[libc] Make str_to_float independent of fenv (#102369)

The str_to_float conversion code doesn't need the features provided by
fenv and the dependency is creating a blocker for hand-in-hand. This
patch uses a workaround to remove this dependency.

Author: michaelrj-google

libc/src/__support/CMakeLists.txt

@@ -190,8 +190,6 @@ add_header_library(
     libc.src.__support.CPP.bit
     libc.src.__support.CPP.limits
     libc.src.__support.CPP.optional
-    libc.src.__support.FPUtil.dyadic_float
-    libc.src.__support.FPUtil.fenv_impl
     libc.src.__support.FPUtil.fp_bits
     libc.src.__support.FPUtil.rounding_mode
     libc.src.errno.errno

libc/src/__support/str_to_float.h

@@ -13,9 +13,7 @@
 #include "src/__support/CPP/limits.h"
 #include "src/__support/CPP/optional.h"
 #include "src/__support/CPP/string_view.h"
-#include "src/__support/FPUtil/FEnvImpl.h"
 #include "src/__support/FPUtil/FPBits.h"
-#include "src/__support/FPUtil/dyadic_float.h"
 #include "src/__support/FPUtil/rounding_mode.h"
 #include "src/__support/common.h"
 #include "src/__support/ctype_utils.h"
@@ -27,6 +25,8 @@
 #include "src/__support/uint128.h"
 #include "src/errno/libc_errno.h" // For ERANGE
 
+#include <stdint.h>
+
 namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 
@@ -525,10 +525,9 @@ clinger_fast_path(ExpandedFloat<T> init_num,
   FPBits result;
   T float_mantissa;
   if constexpr (cpp::is_same_v<StorageType, UInt<128>>) {
-    float_mantissa = static_cast<T>(fputil::DyadicFloat<128>(
-        Sign::POS, 0,
-        fputil::DyadicFloat<128>::MantissaType(
-            {uint64_t(mantissa), uint64_t(mantissa >> 64)})));
+    float_mantissa =
+        (static_cast<T>(uint64_t(mantissa)) * static_cast<T>(0x1.0p64)) +
+        static_cast<T>(uint64_t(mantissa >> 64));
   } else {
     float_mantissa = static_cast<T>(mantissa);
   }

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

@@ -664,8 +664,6 @@ libc_support_library(
         ":__support_cpp_optional",
         ":__support_cpp_string_view",
         ":__support_ctype_utils",
-        ":__support_fputil_dyadic_float",
-        ":__support_fputil_fenv_impl",
         ":__support_fputil_fp_bits",
         ":__support_fputil_rounding_mode",
         ":__support_str_to_integer",