[LLVMdev] SimplifyCFG vs loops

Krzysztof Parzyszek kparzysz at codeaurora.org
Wed Nov 21 07:02:13 PST 2012

On 11/20/2012 4:35 PM, Humphreys, Jonathan wrote:
> What was the resolution to this issue?  Was the patch accepted?
> Couple of thoughts:
>   - how firm is the constraint that all loops must have a single backedge?  I don't know how fundamental this assumption is made in LLVM code.  This really causes some common loops to have a funky shape.

Loops that don't have a single back-edge are not easily optimizable. 
There is no single point that decides on whether the loop continues to 
iterate or not.  Other than introducing an edge going from outside into 
the middle of the loop, this is likely one of the worst loop pessimizations.


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