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Merge pull request #1497 from luke-jr/bugfix_neguint

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CBigNum: Convert negative int64 values in a more well-defined way
This commit is contained in:
Gregory Maxwell 2012-07-11 16:56:18 -07:00
commit 3dbca25b69
1 changed files with 3 additions and 9 deletions
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12
src/bignum.h
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@ -131,15 +131,9 @@ public:
if (sn < (int64)0) if (sn < (int64)0)
{ {
// We negate in 2 steps to avoid signed subtraction overflow, // Since the minimum signed integer cannot be represented as positive so long as its type is signed, and it's not well-defined what happens if you make it unsigned before negating it, we instead increment the negative integer by 1, convert it, then increment the (now positive) unsigned integer by 1 to compensate
// i.e. -(-2^63), which is an undefined operation and causes SIGILL n = -(sn + 1);
// when compiled with -ftrapv. ++n;
//
// Note that uint64_t n = sn, when sn is an int64_t, is a
// well-defined operation and n will be equal to sn + 2^64 when sn
// is negative.
n = sn;
n = -n;
fNegative = true; fNegative = true;
} else { } else {
n = sn; n = sn;