCompileOptionRules.cmake

@@ -79,6 +79,14 @@ function(_get_compile_options_from_config output_var)
     list(APPEND config_options "-DLIBC_ADD_NULL_CHECKS")
   endif()
 
+  if(NOT "${LIBC_CONF_FREXP_INF_NAN_EXPONENT}" STREQUAL "")
+    list(APPEND config_options "-DLIBC_FREXP_INF_NAN_EXPONENT=${LIBC_CONF_FREXP_INF_NAN_EXPONENT}")
+  endif()
+
+  if(LIBC_CONF_MATH_OPTIMIZATIONS)
+    list(APPEND compile_options "-DLIBC_MATH=${LIBC_CONF_MATH_OPTIMIZATIONS}")
+  endif()
+
   set(${output_var} ${config_options} PARENT_SCOPE)
 endfunction(_get_compile_options_from_config)
 
@@ -170,9 +178,6 @@ function(_get_common_compile_options output_var flags)
       list(APPEND compile_options "-Wthread-safety")
       list(APPEND compile_options "-Wglobal-constructors")
     endif()
-    if(LIBC_CONF_MATH_OPTIMIZATIONS)
-      list(APPEND compile_options "-DLIBC_MATH=${LIBC_CONF_MATH_OPTIMIZATIONS}")
-    endif()
   elseif(MSVC)
     list(APPEND compile_options "/EHs-c-")
     list(APPEND compile_options "/GR-")

libc/config/config.json

@@ -87,6 +87,10 @@
     "LIBC_CONF_MATH_OPTIMIZATIONS": {
       "value": 0,
       "doc": "Configures optimizations for math functions. Values accepted are LIBC_MATH_SKIP_ACCURATE_PASS, LIBC_MATH_SMALL_TABLES, LIBC_MATH_NO_ERRNO, LIBC_MATH_NO_EXCEPT, and LIBC_MATH_FAST."
+    },
+    "LIBC_CONF_FREXP_INF_NAN_EXPONENT": {
+      "value": "",
+      "doc": "The value written back to the second parameter when calling frexp/frexpf/frexpl` with `+/-Inf`/`NaN` is unspecified.  Configue an explicit exp value for Inf/NaN inputs."
     }
   },
   "qsort": {

libc/docs/configure.rst

@@ -33,6 +33,7 @@ to learn about the defaults for your platform and target.
 * **"general" options**
     - ``LIBC_ADD_NULL_CHECKS``: Add nullptr checks in the library's implementations to some functions for which passing nullptr is undefined behavior.
 * **"math" options**
+    - ``LIBC_CONF_FREXP_INF_NAN_EXPONENT``: Set the specific exp value for Inf/NaN inputs.
     - ``LIBC_CONF_MATH_OPTIMIZATIONS``: Configures optimizations for math functions. Values accepted are LIBC_MATH_SKIP_ACCURATE_PASS, LIBC_MATH_SMALL_TABLES, LIBC_MATH_NO_ERRNO, LIBC_MATH_NO_EXCEPT, and LIBC_MATH_FAST.
 * **"printf" options**
     - ``LIBC_CONF_PRINTF_DISABLE_FIXED_POINT``: Disable printing fixed point values in printf and friends.

libc/src/__support/FPUtil/ManipulationFunctions.h

@@ -31,8 +31,16 @@ namespace fputil {
 template <typename T, cpp::enable_if_t<cpp::is_floating_point_v<T>, int> = 0>
 LIBC_INLINE T frexp(T x, int &exp) {
   FPBits<T> bits(x);
-  if (bits.is_inf_or_nan())
+  if (bits.is_inf_or_nan()) {
+#ifdef LIBC_FREXP_INF_NAN_EXPONENT
+    // The value written back to the second parameter when calling
+    // frexp/frexpf/frexpl` with `+/-Inf`/`NaN` is unspecified in the standard.
+    // Set the exp value for Inf/NaN inputs explicitly to
+    // LIBC_FREXP_INF_NAN_EXPONENT if it is defined.
+    exp = LIBC_FREXP_INF_NAN_EXPONENT;
+#endif // LIBC_FREXP_INF_NAN_EXPONENT
     return x;
+  }
   if (bits.is_zero()) {
     exp = 0;
     return x;

libc/test/src/math/smoke/FrexpTest.h

@@ -21,8 +21,19 @@ class FrexpTest : public LIBC_NAMESPACE::testing::FEnvSafeTest {
   void testSpecialNumbers(FrexpFunc func) {
     int exponent;
     EXPECT_FP_EQ_ALL_ROUNDING(aNaN, func(aNaN, &exponent));
+#ifdef LIBC_FREXP_INF_NAN_EXPONENT
+    EXPECT_EQ(LIBC_FREXP_INF_NAN_EXPONENT, exponent);
+#endif // LIBC_FREXP_INF_NAN_EXPONENT
+
     EXPECT_FP_EQ_ALL_ROUNDING(inf, func(inf, &exponent));
+#ifdef LIBC_FREXP_INF_NAN_EXPONENT
+    EXPECT_EQ(LIBC_FREXP_INF_NAN_EXPONENT, exponent);
+#endif // LIBC_FREXP_INF_NAN_EXPONENT
+
     EXPECT_FP_EQ_ALL_ROUNDING(neg_inf, func(neg_inf, &exponent));
+#ifdef LIBC_FREXP_INF_NAN_EXPONENT
+    EXPECT_EQ(LIBC_FREXP_INF_NAN_EXPONENT, exponent);
+#endif // LIBC_FREXP_INF_NAN_EXPONENT
 
     EXPECT_FP_EQ_ALL_ROUNDING(zero, func(zero, &exponent));
     EXPECT_EQ(exponent, 0);