[libc-commits] [libc] [libc][math] Fix -Wc++23-extensions warnings in erfcf16 and erff16. (PR #215613)

[via libc-commits]

https://github.com/zhangweize9-cyber created https://github.com/llvm/llvm-project/pull/215613

Change the original logic—which directly returns a fixed value—to use `fputil::cast` for the calculation.

## Problem Description  
I encountered this issue while debugging the `llvm-mc` module. When I running `ninja -C build llvm-mc`, During compilation, the terminal displayed the warning `-Wc++23-extensions warnings`.  

## Solution  
I replaced the original code, which looked something like this, with the following method:
  
```h
// before
return 1.0f16;

// after
return fputil::cast<float16>(1.0f);
```

The conditional expression uses `static expr float16` to store the result in a variable for processing.  

## Test case  
I added two additional tests to the file `libc/test/src/__support/math_extras_test.cpp`—one to test cases greater than 2.0 and another to test cases less than 1.0. I ran the tests using `ninja -C build llvm-lit`.  


>From 60a37d809ac9db25c5e88562b6baadf41a7828c3 Mon Sep 17 00:00:00 2001
From: zhangweize9-cyber <zhangweize9 at gmail.com>
Date: Wed, 12 Aug 2026 00:33:40 +0800
Subject: [PATCH] [libc][math] Fix -Wc++23-extensions warnings in erfcf16 and
 erff16.
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Change the original logic—which directly returns a fixed value—to use `fputil::cast` for the calculation.
---
 libc/src/__support/math/erfcf16.h            |  4 ++--
 libc/src/__support/math/erff16.h             |  7 +++++--
 libc/test/src/__support/math_extras_test.cpp | 20 ++++++++++++++++++++
 3 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/libc/src/__support/math/erfcf16.h b/libc/src/__support/math/erfcf16.h
index ad096527855d5..2de95420b465b 100644
--- a/libc/src/__support/math/erfcf16.h
+++ b/libc/src/__support/math/erfcf16.h
@@ -93,7 +93,7 @@ LIBC_INLINE float16 erfcf16(float16 x) {
   }
 
   if (LIBC_UNLIKELY(x_abs == 0))
-    return 1.0f16;
+    return fputil::cast<float16>(1.0f);
 
   // Asymptotic behavior: erfc(x) rounds to 0 or 2 for |x| >= 4.0.
   if (LIBC_UNLIKELY(x_abs >= 0x4400U)) { // |x| >= 4.0
@@ -109,7 +109,7 @@ LIBC_INLINE float16 erfcf16(float16 x) {
     if (fputil::fenv_is_round_up())
       return FPBits::min_subnormal().get_val();
 #endif
-    return 0.0f16;
+    return fputil::cast<float16>(0.0f);
   }
 
   // Polynomial approximation:
diff --git a/libc/src/__support/math/erff16.h b/libc/src/__support/math/erff16.h
index 8e3a92c092e22..508d137a488d6 100644
--- a/libc/src/__support/math/erff16.h
+++ b/libc/src/__support/math/erff16.h
@@ -134,9 +134,11 @@ LIBC_INLINE float16 erff16(float16 x) {
         return x;
       }
       // Inf -> returns 1.0 or -1.0
-      return is_neg ? -1.0f16 : 1.0f16;
+      static constexpr float16 ONE_F16 = fputil::cast<float16>(1.0f);
+      return is_neg ? -ONE_F16 : ONE_F16;
     }
 
+    // FIXME: Handle f16 constant emission without triggering -Wc++23-extensions
     return fputil::cast<float16>(is_neg ? -1.0f - xf * 0x1.0p-28f
                                         : 1.0f - xf * 0x1.0p-28f);
   }
@@ -144,7 +146,8 @@ LIBC_INLINE float16 erff16(float16 x) {
   // Polynomial approximation:
   //   erf(x) ~ x * (c0 + c1 * x^2 + c2 * x^4 + ... + c7 * x^14)
 
-  int idx = static_cast<int>(xbits.abs().get_val() * 8.0f16);
+  static constexpr float16 EIGHT_F16 = fputil::cast<float16>(8.0f);
+  int idx = static_cast<int>(xbits.abs().get_val() * EIGHT_F16);
 
   float xsq = xf * xf;
   float x4 = xsq * xsq;
diff --git a/libc/test/src/__support/math_extras_test.cpp b/libc/test/src/__support/math_extras_test.cpp
index f5d4dae371827..d04b0447bc379 100644
--- a/libc/test/src/__support/math_extras_test.cpp
+++ b/libc/test/src/__support/math_extras_test.cpp
@@ -8,6 +8,8 @@
 
 #include "src/__support/integer_literals.h"
 #include "src/__support/macros/config.h"
+#include "src/__support/math/erfcf16.h"
+#include "src/__support/math/erff16.h"
 #include "src/__support/math_extras.h"
 #include "src/__support/uint128.h" // UInt<128>
 #include "test/UnitTest/Test.h"
@@ -166,4 +168,22 @@ TYPED_TEST(LlvmLibcBlockMathExtrasTest, sub_overflow, UnsignedTypes) {
   }
 }
 
+TYPED_TEST(LlvmLibcErfcf16Test, SpecialValues) {
+  using LIBC_NAMESPACE_DECL::f16;
+
+  EXPECT_EQ(f16(1.0), LIBC_NAMESPACE_DECL::erfcf16(f16(0.0)));
+
+  EXPECT_TRUE(LIBC_NAMESPACE_DECL::TYPED_TEST(erfcf16(f16(10.0))) >= f16(0.0));
+  EXPECT_TRUE(LIBC_NAMESPACE_DECL::TYPED_TEST(erfcf16(f16(0.0))) <= f16(2.0));
+}
+
+TYPED_TEST(LlvmLibcErff16Test, SpecialValues) {
+  using LIBC_NAMESPACE_DECL::f16;
+
+  EXPECT_EQ(f16(1.0), LIBC_NAMESPACE_DECL::erff16(f16(0.0)));
+
+  EXPECT_TRUE(LIBC_NAMESPACE_DECL::TYPED_TEST(erff16(f16(10.0))) >= f16(0.0));
+  EXPECT_TRUE(LIBC_NAMESPACE_DECL::TYPED_TEST(erff16(f16(0.0))) <= f16(2.0));
+}
+
 } // namespace LIBC_NAMESPACE_DECL