From fba96d6422ce802aa2a79f72e3a5fd947dcb52a2 Mon Sep 17 00:00:00 2001
From: Alex <270190108+CouchRiv@users.noreply.github.com>
Date: Sun, 11 Oct 2026 10:07:47 -0400
Subject: [PATCH] fix(libc): asin(|x| > 1) returns the x86 default NaN and
 leaves errno unchanged

On main asin(2.0) returns 0x7ff8000000000000 and sets errno to 33;
FreeBSD msun (lib/msun/src/e_asin.c) returns 0xfff8000000000000 and
never sets errno. asin now handles |x| > 1 itself with quiet
comparisons, so a NaN input still raises no exception. For |x| <= 1
and for NaN the host function is still called.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 core/libs/prx/libc/src/Math.cpp |  8 +++++++-
 core/libs/tests/GuestMath.cpp   | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/core/libs/prx/libc/src/Math.cpp b/core/libs/prx/libc/src/Math.cpp
index 9d6a8aebd..aef6c0385 100644
--- a/core/libs/prx/libc/src/Math.cpp
+++ b/core/libs/prx/libc/src/Math.cpp
@@ -70,7 +70,13 @@ int APS5_VABI __isnormalf_nid_postfix(float x) { return std::isnormal(x) ? 1 : 0
 int APS5_VABI __isinff_nid_postfix(float x) { return std::isinf(x) ? 1 : 0; }
 
 double APS5_VABI cbrt_nid_postfix(double x) { return std::cbrt(x); }
-double APS5_VABI asin_nid_postfix(double x) { return std::asin(x); }
+double APS5_VABI asin_nid_postfix(double x) {
+    if (std::isgreater(x, 1.0) || std::isless(x, -1.0)) {
+        std::feraiseexcept(FE_INVALID);
+        return -std::numeric_limits<double>::quiet_NaN();
+    }
+    return std::asin(x);
+}
 double APS5_VABI acos_nid_postfix(double x) { return std::acos(x); }
 double APS5_VABI sqrt_nid_postfix(double x) { return std::sqrt(x); }
 float APS5_VABI sqrtf_nid_postfix(float x) { return std::sqrt(x); }
diff --git a/core/libs/tests/GuestMath.cpp b/core/libs/tests/GuestMath.cpp
index 6d56671b5..2179dd60a 100644
--- a/core/libs/tests/GuestMath.cpp
+++ b/core/libs/tests/GuestMath.cpp
@@ -40,6 +40,7 @@ int APS5_VABI __fpclassifyf_nid_postfix(float);
 float APS5_VABI fmodf_nid_postfix(float, float);
 float APS5_VABI asinf_nid_postfix(float);
 float APS5_VABI acosf_nid_postfix(float);
+double APS5_VABI asin_nid_postfix(double);
 float APS5_VABI atan2f_nid_postfix(float, float);
 float APS5_VABI hypotf_nid_postfix(float, float);
 double APS5_VABI hypot_nid_postfix(double, double);
@@ -281,6 +282,36 @@ static void CheckIntegerConversions() {
 
 static bool Near(double value, double expected, double tolerance) { return std::abs(value - expected) <= tolerance * std::abs(expected); }
 
+static void CheckAsin() {
+    const double pi = 3.14159265358979323846;
+    const double pi_2 = pi / 2;
+    const double nan = std::numeric_limits<double>::quiet_NaN();
+    const double infinity = std::numeric_limits<double>::infinity();
+
+    Require(asin_nid_postfix(0.) == 0.);
+    Require(std::signbit(asin_nid_postfix(-0.)));
+    Require(Near(asin_nid_postfix(1.), pi_2, 1e-15));
+    Require(Near(asin_nid_postfix(-1.), -pi_2, 1e-15));
+    Require(std::isnan(asin_nid_postfix(2.0)) && std::signbit(asin_nid_postfix(2.0)));
+    Require(std::isnan(asin_nid_postfix(-2.0)) && std::signbit(asin_nid_postfix(-2.0)));
+    Require(std::isnan(asin_nid_postfix(infinity)) && std::signbit(asin_nid_postfix(infinity)));
+    Require(std::isnan(asin_nid_postfix(-infinity)) && std::signbit(asin_nid_postfix(-infinity)));
+    Require(std::isnan(asin_nid_postfix(nan)));
+    Require(Near(asin_nid_postfix(0.5), pi/6, 1e-15));
+    Require(Near(asin_nid_postfix(-0.5), -pi/6, 1e-15));
+    Require(Near(asin_nid_postfix(1./std::sqrt(2)), pi/4, 1e-15));
+    Require(asin_nid_postfix(1e-300) == 1e-300);
+    Require(asin_nid_postfix(-1e-300) == -1e-300);
+    std::feclearexcept(FE_ALL_EXCEPT);
+    errno = 0;
+    asin_nid_postfix(2.0);
+    Require(std::fetestexcept(FE_INVALID));
+    Require(errno == 0);
+    std::feclearexcept(FE_ALL_EXCEPT);
+    asin_nid_postfix(nan);
+    Require(!std::fetestexcept(FE_INVALID));
+}
+
 static void CheckHyperbolic() {
     const double infinity = std::numeric_limits<double>::infinity();
     Require(sinh_nid_postfix(0.) == 0. && std::signbit(sinh_nid_postfix(-0.)));
@@ -478,6 +509,7 @@ int main() {
     Require(__isnormalf_nid_postfix(std::numeric_limits<float>::denorm_min()) == 0);
     Require(__isnormal_nid_postfix(1.) == 1 && __isnormal_nid_postfix(0.) == 0);
     Require(__isnormal_nid_postfix(std::numeric_limits<double>::denorm_min()) == 0);
+    CheckAsin();
     const auto quotient = div_nid_postfix(7, 2);
     Require(quotient.quot == 3 && quotient.rem == 1);
     const auto negativeNumerator = div_nid_postfix(-7, 2);