[llvm-commits] [llvm-gcc-4.2] r85513 - in /llvm-gcc-4.2/trunk/gcc: llvm-convert.cpp llvm-internal.h

[Bob Wilson]

On Oct 29, 2009, at 1:39 PM, Duncan Sands wrote:

> Hi Bob,
>
>> Add front-end support for indirect branch.  This is mostly copied  
>> from
>> the corresponding code that Chris wrote for clang.  For now, this  
>> is still
>> disabled by default; compile with USEINDIRECTBRANCH defined to  
>> enable it.
>
> this looks awfully complicated.  Does something like dragonegg  
> commit r85525
> not work for llvm-gcc?

I don't think it's any more complicated than what was there before.   
The patch looks messy because instead of modifying the existing  
implementation, I added the alternate version inside conditionals.  As  
you've discovered, there is no codegen support yet so it should not be  
enabled by default.

Besides consistency with clang, the idea behind doing it this way is  
to reduce the number of edges in the CFG.  Indirect branches are  
typically used in things like interpreter loops, where you have a  
large number of labels and a large number of gotos.  If you add an  
edge from every goto to every label, the CFG can blow up.  Instead we  
have a single indirect branch and all the gotos branch to it,  
basically factoring out the CFG.