[libc-commits] [libc] [libc] provide str to float build configs (PR #178780)

[Prabhu Rajasekaran via libc-commits]

https://github.com/Prabhuk created https://github.com/llvm/llvm-project/pull/178780

The str to float code path offers options which can reduce code bloat.
Expose them as build config options. Disable Eisel Lemire for avoiding
code bloat caused by DETAILED_POWERS_OF_TEN look up table.


>From 9e62be0a61093f903c0f79bbdf172192bba43c31 Mon Sep 17 00:00:00 2001
From: prabhukr <prabhukr at google.com>
Date: Thu, 29 Jan 2026 15:46:48 -0800
Subject: [PATCH] [libc] provide str to float build configs

The str to float code path offers options which can reduce code bloat.
Expose them as build config options. Disable Eisel Lemire for avoiding
code bloat caused by DETAILED_POWERS_OF_TEN look up table.
---
 .../modules/LLVMLibCCompileOptionRules.cmake     | 16 ++++++++++++++++
 libc/config/baremetal/config.json                |  5 +++++
 libc/config/config.json                          | 14 ++++++++++++++
 libc/docs/configure.rst                          |  4 ++++
 libc/src/__support/str_to_float.h                |  4 ++--
 5 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index a719f20d532e0..326153dd38a3d 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -77,6 +77,22 @@ endfunction(_get_compile_options_from_flags)
 function(_get_compile_options_from_config output_var)
   set(config_options "")
 
+  if(LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_FLOAT320)
+    list(APPEND config_options "-DLIBC_COPT_PRINTF_FLOAT_TO_STR_USE_FLOAT320")
+  endif()
+
+  if(LIBC_CONF_STRTOFLOAT_DISABLE_EISEL_LEMIRE)
+    list(APPEND config_options "-DLIBC_COPT_STRTOFLOAT_DISABLE_EISEL_LEMIRE")
+  endif()
+
+  if(LIBC_CONF_STRTOFLOAT_DISABLE_SIMPLE_DECIMAL_CONVERSION)
+    list(APPEND config_options "-DLIBC_COPT_STRTOFLOAT_DISABLE_SIMPLE_DECIMAL_CONVERSION")
+  endif()
+
+  if(LIBC_CONF_STRTOFLOAT_DISABLE_CLINGER_FAST_PATH)
+    list(APPEND config_options "-DLIBC_COPT_STRTOFLOAT_DISABLE_CLINGER_FAST_PATH")
+  endif()
+
   if(LIBC_CONF_QSORT_IMPL)
     list(APPEND config_options "-DLIBC_QSORT_IMPL=${LIBC_CONF_QSORT_IMPL}")
   endif()
diff --git a/libc/config/baremetal/config.json b/libc/config/baremetal/config.json
index e3703c28865bb..259bc9a7dff61 100644
--- a/libc/config/baremetal/config.json
+++ b/libc/config/baremetal/config.json
@@ -34,6 +34,11 @@
       "value": "LIBC_QSORT_HEAP_SORT"
     }
   },
+  "str_to_float": {
+    "LIBC_CONF_STRTOFLOAT_DISABLE_EISEL_LEMIRE": {
+      "value": true
+    }
+  },
   "math": {
     "LIBC_CONF_MATH_OPTIMIZATIONS": {
       "value": "(LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES | LIBC_MATH_NO_ERRNO | LIBC_MATH_INTERMEDIATE_COMP_IN_FLOAT)"
diff --git a/libc/config/config.json b/libc/config/config.json
index f981c433b2c7c..171fa185d1cc3 100644
--- a/libc/config/config.json
+++ b/libc/config/config.json
@@ -64,6 +64,20 @@
       "doc": "Disable index mode in the scanf format string."
     }
   },
+  "str_to_float": {
+    "LIBC_CONF_STRTOFLOAT_DISABLE_EISEL_LEMIRE": {
+      "value": false,
+      "doc": "Disable Eisel-Lemire algorithm for string to float conversion."
+    },
+    "LIBC_CONF_STRTOFLOAT_DISABLE_SIMPLE_DECIMAL_CONVERSION": {
+      "value": false,
+      "doc": "Disable simple decimal conversion for string to float conversion."
+    },
+    "LIBC_CONF_STRTOFLOAT_DISABLE_CLINGER_FAST_PATH": {
+      "value": false,
+      "doc": "Disable Clinger's fast path for string to float conversion."
+    }
+  },
   "string": {
     "LIBC_CONF_STRING_LENGTH_IMPL": {
       "value": "element",
diff --git a/libc/docs/configure.rst b/libc/docs/configure.rst
index 81888bb07153c..e083450fe9beb 100644
--- a/libc/docs/configure.rst
+++ b/libc/docs/configure.rst
@@ -60,6 +60,10 @@ to learn about the defaults for your platform and target.
     - ``LIBC_CONF_SCANF_DISABLE_INDEX_MODE``: Disable index mode in the scanf format string.
 * **"setjmp" options**
     - ``LIBC_CONF_SETJMP_AARCH64_RESTORE_PLATFORM_REGISTER``: Make setjmp save the value of x18, and longjmp restore it. The AArch64 ABI delegates this register to platform ABIs, which can choose whether to make it caller-saved.
+* **"str_to_float" options**
+    - ``LIBC_CONF_STRTOFLOAT_DISABLE_CLINGER_FAST_PATH``: Disable Clinger's fast path for string to float conversion.
+    - ``LIBC_CONF_STRTOFLOAT_DISABLE_EISEL_LEMIRE``: Disable Eisel-Lemire algorithm for string to float conversion.
+    - ``LIBC_CONF_STRTOFLOAT_DISABLE_SIMPLE_DECIMAL_CONVERSION``: Disable simple decimal conversion for string to float conversion.
 * **"string" options**
     - ``LIBC_CONF_FIND_FIRST_CHARACTER_IMPL``: Selects the implementation for find-first-character-related functions: 'element', 'word', 'clang_vector', or 'arch_vector'.
     - ``LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING``: Inserts prefetch for write instructions (PREFETCHW) for memset on x86 to recover performance when hardware prefetcher is disabled.
diff --git a/libc/src/__support/str_to_float.h b/libc/src/__support/str_to_float.h
index 54c7aa1d1cb50..7dd446fb3d2fb 100644
--- a/libc/src/__support/str_to_float.h
+++ b/libc/src/__support/str_to_float.h
@@ -684,9 +684,7 @@ LIBC_INLINE FloatConvertReturn<T> decimal_exp_to_float(
     const CharType *__restrict numStart,
     const size_t num_len = cpp::numeric_limits<size_t>::max()) {
   using FPBits = typename fputil::FPBits<T>;
-  using StorageType = typename FPBits::StorageType;
 
-  StorageType mantissa = init_num.mantissa;
   int32_t exp10 = init_num.exponent;
 
   FloatConvertReturn<T> output;
@@ -725,6 +723,8 @@ LIBC_INLINE FloatConvertReturn<T> decimal_exp_to_float(
 
 #ifndef LIBC_COPT_STRTOFLOAT_DISABLE_EISEL_LEMIRE
   // Try Eisel-Lemire
+  using StorageType = typename FPBits::StorageType;
+  StorageType mantissa = init_num.mantissa;
   opt_output = eisel_lemire<T>(init_num, round);
   if (opt_output.has_value()) {
     if (!truncated) {