From: Taylor R Campbell <campbell@mumble.net>
Date: Wed, 23 Jan 2019 07:50:38 +0000 (+0000)
Subject: Check at runtime whether feenableexcept does anything.
X-Git-Tag: mit-scheme-pucked-10.1.20~11^2~66^2~26
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=fff23f74e52f1cc59317c194e844aa826a516608;p=mit-scheme.git

Check at runtime whether feenableexcept does anything.

The bits are defined on aarch64, but apparently some CPUs are
fabricated without support for them so they just read back as zers.
Bummer!
---

diff --git a/src/microcode/floenv.c b/src/microcode/floenv.c
index 82e0f49dc..2672996d3 100644
--- a/src/microcode/floenv.c
+++ b/src/microcode/floenv.c
@@ -594,7 +594,22 @@ DEFINE_PRIMITIVE ("HAVE-FLOAT-TRAP-ENABLE/DISABLE?", Prim_have_float_trap_enable
 {
   PRIMITIVE_HEADER (0);
 #if ((defined (HAVE_FEENABLEEXCEPT)) && (defined (HAVE_FEDISABLEEXCEPT)))
-  PRIMITIVE_RETURN (SHARP_T);
+  static int have = -1;
+  if (have == -1)
+    {
+      fenv_t fenv;
+      int excepts = (fegetexcept ());
+      /* Prevent traps while we futz with stuff.  */
+      feholdexcept (&fenv);
+      /* Reverse the sense.  */
+      feenableexcept (FE_ALL_EXCEPT &~ excepts);
+      fedisableexcept (excepts);
+      /* Check whether that had any effect.  */
+      have = ((fegetexcept ()) != excepts);
+      /* Restore the environment without raising exceptions.  */
+      fesetenv (&fenv);
+    }
+  PRIMITIVE_RETURN (have ? SHARP_T : SHARP_F);
 #else
   PRIMITIVE_RETURN (SHARP_F);
 #endif