[LLVMdev] LLVM Code Generation on flattened IR code

[Andrew Trick]

> On Jul 17, 2014, at 11:41 AM, charlessl at free.fr wrote:
> 
> Hello,
> 
> I made two flattening transformation filters, one use a switch instruction for the dispatcher, and the other an indirectbranch instruction. Both work well but llc is very time consuming in the second case : 10 minutes for a 500 basic block function, while it takes 10 seconds for the switchcase implementation . The instrumentation of llc shows that most of the time is passed in  the Machine Block Frequency Analysis pass
> 
> 
> I don't understand why there is so huge difference. Is there a way to speed up llc ?

That’s a bug. Can you try with trunk and provide the LLVM IR test case? You can file a PR it at llvm.org/bugs. 

There’s been some work on the block frequency algorithm recently, which is shared between LLVM IR and Machine IR, so it would be interesting to get that test case.

-Andy


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