[llvm-dev] LoopSimplify pass prevents loop unrolling

[Friedman, Eli via llvm-dev]

On 6/30/2017 7:48 AM, Balaram Makam via llvm-dev wrote:
>
> Edit. Predecessor -> successor.
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf 
> Of *Balaram Makam via llvm-dev
> *Sent:* Friday, June 30, 2017 10:47 AM
> *To:* llvm-dev at lists.llvm.org
> *Subject:* [llvm-dev] LoopSimplify pass prevents loop unrolling
>
> Hi All,
>
> In the attached test case there, is an unnested loop with 2 
> iterations. The loop latch block is terminated by an unconditional 
> branch, so simplifycfg folds the almost empty latch block into its 
> *successor* which is the loop header. This results in an additional 
> backedge in the CFG, so when LoopRotate pass is called it 
> canonicalizes the loop into a nested loop. However, now the loop trip 
> count is unpredictable as the BackedgeTakenCount for the outer loop is 
> not loop invariant. As a result the loop cannot be unrolled. Is this 
> the intended canonicalization for this loop or is the loopsimplify 
> canonicalizing incorrectly? Should simplifycfg skip folding the latch 
> block into the loop header if this results in additional backedges and 
> let the empty blocks be folded during CGP? More details in 
> https://bugs.llvm.org/show_bug.cgi?id=33605.
>

We have code that's supposed to prevent this from happening; see 
https://reviews.llvm.org/rL264697 .  Maybe it also needs to check 
whether the destination of the branch is a loop header?

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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