[LLVMdev] Separating loop nests based on profile information?
Philip Reames
listmail at philipreames.com
Mon Jan 19 16:44:22 PST 2015
(Splitting this out from "Re: [LLVMdev] Separating loop nests based on
profile information?" since it's now a fairly different topic.)
On 01/09/2015 11:39 AM, Philip Reames wrote:
> On 01/08/2015 12:33 PM, Philip Reames wrote:
>> 2) We need PRE to look back through merge blocks. This should
>> possibly even live in MemoryDependenceAnalysis. Alternatively, we
>> could create a "deconstruct loop simplify form" pass to run right
>> before GVN to side step this issue.
> Point 2 here may be inaccurate. I've definitely seen problems here in
> the past, but when I sat down to write some test cases, it looks like
> the simple merge block for latch (with the load being considered in
> the loop header) works just fine. I suspect my original example is
> actually blocked by something else; I'm just not sure what that is yet.
I went back and looked. One - of possibly several - cases that were
giving me trouble was when we had a loop with multiple latches before
loop-simplify, loop simplify creates a unified latch, and we had not
peeled the first iteration of the loop when we get to PRE. This creates
a tree of merges that PRE doesn't deal with.
I took a stab at addressing this with a PRE patch. It can be found
here: http://reviews.llvm.org/D7061
As I should have expected, it turned out to be quite a bit more involved
that it first looked. :)
Philip
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