[LLVMbugs] [Bug 19174] -O1 breaks std::abs

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Mar 25 10:05:37 PDT 2014


Benjamin Kramer <benny.kra at gmail.com> changed:

           What    |Removed                     |Added
             Status|NEW                         |RESOLVED
                 CC|                            |benny.kra at gmail.com
         Resolution|---                         |INVALID

--- Comment #1 from Benjamin Kramer <benny.kra at gmail.com> ---
On linux, rand() without seed returns 1804289383. Multiplying this by 4
overflows a 32 bit integer, which is undefined behavior. The optimizer can use
that fact to simplify code.

To avoid the undefined behavior you can pass -fwrapv to clang.

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