[llvm-bugs] [Bug 26395] New: Analyzer says undefined behavior in uniform_int_distribution<size_t>

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Sat Jan 30 10:59:07 PST 2016


            Bug ID: 26395
           Summary: Analyzer says undefined behavior in
           Product: libc++
           Version: 3.7
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jared.grubb at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com
    Classification: Unclassified

The clang static analyzer flags this code as having undefined behavior. Note
that if I change "size_t" to "unsigned" then things look fine. It looks like
the engine is 32-bit but it's going to try a 64-bit shift maybe? I'm totally
guessing here.

 warning: The result of the '>>' expression is undefined
    __mask0_ = __w0_ > 0 ? _Engine_result_type(~0) >> (_EDt - __w0_) :

To reproduce:

$ cat x.cpp
#include <random>
#include <cstddef>

size_t genRandom(size_t maxValue) {
        std::minstd_rand engine;
        std::uniform_int_distribution<size_t> dist{0, maxValue};
        return dist(engine);

$ clang++ x.cpp -std=c++14 --analyze

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