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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
This commit is contained in:
Jeff Walden
2019-08-28 16:35:22 +00:00
parent 7ebb8f61e7
commit adb48c068f
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: * The clamp is written carefully. Note that if we had said:
* *
* if (skipCount > SIZE_MAX) * if (skipCount > double(SIZE_MAX))
* skipCount = SIZE_MAX; * mSkipCount = SIZE_MAX;
* else
* mSkipCount = skipCount;
* *
* that leads to undefined behavior 64-bit machines: SIZE_MAX coerced to * 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 * double can equal 2^64, so if skipCount equaled 2^64 converting it to
* value that can be safely assigned to mSkipCount. * size_t would induce undefined behavior.
* *
* Jakob Olesen cleverly suggested flipping the sense of the comparison: if * Jakob Olesen cleverly suggested flipping the sense of the comparison to
* we require that skipCount < SIZE_MAX, then because of the gaps (2048) * skipCount < double(SIZE_MAX). The conversion will evaluate to 2^64 or
* between doubles at that magnitude, the highest double less than 2^64 is * the double just below it: either way, skipCount is guaranteed to have a
* 2^64 - 2048, which is fine to store in a size_t. * value that's safely convertible to size_t.
* *
* (On 32-bit machines, all size_t values can be represented exactly in * (On 32-bit machines, all size_t values can be represented exactly in
* double, so all is well.) * double, so all is well.)
*/ */
double skipCount = double skipCount =
std::floor(std::log(mGenerator.nextDouble()) * mInvLogNotProbability); std::floor(std::log(mGenerator.nextDouble()) * mInvLogNotProbability);
if (skipCount < SIZE_MAX) if (skipCount < double(SIZE_MAX))
mSkipCount = skipCount; mSkipCount = skipCount;
else else
mSkipCount = SIZE_MAX; mSkipCount = SIZE_MAX;