[libc-commits] [libc] 1cb219f - [libc][math] Fix -Wc++23-extensions warnings in erfcf16 and erff16. (#215613)

[via libc-commits]

Author: original-cooling-space
Date: 2026-08-14T09:07:15-07:00
New Revision: 1cb219f75aaeb7c2b98d85ec87d92d3c602b7a78

URL: https://github.com/llvm/llvm-project/commit/1cb219f75aaeb7c2b98d85ec87d92d3c602b7a78
DIFF: https://github.com/llvm/llvm-project/commit/1cb219f75aaeb7c2b98d85ec87d92d3c602b7a78.diff

LOG: [libc][math] Fix -Wc++23-extensions warnings in erfcf16 and erff16. (#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
was 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 0.0f16;

// The original approach
return fputil::cast<float16>(0.0f);

// Intue suggested
FPBits::zero().get_val();
```

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

## Test case  
I added two additional tests to the two files.  

- `‎libc/src/__support/math/erfcf16.h‎`
- `‎libc/src/__support/math/erff16.h‎`

0ne is for test cases greater than 2.0 and another to test cases less
than 1.0. This test also added test cases with negative values. I ran
the tests using `ninja -C build llvm-lit`.

Added: 
    

Modified: 
    libc/src/__support/math/erfcf16.h
    libc/src/__support/math/erff16.h

Removed: 
    


################################################################################
diff  --git a/libc/src/__support/math/erfcf16.h b/libc/src/__support/math/erfcf16.h
index ad096527855d5..7608586001996 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 FPBits::zero().get_val();
   }
 
   // Polynomial approximation:

diff  --git a/libc/src/__support/math/erff16.h b/libc/src/__support/math/erff16.h
index 8e3a92c092e22..68b6575957989 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;
+      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);
+  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;