[llvm] r283912 - [Support] Fix undefined behavior in RandomNumberGenerator.

[Zachary Turner via llvm-commits]

Author: zturner
Date: Tue Oct 11 13:17:26 2016
New Revision: 283912

URL: http://llvm.org/viewvc/llvm-project?rev=283912&view=rev
Log:
[Support] Fix undefined behavior in RandomNumberGenerator.

This has existed pretty much forever AFAICT, but the code was
never being exercised because nobody was using the class.  A
user of this class surfaced, and now we're breaking with UB.
The code was obviously wrong, so it's fixed here.

Modified:
    llvm/trunk/lib/Support/RandomNumberGenerator.cpp

Modified: llvm/trunk/lib/Support/RandomNumberGenerator.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/RandomNumberGenerator.cpp?rev=283912&r1=283911&r2=283912&view=diff
==============================================================================
--- llvm/trunk/lib/Support/RandomNumberGenerator.cpp (original)
+++ llvm/trunk/lib/Support/RandomNumberGenerator.cpp Tue Oct 11 13:17:26 2016
@@ -47,11 +47,11 @@ RandomNumberGenerator::RandomNumberGener
   // are using a 64-bit RNG. This isn't a problem since the Mersenne
   // twister constructor copies these correctly into its initial state.
   std::vector<uint32_t> Data;
-  Data.reserve(2 + Salt.size());
-  Data.push_back(Seed);
-  Data.push_back(Seed >> 32);
+  Data.resize(2 + Salt.size());
+  Data[0] = Seed;
+  Data[1] = Seed >> 32;
 
-  std::copy(Salt.begin(), Salt.end(), Data.end());
+  std::copy(Salt.begin(), Salt.end(), Data.begin() + 2);
 
   std::seed_seq SeedSeq(Data.begin(), Data.end());
   Generator.seed(SeedSeq);