From: Taylor R Campbell <campbell@mumble.net>
Date: Fri, 14 Dec 2018 16:24:58 +0000 (+0000)
Subject: Disable FLONUM-EXP open-coding on i386.
X-Git-Tag: mit-scheme-pucked-10.1.9~3^2~35^2~14
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=cb341bd1d790eb1f8c49fc683e3a3b8428bd01ea;p=mit-scheme.git

Disable FLONUM-EXP open-coding on i386.

The instruction sequence we had was not very good, e.g. (exp -708)
was off by far more than 1ulp.  It can be done that way, but we need
to use 64-bit precision for it, and setting the i387 precision here
is getting a bit far afield from open-coding.
---

diff --git a/src/compiler/machines/i386/machin.scm b/src/compiler/machines/i386/machin.scm
index 17dd499dd..2643098eb 100644
--- a/src/compiler/machines/i386/machin.scm
+++ b/src/compiler/machines/i386/machin.scm
@@ -357,6 +357,9 @@ USA.
 		  ;; <= pi/4.  Correct argument reduction requires a
 		  ;; better approximation of pi than the i387 has.
 		  FLONUM-SIN FLONUM-COS FLONUM-TAN
+		  ;; Disabled: exp is too much trouble to get right in
+		  ;; i387; need 64-bit precision.  Let libm do it.
+		  FLONUM-EXP
 		  VECTOR-CONS STRING-ALLOCATE FLOATING-VECTOR-CONS
 		  FLONUM-IS-LESS? FLONUM-IS-LESS-OR-EQUAL?
 		  FLONUM-IS-GREATER? FLONUM-IS-GREATER-OR-EQUAL?
diff --git a/src/compiler/machines/i386/rulflo.scm b/src/compiler/machines/i386/rulflo.scm
index 7bd2aaf69..d4ba63aab 100644
--- a/src/compiler/machines/i386/rulflo.scm
+++ b/src/compiler/machines/i386/rulflo.scm
@@ -335,7 +335,10 @@ USA.
      (LAP (FLDL2E)
 	  (FMULP (ST 1) (ST 0))
 	  (F2XM1)))))
+
+;; XXX This is disabled because it is inadequate in 53-bit precision.
 
+#;
 (define-arithmetic-method 'FLONUM-EXP flonum-methods/1-arg
   (flonum-unary-operation/stack-top
    (lambda ()