bug in (log)

Jeronimo Pellegrini jpl at randomnode.info
Sun Jul 3 13:57:52 UTC 2022

Out of curiosity -- the spec mentions "floating point approximations",
and in this case I understand it is reasonable (a very small number is
treated like zero).

But is floating-point infinity also considered an approximation to very
large numbers?

(setq x (expt 10 5000))
(log x)   =>    11512.926

ECL, SBCL, Clisp, CCL, CMUCL return 11512.926.

ABCL doesn't (it actually invokes a TYPE-ERROR condition, because the
number is too large to be converted to DOUBLE-FLOAT) -- but it does
not return float infinity.

GCL's behavior is a bit more complicated:

(setq x (expt 10 5000))
(progn (setq y (log x)) 'whatever)    ;; don't let GCL print it yet!

(type-of y)

But If I ask for the value of y on the REPL:

Fast links are on: do (si::use-fast-links nil) for debugging
Condition in SYSTEM::GCL-TOP-LEVEL [or a callee]: INTERNAL-SIMPLE-ERROR: Can't print a non-number.

(So I produced a non-printable LONG-FLOAT object by just calling
 `log` on an integer.)

I did not test Clasp.

Anyway - I was just wondering how to interpret the standard in this


Stas Boukarev <stassats at gmail.com> writes:

> This is permitted by
> http://www.lispworks.com/documentation/HyperSpec/Body/12_acc.htm
> On Sun, Jul 3, 2022 at 12:10 AM James Cloos <cloos at jhcloos.com> wrote:
>> (gitlab is uusable; i have to report here.)
>> ecl 21.2.1 gives:
>> > (log 1/6319748715279270675921934218987893281199411530039296)
>> Debugger received error of type: DIVISION-BY-ZERO
>> #<a DIVISION-BY-ZERO 0x7ff5afe6c540>
>> Error flushed.
>> whereas other cl's (i testyed sbcl and ccl) give results like:
>> ? (log 1/6319748715279270675921934218987893281199411530039296)
>> -119.27552
>> I tested on amd64 (gentoo) and arm64 (debian and netbsd) with identical
>> results.  i did not have a musl box or other bsd to test on.
>> run with --no-trap-fpe, the result is #<single-float negative infinity>.
>> another example is:
>> (truncate (log 1/6319748715279270675921934218987893281199418867))
>> Debugger received error of type: ARITHMETIC-ERROR
>> #<a ARITHMETIC-ERROR 0xffff8f688a40>
>> whereas this works:
>> (truncate (log 1/631974871527927067592193421898789328119941867))
>> -103
>> -0.27893066
>> which of course suggests that the issue is c's long double's precision.
>> it looks like ecl could use an mpq log function;
>> https://github.com/linas/anant might work.
>> -JimC
>> --
>> James Cloos <cloos at jhcloos.com>         OpenPGP: 0x997A9F17ED7DAEA6

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