Raymond Toy pushed to branch issue-274-make-float-of-huge-numbers at cmucl / cmucl

Commits: 6bc0a576 by Raymond Toy at 2024-02-09T14:17:52-08:00 Use integer-length instead of log2

For positive integers, `integer-length` is close enough to `log2` for what we want; we only need an approximate value for log2(x) to know if x is too small or too large to fit in a float. This should make things a bit faster so this change has less of an impact on the speed of reading floats.

- - - - -

1 changed file:

- src/code/reader.lisp

Changes:

===================================== src/code/reader.lisp ===================================== @@ -1833,25 +1833,31 @@ the end of the stream." ;; super accurate with the limits. The rest of the code will handle ;; it correctly, even if we're too small or too large. (unless (zerop number) - (let ((log2-num (+ (kernel::log2 (float (/ number divisor) 1d0)) - (* exponent #.(kernel::log2 10d0))))) - (multiple-value-bind (log2-low log2-high) - (ecase float-format - ((short-float single-float) - ;; Single-float exponents range is -149 to 127 - (values (* 2 -149) (* 2 127))) - ((double-float long-float - #+double-double kernel:double-double-float) - ;; Double-float exponent range is -1074 to -1023 - (values (* 2 -1074) (* 2 1023)))) - (when (< log2-num log2-low) - ;; If the number is too small, just return 0. - (return-from make-float-aux (coerce 0 float-format))) + (flet ((fast-log2 (n) + ;; For an integer, the integer-length is close enough to + ;; the log2 of the number. + (integer-length n))) + ;; log2(x) = exponent*log2(10) + log2(number)-log2(divisor) + (let ((log2-num (+ (* exponent #.(kernel::log2 10d0)) + (fast-log2 number) + (- (fast-log2 divisor))))) + (multiple-value-bind (log2-low log2-high) + (ecase float-format + ((short-float single-float) + ;; Single-float exponents range is -149 to 127 + (values (* 2 -149) (* 2 127))) + ((double-float long-float + #+double-double kernel:double-double-float) + ;; Double-float exponent range is -1074 to -1023 + (values (* 2 -1074) (* 2 1023)))) + (when (< log2-num log2-low) + ;; If the number is too small, just return 0. + (return-from make-float-aux (coerce 0 float-format)))

- (when (> log2-num log2-high) - ;; The numbe is definitely too large to fit. Signal an error. - (%reader-error stream _"Number not representable as a ~S: ~S" - float-format (read-buffer-to-string)))))) + (when (> log2-num log2-high) + ;; The numbe is definitely too large to fit. Signal an error. + (%reader-error stream _"Number not representable as a ~S: ~S" + float-format (read-buffer-to-string)))))))

;; Otherwise the number might fit, so we carefully compute the result (handler-case

View it on GitLab: https://gitlab.common-lisp.net/cmucl/cmucl/-/commit/6bc0a576b1b525875f6c6bb4...