Revision: 21828
Author: [email protected]
Date: Fri Jun 13 06:36:09 2014 UTC
Log: Fixed undefined behavior in RNG.
We're basically trading undefined behavior for implementation defined
behavior, which should be OK for UBSan. :-) The generated code should
be identical, at least I checked that for GCC 4.6.3 on x64.
BUG=377790
LOG=y
[email protected]
Review URL: https://codereview.chromium.org/332733002
http://code.google.com/p/v8/source/detail?r=21828
Modified:
/branches/bleeding_edge/src/utils/random-number-generator.cc
=======================================
--- /branches/bleeding_edge/src/utils/random-number-generator.cc Tue Jun 3
08:12:43 2014 UTC
+++ /branches/bleeding_edge/src/utils/random-number-generator.cc Fri Jun 13
06:36:09 2014 UTC
@@ -117,7 +117,13 @@
int RandomNumberGenerator::Next(int bits) {
ASSERT_LT(0, bits);
ASSERT_GE(32, bits);
- int64_t seed = (seed_ * kMultiplier + kAddend) & kMask;
+ // Do unsigned multiplication, which has the intended modulo semantics,
while
+ // signed multiplication would expose undefined behavior.
+ uint64_t product = static_cast<uint64_t>(seed_) * kMultiplier;
+ // Assigning a uint64_t to an int64_t is implementation defined, but this
+ // should be OK. Use a static_cast to explicitly state that we know what
we're
+ // doing. (Famous last words...)
+ int64_t seed = static_cast<int64_t>((product + kAddend) & kMask);
seed_ = seed;
return static_cast<int>(seed >> (48 - bits));
}
--
--
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