[cfe-dev] selective/fine grained optimizations control

[Domagoj Saric]

On 28.8.2013. 18:45, Eli Friedman wrote:
> On Wed, Aug 28, 2013 at 12:50 AM, Domagoj Saric <domagoj.saric at littleendian.com
> <mailto:domagoj.saric at littleendian.com>> wrote:
>
>
>     We have a project which Clang miscompiles in release builds (in short "the
>     GUI shows up empty"). "It all worked fine" up until and including Xcode 4.2,
>     with all later versions (of Xcode and consequently of Clang shipped with it)
>     this bug manifests itself. As we officially switched to Xcode 4.6.3 I have
>     to find a way to workaround or fix this problem.
>
>     The bug manifests already with -O1 while it does not with -O0 so I need a
>     way to selectively enable or disable optimizations turned on at O1. GCC
>     nicely documents which optimizations are turned on by individual -O levels
>     and offers the various -fno-* options for fine grained control but
>     unfortunately Clang does not actually support GCC's command line options
>     other than the basic ones.
>
>     I did find this
>     http://stackoverflow.com/__questions/15548023/clang-__optimization-levels
>     <http://stackoverflow.com/questions/15548023/clang-optimization-levels>
>     but it didn't help: I tried -O0 -mllvm -sroa and got:
>     clang (LLVM option parsing): Unknown command line argument '-sroa'.  Try:
>     'clang (LLVM option parsing) -help'
>     clang (LLVM option parsing): Did you mean '-help'?
>
>
>     Is there any other way to achieve fine grained optimizations control?
>
>
> clang intentionally doesn't expose fine-grained control over optimization passes.

Can I ask for the rationale behind the intention?


> It's probably a better idea to figure out which file is getting incorrectly
> compiled, and change the optimization level for that file,instead of messing
> with the optimization levels for the whole project.  Also, this sort of issue is
> almost always caused by code that has undefined behavior, not by a bug in the
> compiler.

I exactly did that immediately after sending the original email just didn't find 
the time until now to report...

The issue was caused by

template <int ID>
__attribute__(( const ))
SomeClass const & someFunction()
{
     static SomeClass const someObject( ID );
     return someObject;
}

removing the const function attribute the bug disappeared...so who's bug is it? ;-D


-- 
Domagoj Saric
Software Architect
www.LittleEndian.com