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Bug 1577051 - Fix an implicit-conversion-changes-value warning with new-enough clang by explicitly converting. r=froydnj

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Differential Revision: https://phabricator.services.mozilla.com/D43701

--HG--
extra : moz-landing-system : lando
This commit is contained in:
Jeff Walden 2019-08-28 16:35:22 +00:00
parent 746abdeae8
commit aeb4f741d0
1 changed files with 11 additions and 9 deletions
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20
mfbt/FastBernoulliTrial.h
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@ -348,24 +348,26 @@ class FastBernoulliTrial {
*
* The clamp is written carefully. Note that if we had said:
*
* if (skipCount > SIZE_MAX)
* skipCount = SIZE_MAX;
* if (skipCount > double(SIZE_MAX))
* mSkipCount = SIZE_MAX;
* else
* mSkipCount = skipCount;
*
* that leads to undefined behavior 64-bit machines: SIZE_MAX coerced to
* double is 2^64, not 2^64-1, so this doesn't actually set skipCount to a
* value that can be safely assigned to mSkipCount.
* double can equal 2^64, so if skipCount equaled 2^64 converting it to
* size_t would induce undefined behavior.
*
* Jakob Olesen cleverly suggested flipping the sense of the comparison: if
* we require that skipCount < SIZE_MAX, then because of the gaps (2048)
* between doubles at that magnitude, the highest double less than 2^64 is
* 2^64 - 2048, which is fine to store in a size_t.
* Jakob Olesen cleverly suggested flipping the sense of the comparison to
* skipCount < double(SIZE_MAX). The conversion will evaluate to 2^64 or
* the double just below it: either way, skipCount is guaranteed to have a
* value that's safely convertible to size_t.
*
* (On 32-bit machines, all size_t values can be represented exactly in
* double, so all is well.)
*/
double skipCount =
std::floor(std::log(mGenerator.nextDouble()) * mInvLogNotProbability);
if (skipCount < SIZE_MAX)
if (skipCount < double(SIZE_MAX))
mSkipCount = skipCount;
else
mSkipCount = SIZE_MAX;