[LLVMdev] Major i386 optimization bug in Clang++?

[Alexandre Colucci]

The issue appears indeed to be fixed in clang trunk. Hopefully Apple will include the fix in Xcode 4.2 final or will provide soon a Xcode 4.2.1 beta.
Right now Xcode 4.2 GM (October 4, 2011) is simply unusable for compiling C++ for i386.

Thanks,
Alexandre


> 
> On 10.10.2011, at 16:10, Alexandre Colucci wrote:
> 
>> Hi,
>> 
>> I am currently making the transition from gcc 4.2 to clang for the projects (mostly C++) I am working on.
>> I think I discovered a major optimization bug in Clang++. I managed to create a simple (60 lines of code) test case which exhibits the issue.
>> 
>> When you compile this file for i386 with optimizations turned on (O2, O3 or Os), you get an unexpected result. When you compile it for x86_64, or for i386 with optimizations turned off (O0 or O1), you get the expected behavior. I can reproduce this issue with Clang shipping in Xcode 4.2 GM as well as the Clang binaries from LLVM 2.9 (http://llvm.org/releases/download.html#svn). The tests were made on a Mac running 10.7.1.
>> 
>> Here is what you see if you compile the example with O2:
>> 
>> $ /Developer/usr/bin/clang++ -arch i386 -o testLLVM testLLVM.cp -O2
>> $ ./testLLVM                                                       
>> Unexpected Value: 3222105416 (at index: 2)
>> Expected Value: 3 (at index: 2)
>> 
>> 
>> I haven't found any bugs similar in the LLVM bug database. It is a known issue? This looks like a major optimization issue which causes us to not be able to use Clang for our products.
> 
> I can reproduce your problem with the version of clang that ships with xcode 4.2gm but I can't reproduce it with clang trunk. The bug was probably fixed in the meantime.
> 
> - Ben
> 
>