[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

[Bob Wilson]

On Oct 27, 2009, at 6:15 PM, Evan Cheng wrote:

>
> On Oct 27, 2009, at 4:49 PM, Bob Wilson wrote:
>
>> Author: bwilson
>> Date: Tue Oct 27 18:49:38 2009
>> New Revision: 85346
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=85346&view=rev
>> Log:
>> Record CodeGen optimization level in the BranchFolding pass so that  
>> we can
>> use it to control tail merging when there is a tradeoff between  
>> performance
>> and code size.  When there is only 1 instruction in the common  
>> tail, we have
>> been merging.  That can be good for code size but is a definite  
>> loss for
>> performance.  Now we will avoid tail merging in that case when the
>> optimization level is "Aggressive", i.e., "-O3".  Radar 7338114.
> Just then controlling this with optimization level, we should only  
> do it when the function is marked OptSize. Would that work?

I'm not sure I understand what you're asking.  Are you suggesting that  
instead of disabling tail merging of 1 instruction for CodeGenOpt  
level "Aggressive" (-O3) that we change so that case is disabled by  
default and only enabled for functions with the OptimizeForSize  
attribute (which is set by the front-end when using -Os)?

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?