[llvm-commits] [llvm] r85346 - in /llvm/trunk: include/llvm/CodeGen/Passes.h lib/CodeGen/BranchFolding.cpp lib/CodeGen/BranchFolding.h lib/CodeGen/IfConversion.cpp lib/CodeGen/LLVMTargetMachine.cpp lib/Target/ARM/ARMTargetMachine.cpp

Evan Cheng evan.cheng at apple.com
Wed Oct 28 11:10:15 PDT 2009


On Oct 28, 2009, at 10:38 AM, Chris Lattner wrote:

> 
> On Oct 28, 2009, at 10:21 AM, Dale Johannesen wrote:
> 
>>> If so, yes, I think that would work.  In fact, I think it would be a
>>> good idea.  At -O2, that would mean that we would be optimizing for
>>> performance at a small cost in code size.  That's a more aggressive
>>> change than mine, but I think it would be a good idea.
>>> 
>>> Shall I go ahead with that?
>> 
>> Branch folding is a size optimization.  It would not surprise me if
>> turning it off completely at
>> -O3 was the right thing for performance.  My only strong belief is
>> that any change should be based on measurements, not arguments.
> 
> Doesn't it also do some code layout to encourage better fall-throughs  
> etc?
> 
That has been removed. The heuristics have shown to do as much harm on x86 as good. On ARM (especially Thumb), it's badness. Block layout optimizations are now done in CodePlacementOpt.cpp.

Evan


> -Chris
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