[llvm-commits] [PATCH] Fix llvm.invariant support

[Kenneth Uildriks]

On Sun, Dec 5, 2010 at 4:49 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
> Kenneth Uildriks wrote:
>>
>> Here's the updated version.
>
> Firstly, could you please reduce your tests? Try writing tests as .ll files
> by hand; they should be absolutely minimal to expose the optimization (or
> verify that we don't do an unsafe transformation).
>
> +    DEBUG(errs() << "Invariant value is (" << *(BI->first) << ", "
> +        << BI->second.Size << ")\n");
> +    DEBUG(errs() << "Target is (" << *MemLoc.Ptr << ", " << MemLoc.Size
> +        << ")\n");
>
> These should be DEBUG(dbgs() << ...) not errs(). Please check all of them.
>
> Nick
>
After reducing the tests, I found cases that it missed, so I reworked
the algorithm.  Now it recognizes invariant regions as regions of code
dominated by an llvm.invariant.begin that aren't reachable from the
matching llvm.invariant.end without going back through the
llvm.invariant.begin.  It brings in the DominatorTree and
DominanceFrontier analyses to figure this out more quickly and
cleanly.

In MemDepPrereqs.patch, there are changes to DominanceFrontier and
BreakCritical edges that were needed for my MemoryDependenceAnalysis
changes to pass all the nightly tests.  Since DominanceFrontier is
really a cache of frontier analysis results, it seems to me that it
shouldn't be asserting that an add attempt isn't redundant (and a
later pass was trying to add something that my algorithm had already
looked up in a few cases).  Also, there was a latent bug in
BreakCriticalEdges that allows an llvm_unreachable assertion to
occasionally hit if DominanceFrontier is in play and a block is
unreachable.
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