[LLVMdev] [RFC] Invariants in LLVM

[Philip Reames]

On 07/17/2014 01:51 PM, Chandler Carruth wrote:
>
>
>     2. Would adding a canonicalization of if(c) { unreachable } to
>     llvm.invariant(c) would be worthwhile?
>
>
> There was a long and painful attempt to implement invariants based on 
> the branch-to-unreachable pattern. It didn't work. I don't expect 
> these patterns to show up often organically and to go away too soon in 
> the optimizer to be useful. The whole point of 'llvm.invariant' 
> instead of the if construct is to distinguish between the case the 
> optimizer should try to remove (a branch to unreachable) and the case 
> the optimizer should try to preserve because of some specific utility. 
> We shouldn't lose this important distinction.
On first thought, I disagree.  I may not be understanding your point though.

My understanding of the previous work was that it tried to use 
"branch-to-unreachable" as the canonical form.  This is inherently 
problematic in an IR based on basic blocks since it split basic blocks 
into many smaller chunks.  It might work out better if we used extended 
basic blocks, but we don't.

I don't see any harm in canonicalizing to "llvm.invariant" from "if(c) 
unreachable".  In either case, we remove the branch and can merge the 
basic blocks.  In the former, we preserve more information for later 
passes.  The only real downside is potentially keeping more Values alive 
and thus forcing the compiler to do more work.

Can you spell out your objections more?


>
>
>      2. We might want to have passes precompute the Value->(set of
>     Invariants) map, and update it as necessary instead of doing
>     transitive-user searches [a suggestion by Chandler].
>
>
> I think this is mostly just a small API tweak to make the code more 
> maintainable. I'm specifically *not* imagining this as a formal 
> analysis pass or anything of the sort.
I would support this.

I'm not seeing the downside to an analysis pass.  Probably not worth 
implementing up front, but might be worthwhile down the road.

Philip
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