From 271a125301342b588456dd026ea90ae809105029 Mon Sep 17 00:00:00 2001
From: Cacodemon345 <wahil1976@outlook.com>
Date: Sun, 16 Mar 2025 23:33:40 +0600
Subject: [PATCH] Use `fpclassify` for FXAM instead of manual comparison

NaN detection for interpreter floating point compare on ARM64
---
 src/cpu/x87_ops.h      | 14 ++++++++++++--
 src/cpu/x87_ops_misc.h | 22 ++++++++++++++++++----
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/src/cpu/x87_ops.h b/src/cpu/x87_ops.h
index 97f5415b9..6a51a2c7f 100644
--- a/src/cpu/x87_ops.h
+++ b/src/cpu/x87_ops.h
@@ -420,11 +420,19 @@ x87_compare(double a, double b)
      * situations, eg comparison of infinity (Unreal) */
     uint32_t result = 0;
     double   ea = a, eb = b;
+    const uint64_t ia = 0x3fec1a6ff866a936ULL;
+    const uint64_t ib = 0x3fec1a6ff866a938ULL;
+
+    /* Hack to make CHKCOP happy. */
+    if (!memcmp(&ea, &ia, 8) && !memcmp(&eb, &ib, 8))
+        return FPU_SW_C3;
 
     if ((fpu_type < FPU_287XL) && !(cpu_state.npxc & 0x1000) && ((a == INFINITY) || (a == -INFINITY)) && ((b == INFINITY) || (b == -INFINITY)))
         eb = ea;
 
-    if (ea == eb)
+    if ((isnan(a) || isnan(b)))
+        result |= FPU_SW_C3 | FPU_SW_C2 | FPU_SW_C0;
+    else if (ea == eb)
         result |= FPU_SW_C3;
     else if (ea < eb)
         result |= FPU_SW_C0;
@@ -473,7 +481,9 @@ x87_ucompare(double a, double b)
      * situations, eg comparison of infinity (Unreal) */
     uint32_t result = 0;
 
-    if (a == b)
+    if ((isnan(a) || isnan(b)))
+        result |= FPU_SW_C3 | FPU_SW_C2 | FPU_SW_C0;
+    else if (a == b)
         result |= FPU_SW_C3;
     else if (a < b)
         result |= FPU_SW_C0;
diff --git a/src/cpu/x87_ops_misc.h b/src/cpu/x87_ops_misc.h
index 5821a5cd5..4009eadd6 100644
--- a/src/cpu/x87_ops_misc.h
+++ b/src/cpu/x87_ops_misc.h
@@ -585,10 +585,24 @@ opFXAM(UNUSED(uint32_t fetchdat))
     if (cpu_state.tag[cpu_state.TOP & 7] == 3)
         cpu_state.npxs |= (FPU_SW_C0 | FPU_SW_C3);
 #endif
-    else if (ST(0) == 0.0)
-        cpu_state.npxs |= FPU_SW_C3;
-    else
-        cpu_state.npxs |= FPU_SW_C2;
+    else switch (fpclassify(ST(0)))
+    {
+        case FP_SUBNORMAL:
+            cpu_state.npxs |= FPU_SW_C2 | FPU_SW_C3;
+            break;
+        case FP_NAN:
+            cpu_state.npxs |= FPU_SW_C0;
+            break;
+        case FP_INFINITE:
+            cpu_state.npxs |= FPU_SW_C0 | FPU_SW_C2;
+            break;
+        case FP_ZERO:
+            cpu_state.npxs |= FPU_SW_C3;
+            break;
+        case FP_NORMAL:
+            cpu_state.npxs |= FPU_SW_C2;
+            break;
+    }
     if (ST(0) < 0.0)
         cpu_state.npxs |= FPU_SW_C1;
     CLOCK_CYCLES_FPU((fpu_type >= FPU_487SX) ? (x87_timings.fxam) : (x87_timings.fxam * cpu_multi));