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Subject: Re: [boost] [multiprecision review] Possible use of uninitialisedvalue
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-06-14 11:43:27

> resize() doesn't zero-initialise the new storage. In this particular
> case, a == result because the expression being evaluated is something like
> result = result + other. So resizing result is also resizing a. Hence
> the compare_unsigned looks at the limbs of a that have just been added by
> the resize, which are undefined.
> It appears that this could be fixed by zeroing the new limbs in resize(),
> but that is unnecessary in other cases. Is it even correct that a ==
> result is possible in this code?

Yes, inplace subtraction is OK.

Here's the fix I'm testing:

Index: cpp_int.hpp
--- cpp_int.hpp (revision 78943)
+++ cpp_int.hpp (working copy)
@@ -964,12 +964,16 @@
+ // This isn't used till later, but comparison has to occur before we
resize the result,
+ // as that may also resize a or b if this is an inplace operation:
+ int c = a.compare_unsigned(b);
+ // Set up the result vector:
+ // Now that a, b, and result are stable, get pointers to their limbs:
    typename cpp_int_backend<MinBits, Signed, Allocator>::const_limb_pointer
pa = a.limbs();
    typename cpp_int_backend<MinBits, Signed, Allocator>::const_limb_pointer
pb = b.limbs();
    typename cpp_int_backend<MinBits, Signed, Allocator>::limb_pointer pr =
    bool swapped = false;
- int c = a.compare_unsigned(b);
    if(c < 0)
       swap(pa, pb);


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