[libc][math][c23] Fix undefined behavior in expxf16.h

[overmighty]

Fixes the left-shifting of potentially negative signed integers.

[llvmbot]

@llvm/pr-subscribers-libc

Author: OverMighty (overmighty)

Changes

Fixes the left-shifting of potentially negative signed integers.

---

Full diff: https://github.com/llvm/llvm-project/pull/112734.diff

1 Files Affected:

• (modified) libc/src/math/generic/expxf16.h (+4-4)

diff --git a/libc/src/math/generic/expxf16.h b/libc/src/math/generic/expxf16.h
index 8de329bd2ab07f..aba99a2914b41d 100644
--- a/libc/src/math/generic/expxf16.h
+++ b/libc/src/math/generic/expxf16.h
@@ -108,8 +108,8 @@ LIBC_INLINE ExpRangeReduction exp2_range_reduction(float16 x) {
   float xf = x;
   float kf = fputil::nearest_integer(xf * 0x1.0p+3f);
   int x_hi_mid = static_cast<int>(kf);
-  int x_hi = x_hi_mid >> 3;
-  int x_mid = x_hi_mid & 0x7;
+  unsigned x_hi = static_cast<unsigned>(x_hi_mid) >> 3;
+  unsigned x_mid = static_cast<unsigned>(x_hi_mid) & 0x7;
   // lo = x - (hi + mid) = round(x * 2^3) * (-2^(-3)) + x
   float lo = fputil::multiply_add(kf, -0x1.0p-3f, xf);
 
@@ -155,8 +155,8 @@ LIBC_INLINE ExpRangeReduction exp10_range_reduction(float16 x) {
   float xf = x;
   float kf = fputil::nearest_integer(xf * (LOG2F_10 * 0x1.0p+3f));
   int x_hi_mid = static_cast<int>(kf);
-  int x_hi = x_hi_mid >> 3;
-  int x_mid = x_hi_mid & 0x7;
+  unsigned x_hi = static_cast<unsigned>(x_hi_mid) >> 3;
+  unsigned x_mid = static_cast<unsigned>(x_hi_mid) & 0x7;
   // lo = x - (hi + mid) = round(x * 2^3 * log2(10)) * log10(2) * (-2^(-3)) + x
   float lo = fputil::multiply_add(kf, LOG10F_2 * -0x1.0p-3f, xf);
 

[overmighty]

Casting kf (potentially negative float) to unsigned directly would also be UB: https://eel.is/c++draft/conv.fpint#1. Hence why I left x_hi_mid as int.

We really only need x_hi to be unsigned. I made x_mid unsigned too for consistency.