Re: Strange problems with data-tests.el

[Paul Eggert <eggert@cs.ucla.edu>]

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Eli Zaretskii wrote:
> This all looks like a compiler bug

Yes and no. In this area, GCC deliberately doesn't conform to the C standard 
unless you use an option like -fexcess-precision=standard that causes GCC to 
conform but often generate worse code.

The odd results you observed can occur when a C 'double' variable spontaneously 
changes value, e.g., when an 80-bit floating-point register is spilled to a 
64-bit memory location. I reproduced the problem on Fedora 28 and fixed it by 
installing the attached patch into master; please give it a try.

I suspect that it's only bad luck that exposed the problem, that is, that the 
recent bignum or misc changes triggered the bug only because they caused GCC to 
optimize in a different way. In theory if we're unlucky the bug could recur, 
since GCC doesn't make promises in this area. However, I changed arithcompare in 
such a way that GCC would have to be pretty perverse to cause the problem to 
resurface, and similarly for other C compilers.

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From a84cef90957f2379cc0df6bd908317fc441971ce Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 12 Aug 2018 15:28:20 -0700
Subject: [PATCH] Port better to x86 -fexcess-precision=fast

Problem reported by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2018-08/msg00380.html
* src/data.c (arithcompare): Work around incompatibility
between gcc -fexcess-precision=fast and the C standard on x86,
by capturing the results of floating-point comparisons before
the excess precision spontaneously decays.  Although this fix
might not work in general, it does work here and is probably
good enough for the platforms we care about.
---
 src/data.c | 43 +++++++++++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 14 deletions(-)

diff --git a/src/data.c b/src/data.c
index c8a9c6b378..7b8dd45c94 100644
--- a/src/data.c
+++ b/src/data.c
@@ -2492,7 +2492,7 @@ arithcompare (Lisp_Object num1, Lisp_Object num2,
 {
   double f1, f2;
   EMACS_INT i1, i2;
-  bool fneq;
+  bool lt, eq, gt;
   bool test;
 
   CHECK_NUMBER_COERCE_MARKER (num1);
@@ -2502,10 +2502,13 @@ arithcompare (Lisp_Object num1, Lisp_Object num2,
     return bignumcompare (num1, num2, comparison);
 
   /* If either arg is floating point, set F1 and F2 to the 'double'
-     approximations of the two arguments, and set FNEQ if floating-point
-     comparison reports that F1 is not equal to F2, possibly because F1
-     or F2 is a NaN.  Regardless, set I1 and I2 to integers that break
-     ties if the floating-point comparison is either not done or reports
+     approximations of the two arguments, and set LT, EQ, and GT to
+     the <, ==, > floating-point comparisons of F1 and F2
+     respectively, taking care to avoid problems if either is a NaN,
+     and trying to avoid problems on platforms where variables (in
+     violation of the C standard) can contain excess precision.
+     Regardless, set I1 and I2 to integers that break ties if the
+     floating-point comparison is either not done or reports
      equality.  */
 
   if (FLOATP (num1))
@@ -2528,7 +2531,9 @@ arithcompare (Lisp_Object num1, Lisp_Object num2,
 	     to I2 will break the tie correctly.  */
 	  i1 = f2 = i2 = XFIXNUM (num2);
 	}
-      fneq = f1 != f2;
+      lt = f1 < f2;
+      eq = f1 == f2;
+      gt = f1 > f2;
     }
   else
     {
@@ -2539,39 +2544,49 @@ arithcompare (Lisp_Object num1, Lisp_Object num2,
 	     converse of comparing float to integer (see above).  */
 	  i2 = f1 = i1;
 	  f2 = XFLOAT_DATA (num2);
-	  fneq = f1 != f2;
+	  lt = f1 < f2;
+	  eq = f1 == f2;
+	  gt = f1 > f2;
 	}
       else
 	{
 	  i2 = XFIXNUM (num2);
-	  fneq = false;
+	  eq = true;
 	}
     }
 
+  if (eq)
+    {
+      /* Break a floating-point tie by comparing the integers.  */
+      lt = i1 < i2;
+      eq = i1 == i2;
+      gt = i1 > i2;
+    }
+
   switch (comparison)
     {
     case ARITH_EQUAL:
-      test = !fneq && i1 == i2;
+      test = eq;
       break;
 
     case ARITH_NOTEQUAL:
-      test = fneq || i1 != i2;
+      test = !eq;
       break;
 
     case ARITH_LESS:
-      test = fneq ? f1 < f2 : i1 < i2;
+      test = lt;
       break;
 
     case ARITH_LESS_OR_EQUAL:
-      test = fneq ? f1 <= f2 : i1 <= i2;
+      test = lt | eq;
       break;
 
     case ARITH_GRTR:
-      test = fneq ? f1 > f2 : i1 > i2;
+      test = gt;
       break;
 
     case ARITH_GRTR_OR_EQUAL:
-      test = fneq ? f1 >= f2 : i1 >= i2;
+      test = gt | eq;
       break;
 
     default:
-- 
2.17.1