[llvm-commits] [llvm] r159694 - in /llvm/trunk: include/llvm/CodeGen/Passes.h include/llvm/InitializePasses.h include/llvm/Target/TargetInstrInfo.h lib/CodeGen/CMakeLists.txt lib/CodeGen/CodeGen.cpp lib/CodeGen/EarlyIfConversion.cpp lib/CodeGen/Passes.cpp

[Jakob Stoklund Olesen]

On Jul 3, 2012, at 5:26 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>> Add an experimental early if-conversion pass, off by default.
> 
> Cool. I can probably use this on PPC as well.

I expect you can.

> How does this differ from CodeGen/IfConversion.cpp?

The late if-conversion pass uses predication, so it only works on architectures like ARM that can predicate most instructions. The predication means that the pass can run after register allocation, it will never increase register pressure.

The early if-conversion pass will speculatively execute both sides of a branch, and then insert a select instruction to choose the result. It only really makes sense for out-of-order CPUs where the speculated instructions can be hidden as extra ILP.

The TBD heuristics will assume an out-of-order super-scalar CPU, but I suppose an in-order super-scalar micro-architecture can benefit as well once Andy's MI scheduler is up and running.

/jakob

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20120703/5145e8e5/attachment.html>