https://bugs.llvm.org/show_bug.cgi?id=39209
Bug ID: 39209
Summary: std::uniform_int_distribution erroneously rejects
generator values
Product: libc++
Version: 6.0
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: All Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: l...@christoph-conrads.name
CC: llvm-bugs@lists.llvm.org, mclow.li...@gmail.com
Created attachment 20974
--> https://bugs.llvm.org/attachment.cgi?id=20974&action=edit
Demonstration of bug
Given a generator returning values in the interval [0, r] and
`std::uniform_int_distribution(0, b)`, let r,b such that r+1 multiple of b+1.
In this case `std::uniform_int_distribution` may erroneously reject generator
values although no rejection sampling is needed.
In the demonstration program in the attachment, r=19, b=9 so that half of all
generator values are rejected. The bug does not occur if r+1 and b+1 are powers
of two.
Note that the GNU C++ Library contains a similar bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80977
$ clang++ --version
clang version 6.0.1 (tags/RELEASE_601/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm/6/bin
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