[LLVMdev] [RFC] Invariants in LLVM

[Chandler Carruth]

On Thu, Jul 17, 2014 at 5:38 PM, Philip Reames <listmail at philipreames.com>
wrote:

> 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?


Adding an invariant has a cost. Just because we nuked a branch doesn't mean
that the condition feeding the branch is definitely a high-value invariant
to preserve in the IR.

I wouldn't mind an experiment to see if this ended up being profitable, but
I'm dubious about it's profitability so far, and would want to see data to
back up this change as I worry about significantly increasing the density
of invariants (and thus their compile time cost and cost on hasOneUse
failures) without any real benefit.
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