[LLVMdev] is createCFGSimplificationPass unused?

[Chris Lattner]

On Thu, 2 Nov 2006, [UTF-8] Rafael Esp?ndola wrote:
>> If there are specific cases like this where simplify cfg produces better
>> code than branch folding (with the hooks implemented) please file
>> bugzilla bugs to track them.

> The branch folding works after instruction selection, right? It looks
> strange to me that it will be able to convert a branch sequence into a
> select. IMHO this transformation can be better performed with a higher
> level representation.

Yes, for this specific one, you're right.  Simplifycfg is run multiple 
times in the standard llvm-gcc -Ox pipeline.  We don't expect the code 
generator to magically produce optimal code for random .ll fragments, we 
expect that it has been optimized a bit first.  :)

If there is a specific example like this that isn't picked up by -O2, 
please let me know.

>> Long term, I'd like to get the code generator to the point where running
>> the code generator doesn't modify from the input LLVM IR. Running
>> llvm->llvm passes breaks this property.

> I see that this is a desired property. If I recall correctly, you need
> this to remove the remaining annotations.

The desire is driven more by my abstract notion of purity than anything 
practical :).  However, it is needed to do a credible job of 
reoptimization in a JIT context, is needed if you want to codegen to 
multiple target variants at once, etc.  These are all things that LLVM can 
and should do, but I personally don't have a short term driver for them.

> Maybe llvm-gcc should run CFGSimplification. It would then compile "if
> (a) return 0; else return 1" into
>
> %tmp = seteq int %a, 0
> %tmp1 = select bool %tmp, int 0, int 1
> ret int %tmp1

Yep, it definitely does.  Make sure you're using llvm-gcc4 and passing -O2 
or higher.

-Chris

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