Merge pull request #109 from herbie-fp/fix-105

Fix near-underflows in `ival-pow`

Author: pavpanchekha

ops/pow.rkt

@@ -52,17 +52,20 @@
          (match-define (endpoint cval c!) c)
          (match-define (endpoint dval d!) d)
 
-         ;; Important: exp2-overflow-threshold is an exact power of 2, so we can use >=
+         ;; Can't use >=, even though exp2-overflow-threshold is a
+         ;; power of 2, because mpfr-exp is offset by 1 from the real
+         ;; exponent, which matters when we add them.
          (define must-overflow
            (and (bfinfinite? hi)
                 (= (* x-class y-class) 1)
-                (>= (+ (mpfr-exp bval) (mpfr-exp (rnd 'zero bflog2 aval)))
-                    (mpfr-exp exp2-overflow-threshold))))
+                (> (+ (mpfr-exp bval) (mpfr-exp (rnd 'zero bflog2 aval)))
+                   (mpfr-exp exp2-overflow-threshold))))
+
          (define must-underflow
            (and (bfzero? lo)
                 (= (* x-class y-class) -1)
-                (>= (+ (mpfr-exp dval) (mpfr-exp (rnd 'zero bflog2 cval)))
-                    (mpfr-exp exp2-overflow-threshold))))
+                (> (+ (mpfr-exp dval) (mpfr-exp (rnd 'zero bflog2 cval)))
+                   (mpfr-exp exp2-overflow-threshold))))
 
          (define real-lo! (or lo! must-underflow (and (bfzero? lo) a! b!)))
          (define real-hi! (or hi! must-underflow must-overflow (and (bfinfinite? hi) c! d!)))

test.rkt

@@ -21,6 +21,7 @@
 (require "main.rkt"
          "mpfr.rkt")
 (provide ival-valid?
+         ival-refines?
          function-table
          sample-interval
          slow-tests