math: Pow(x,y) fails for large values of y

[gopherbot]

by nicolas.riesch:

What steps will reproduce the problem?

see http://play.golang.org/p/rcVquN_0xc


What is the expected output?

for x=2.0 and y>=2147483648.0, math.Pow(x,y) should return +Inf

What do you see instead?

It returns 0.0


Please provide any additional information below.

On my 64bits computer, "Linux VBox-Mint15-64bits 3.8.0-19-generic #29-Ubuntu SMP
Wed Apr 17 18:16:28 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux"
with "go version go1.2rc5 linux/amd64", I have a similar problem :

for x = 2.0 and y = 9223372036854775808.0, math.Pow(x,y) should return +Inf
but it returns 0.0

[minux]

Comment 1:

Labels changed: added release-go1.3, repo-main.

Status changed to Accepted.

[minux]

Comment 2:

the msun implementation is correct in this regard, should we
move to a rewrite of the msun implementation? or just
fix the problem before calling Ldexp if the exponent is
too large?

[rsc]

Comment 3:

It's probably too late to rewrite Pow completely, but I wouldn't object to a simple bug
fix. I don't see anything there that uses an int32, but I imagine it should be a fairly
simple fix.

[gopherbot]

Comment 4 by pcaven:

I was looking for a simple Go 1.3 issue to help out with the release effort, so I
thought I'd start with this one. It didn't look like anyone had investigated it yet.
I've reproduced the problem on GOARCH="386" (Linux), but it seems to work correctly on
GOARCH=amd64 (Windows). I think that explains why it fails on the Playground, but not
why the original submitter saw the problem on linux/amd64.
I don't have suggested fix yet, but I've tracked it down to the use of 'int' (32 bits on
'386, 64 bits on amd64), which overflows an int variable in the math.Pow code at some
point after the call to math.Frexp. At the end of math.Pow it is passed as a negative
int value to math.Ldexp, which at line #29, after normalization of the mantissa,
executes:
return Copysign(0, frac) // underflow
I'll look at it some more tonight. I think there might be a simple fix in math.Pow that
avoids the call to math.Ldexp in that case, but I haven't implemented it or run any
tests yet.

[ianlancetaylor]

Comment 5:

Owner changed to @griesemer.

[griesemer]

Comment 6:

Also, Exp(1<<30) returns +Inf, but Exp(1<<31) returns 0. There may be more
cases like this.

Labels changed: added release-go1.3maybe, removed release-go1.3.

[griesemer]

Comment 7:

Labels changed: added release-go1.4, removed release-go1.3maybe.

[griesemer]

Comment 8:

Labels changed: added release-none, removed release-go1.4.