[libc-commits] [libc] [libc][math] Preserve the sign of tiny asinhf16 zeros (PR #236479)

[via libc-commits]

https://github.com/sriramshastry updated https://github.com/llvm/llvm-project/pull/236479

>From 21c48f83ed0d3ced9fec6df11e4df8dcce3a4df2 Mon Sep 17 00:00:00 2001
From: sriramshastry <sriramshastry at gmail.com>
Date: Wed, 28 Oct 2026 16:21:31 +0000
Subject: [PATCH] [libc][math] Preserve the sign of tiny asinhf16 zeros

Restore the input sign after the existing tiny correction rounds to zero.
Cover negative minimum subnormal in upward and toward-zero rounding and
positive minimum subnormal in downward rounding.

Keep the selected magnitude and approximation unchanged. This is not a
general underflow-reporting repair.

Signed-off-by: sriramshastry <sriramshastry at gmail.com>
---
 libc/src/__support/math/asinhf16.h         | 14 ++++++++++----
 libc/test/src/math/smoke/asinhf16_test.cpp | 15 +++++++++++++++
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/libc/src/__support/math/asinhf16.h b/libc/src/__support/math/asinhf16.h
index e6f108763969bc..6c38159c20a766 100644
--- a/libc/src/__support/math/asinhf16.h
+++ b/libc/src/__support/math/asinhf16.h
@@ -89,10 +89,16 @@ LIBC_INLINE constexpr float16 asinhf16(float16 x) {
     if (LIBC_UNLIKELY(x_abs < 0x29c6)) {
 #ifndef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
       int rounding = fputil::quick_get_round();
-      if ((rounding == FE_UPWARD || rounding == FE_TOWARDZERO) && xf < 0)
-        return fputil::cast<float16>(xf + 0x1p-24f);
-      if ((rounding == FE_DOWNWARD || rounding == FE_TOWARDZERO) && xf > 0)
-        return fputil::cast<float16>(xf - 0x1p-24f);
+      if ((rounding == FE_UPWARD || rounding == FE_TOWARDZERO) && xf < 0) {
+        FPBits result(fputil::cast<float16>(xf + 0x1p-24f));
+        result.set_sign(xbits.sign());
+        return result.get_val();
+      }
+      if ((rounding == FE_DOWNWARD || rounding == FE_TOWARDZERO) && xf > 0) {
+        FPBits result(fputil::cast<float16>(xf - 0x1p-24f));
+        result.set_sign(xbits.sign());
+        return result.get_val();
+      }
 #endif
       return fputil::cast<float16>(xf);
     }
diff --git a/libc/test/src/math/smoke/asinhf16_test.cpp b/libc/test/src/math/smoke/asinhf16_test.cpp
index f17300cf980e25..3206320f6278fa 100644
--- a/libc/test/src/math/smoke/asinhf16_test.cpp
+++ b/libc/test/src/math/smoke/asinhf16_test.cpp
@@ -32,3 +32,18 @@ TEST_F(LlvmLibcAsinhf16Test, SpecialNumbers) {
   EXPECT_FP_EQ(neg_inf, LIBC_NAMESPACE::asinhf16(neg_inf));
   EXPECT_MATH_ERRNO(0);
 }
+
+#ifndef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
+TEST_F(LlvmLibcAsinhf16Test, TinySignedZeros) {
+  using Bits = LIBC_NAMESPACE::fputil::FPBits<float16>;
+  EXPECT_FP_EQ_ROUNDING_UPWARD(
+      Bits::zero(LIBC_NAMESPACE::Sign::NEG).get_val(),
+      LIBC_NAMESPACE::asinhf16(Bits(uint16_t(0x8001)).get_val()));
+  EXPECT_FP_EQ_ROUNDING_TOWARD_ZERO(
+      Bits::zero(LIBC_NAMESPACE::Sign::NEG).get_val(),
+      LIBC_NAMESPACE::asinhf16(Bits(uint16_t(0x8001)).get_val()));
+  EXPECT_FP_EQ_ROUNDING_DOWNWARD(
+      Bits::zero().get_val(),
+      LIBC_NAMESPACE::asinhf16(Bits(uint16_t(1)).get_val()));
+}
+#endif