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math/big: incorrect result from Exp on 386 architecture #13515

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ncw opened this issue Dec 6, 2015 · 9 comments
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math/big: incorrect result from Exp on 386 architecture #13515

ncw opened this issue Dec 6, 2015 · 9 comments
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@ncw
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ncw commented Dec 6, 2015

import (
    "fmt"
    "math/big"
)

func main() {
    A := new(big.Int)
    B := new(big.Int)
    P := new(big.Int)
    C := new(big.Int)
    A.SetUint64(0xAAAAAAAA00000000)
    B.SetUint64(0xAAAAAAAA00000000)
    P.SetUint64(0xFFFFFFFF00000001)
    C.Exp(A, B, P)
    fmt.Printf("0x%x ** 0x%x %% 0x%x = 0x%x\n", A, B, P, C)
}

I get different results whether I run it in amd64 mode or 386 mode

$ go run exptest.go
0xaaaaaaaa00000000 ** 0xaaaaaaaa00000000 % 0xffffffff00000001 = 0x1
$ GOARCH=386 go run exptest.go
0xaaaaaaaa00000000 ** 0xaaaaaaaa00000000 % 0xffffffff00000001 = 0xc1540908df2582c2

The first one is correct (you can verify this with python thus)

$ python
Python 2.7.10 (default, Oct 14 2015, 16:09:02) 
[GCC 5.2.1 20151010] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> pow(0xAAAAAAAA00000000 , 0xAAAAAAAA00000000 , 0xFFFFFFFF00000001)
1L

The playground also gives the incorrect results.

Tested with go 1.5.1 and tip

go version devel +b2963a5 Sun Dec 6 06:28:33 2015 +0000 linux/amd64
go version go1.5.1 linux/amd64
@ncw ncw changed the title math/big.Exp: Incorrect results on 32 bit platforms math/big.Exp: Incorrect results on 386 architecture Dec 6, 2015
@ncw ncw closed this as completed Dec 6, 2015
@griesemer griesemer self-assigned this Dec 6, 2015
@griesemer griesemer added this to the Go1.6 milestone Dec 6, 2015
@griesemer
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Just noticing that you @ncw closed this already. Not a bug? (Haven't looked into it yet).

@bradfitz bradfitz reopened this Dec 6, 2015
@davecgh
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davecgh commented Dec 7, 2015

This is a legitimate issue. I'm seeing the same behavior with different values:

package main

import (
    "fmt"
    "math/big"
)

func main() {
    x, _ := new(big.Int).SetString("b62ca285f97d2532089ea7b6c0795e692c5105f316c65a5cbdd4719dd20162b6b42ceab81f1178111fa981c05602bdf7ae274891ba35a53cb01784fbf0e612658baa96c07e7026374bed5d71e49a19c44c4da22b6ca4d6fe98de28291cc5b64f", 16)
    y, _ := new(big.Int).SetString("3fffffffffffffffffffffffffffffffffffffffffffffffffffffffbfffff0c", 16)
    p, _ := new(big.Int).SetString("fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f", 16)
    fmt.Printf("x: %x\ny: %x\np: %x\n", x, y, p)
    fmt.Printf("result: %x\n", new(big.Int).Exp(x, y, p))
}

Output:

$ GOARCH=amd64 go run bigintfail.go; GOARCH=386 go run bigintfail.go
x: b62ca285f97d2532089ea7b6c0795e692c5105f316c65a5cbdd4719dd20162b6b42ceab81f1178111fa981c05602bdf7ae274891ba35a53cb01784fbf0e612658baa96c07e7026374bed5d71e49a19c44c4da22b6ca4d6fe98de28291cc5b64f
y: 3fffffffffffffffffffffffffffffffffffffffffffffffffffffffbfffff0c
p: fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f

result: 77c873fcee4663289da06a660417b6a9831c77875632199b2d1c0f4372606c84

x: b62ca285f97d2532089ea7b6c0795e692c5105f316c65a5cbdd4719dd20162b6b42ceab81f1178111fa981c05602bdf7ae274891ba35a53cb01784fbf0e612658baa96c07e7026374bed5d71e49a19c44c4da22b6ca4d6fe98de28291cc5b64f
y: 3fffffffffffffffffffffffffffffffffffffffffffffffffffffffbfffff0c
p: fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f

result: ef03bdbf3d5e1c37142f762042f3c49d3e1ba5a71ba1e238c54aa913b00fbe4f

EDIT: I should note the amd64 result is the correct one.

@rsc
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rsc commented Dec 7, 2015

Yes, it's a real bug. I am preparing a fix CL.

@rsc rsc assigned rsc and unassigned griesemer Dec 7, 2015
@rsc rsc changed the title math/big.Exp: Incorrect results on 386 architecture math/big: incorrect results on 386 architecture Dec 7, 2015
@rsc rsc changed the title math/big: incorrect results on 386 architecture math/big: incorrect result from Exp on 386 architecture Dec 7, 2015
@gopherbot
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CL https://golang.org/cl/17673 mentions this issue.

@gopherbot
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CL https://golang.org/cl/17672 mentions this issue.

@rsc rsc closed this as completed in 4306352 Dec 11, 2015
rsc added a commit that referenced this issue Dec 16, 2015
Also fix bug reported in CL 17510.

Found during fix of #13515 in CL 17672, but separate from the fix.

Change-Id: I4b1024569a98f5cfd2ebb442ec3d64356164d284
Reviewed-on: https://go-review.googlesource.com/17673
Reviewed-by: Robert Griesemer <[email protected]>
@ysmolski
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I wonder if this is related to this case: http://play.golang.org/p/QVWwgKMdCu ?
I expect result to be equal zero, but I see result equal M.

Should I open separate issue for this matter?

Thanks.

@rsc
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rsc commented Jan 11, 2016

@ysmolsky, thanks. I will file a new issue.

@ysmolski
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Thanks, Russ!

@gopherbot
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CL https://golang.org/cl/18585 mentions this issue.

gopherbot pushed a commit that referenced this issue Jan 13, 2016
…tgomery code

Fixes #13515.

Change-Id: I7dd5fbc816e5ea135f7d81f6735e7601f636fe4f
Reviewed-on: https://go-review.googlesource.com/17672
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-on: https://go-review.googlesource.com/18585
@golang golang locked and limited conversation to collaborators Jan 13, 2017
@rsc rsc removed their assignment Jun 23, 2022
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