[LLVMbugs] [Bug 18591] Incorrect optimization of is_simm32 C++ function from HotSpot JVM sources

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18591

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> ---
Sadly this is not really a miscompilation but yet another instance of undefined
behavior being optimized away.

min_simm(32) compiles down to "return -INT_MIN;", which is undefined.
max_simm(32) compiles down to "return INT_MIN-1;", also undefined.

A simple fix would be to change the intptr_t(1) instances to uintptr_t(1),
unsigned overflow is defined and you'll get the intended results. You can also
globally disable this kind of optimization with -fno-strict-overflow or
-fwrapv. Clang has a tool to detect undefined behavior at runtime, try
-fsanitize=undefined.

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