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- Log ----------------------------------------------------------------- commit 315ea23408772763fc0f141d51bf5038269f0b7c Author: Raymond Toy toy.raymond@gmail.com Date: Fri Sep 26 20:20:54 2014 -0700
Fix incorrect value from INTEGER-DECODE-FLOAT for denormals.
This is a long standing bug where INTEGER-DECODE-FLOAT returned incorrect values for denormal double-float numbers. The lower 32 bits of the double were not shifted enough when creating the integer result.
As a side-effect, DECODE-FLOAT decodes denormal double-float's correctly and SCALE-FLOAT scales denormals correctly too.
* src/code/float.lisp: * Shift the lower 32-bit of the fraction one more time to adjust for the fact that the upper 20 bits were shifted an extra time. * tests/float.lisp: * Add several tests for INTEGER-DECODE-FLOAT, SCALE-FLOAT, and DECODE-FLOAT. * src/general-info/release-20f.txt: * Add this bug fix to the notes.
diff --git a/src/code/float.lisp b/src/code/float.lisp index cace1a4..3ac6512 100644 --- a/src/code/float.lisp +++ b/src/code/float.lisp @@ -581,7 +581,7 @@ (return)) (setq sig (ash sig 1)) (incf extra-bias)) - (values (logior (ash sig 32) (ash low-bits (1- extra-bias))) + (values (logior (ash sig 32) (ash low-bits (1+ extra-bias))) (truly-the fixnum (- biased extra-bias)) sign)))))
diff --git a/src/general-info/release-20f.txt b/src/general-info/release-20f.txt index 0327f5b..4fd57e3 100644 --- a/src/general-info/release-20f.txt +++ b/src/general-info/release-20f.txt @@ -123,6 +123,10 @@ New in this release: * For linux, motifd is no longer a 64-bit app. * (exp 1d0) now returns the correctly rounded value of e. Previously, it was off by one bit. + * INTEGER-DECODE-FLOAT returns the correct values for denormal + doubles. As a side-effect of this fix, DECODE-FLOAT returns the + correct values for denormals, and SCALE-FLOAT scales denormals + correctly.
* Trac Tickets: * Ticket #90 fixed. diff --git a/tests/float.lisp b/tests/float.lisp index 55b9013..71cdeb8 100644 --- a/tests/float.lisp +++ b/tests/float.lisp @@ -42,4 +42,77 @@ (assert-equalp k (clog10 x)))))
+(define-test integer-decode-float.double + ;; Generate 100 random denormal values and compare what + ;; integer-decode-float returns against what it should return. + (dotimes (k 100) + (let ((x (random least-positive-normalized-double-float))) + (multiple-value-bind (hi lo) + (kernel:double-float-bits x) + ;; Verify that the exponent is 0, which it must be for a + ;; denormal number. + (assert-equal 0 + (ldb vm:double-float-exponent-byte hi) + x) + ;; Print out the fraction bits, and the bits returned by + ;; INTEGER-DECODE-FLOAT. We could do this differently, but + ;; this has the nice side effect of making it easy to see what + ;; is expected and what went wrong. + (let* ((expected (format nil "~b~32,'0b" hi lo)) + (actual (format nil "~b" (integer-decode-float x))) + (tail (subseq actual (length expected)))) + ;; If everything is working correctly, the beginning of the + ;; actual bits must exactly match the expected bits. + (assert-true (every #'char= + expected + actual) + x + expected + actual) + ;; And finally, the trailing part of the actual bits must be + ;; all zeroes, but this is a denormal number. + (assert-true (every #'(lambda (c) + (char= c #\0)) + tail) + x + tail)))))) + +(define-test scale-float.double + ;; As a side-effect of fixing INTEGER-DECODE-FLOAT, SCALE-FLOAT + ;; should return the correct values now when scaling + ;; denormals. Check a few denormal values. + (dotimes (k 100) + (let* ((x (random least-positive-normalized-double-float)) + (scaled (scale-float x 54)) + (mult (* x (scale-float 1d0 54)))) + ;; The result of SCALE-FLOAT and the multiplication should be + ;; exactly equal because the multiplication by 2^54 is exactly + ;; representable. + (assert-equal scaled + mult + x + scaled + mult))) + ;; Add the test caused the investigation of SCALE-FLOAT + (let* ((x 1d-310) + (scaled (scale-float x 54)) + (mult (* x (scale-float 1d0 54)))) + (assert-equal scaled + mult + x + scaled + mult))) + +(define-test decode-float.double + ;; As a side-effect of fixing INTEGER-DECODE-FLOAT, DECODE-FLOAT + ;; should return the correct values now. We just spot check one + ;; value here. + (dotimes (k 100) + (let ((x (random least-positive-normalized-double-float))) + (multiple-value-bind (f e) + (decode-float x) + (assert-equal x + (scale-float f e) + f + e)))))
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Summary of changes: src/code/float.lisp | 2 +- src/general-info/release-20f.txt | 4 +++ tests/float.lisp | 73 ++++++++++++++++++++++++++++++++++++++ 3 files changed, 78 insertions(+), 1 deletion(-)
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