[llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default
Maxim Kazantsev via llvm-commits
llvm-commits at lists.llvm.org
Thu Feb 21 03:55:36 PST 2019
Hi again,
Actually I’m in a little harsh about that. Tomorrow is my last day at Azul, and then I’ll be not available for a week or so due to a move. ☺ I’ve added my personal email to be able to continue this discussion after that.
I tried using MSSA->applyUpdates to solve this (the comment claims that it’s the exactly the way how it should work). For some reason, I am hitting the same assert on almost all tests where I try it. Here is the patch I tried (with some extra debug messages): https://reviews.llvm.org/D58500
The failure on test test/Transforms/LoopSimplifyCFG/lcssa.ll happens in the very first function when we do updates at all (handleDeadExits). We accumulate updates corresponding to edges we add/remove. Then, we apply these updates to DTUpdater, and DT is able to pass verification after that (that means the updates are correct). But when we then try to apply the same set up updates to MSSAU, it fails with the assertion:
opt: /home/mkazantsev/work/llvm/llvm/lib/Analysis/MemorySSAUpdater.cpp:848: void llvm::MemorySSAUpdater::applyInsertUpdates(llvm::ArrayRef<llvm::cfg::Update<llvm::BasicBlock*> >, llvm::DominatorTree&, const llvm::GraphDiff<llvm::BasicBlock*, false>*): Assertion `DT.dominates(NewIDom, PrevIDom) && "New idom should dominate old idom"' failed.
I am not familiar with the ways how MSSA works at all, and I don’t understand why this assert is there and if it is possible to live without it. I also wasn’t able to find a method that would do full “recalculation” as a temporary fix for that. ☹
I’ve committed the test that Roman has provided as XFAIL. Alina, could you please take a look into my debug patch above? Maybe you’ll have some insights what is missing there. Is there a way to fully recalculate MSSAU until we find a way to make it efficiently?
If there is no good solution for that, I can only propose to turn off terminator folding when we need to update MSSAU (which would be a bad thing to do).
--Max
From: rtereshin at apple.com <rtereshin at apple.com>
Sent: Thursday, February 21, 2019 3:51 PM
To: Maxim Kazantsev <max.kazantsev at azul.com>; Alina Sbirlea <alina.sbirlea at gmail.com>
Cc: dberlin at dberlin.org; llvm-commits <llvm-commits at lists.llvm.org>; asbirlea at google.com
Subject: Re: [llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default
Yes, I saw her reply too (Hi Alina!). No rush, unless MemorySSA is forced on for loop passes (-enable-mssa-loop-dependency=true) it doesn’t break anything I’m aware of.
I noticed the problem because I’m working on a change to LoopRotate, which is also supposed to preserve MemorySSA, so while testing that part I hit a few failures not related to my changes.
I can just ignore them for time being or force -enable-loop-simplifycfg-term-folding off for testing.
Also, I’m not going to be available for at least 10 hours (time zones).
Thank you,
Roman
P.S. I’m also having difficulties with updating MemorySSA properly. Looks like MemorySSAUpdater contains rather per-case high level APIs, which can for instance handle a BB being split or merged into its necessarily unique predecessor.
What I’m doing is more like SplitBlockAndInsertIfThen, which creates a new BB with 2 predecessors. MemorySSAUpdater::applyUpdates crashes with assert(AddedBlockSet.size() == 1 && "Can only handle adding one predecessor to a new block.”)
if I try updating all edges in one call, or seemingly finishes fine if I do them one by one, but then MemorySSA verifier complains on an incomplete MemoryPhi, which makes me think applyUpdates didn’t really do the right thing anyway.
What surprised me the most is that MemorySSA::createMemoryPhi is a private member function. After some running in circles I think I found a way to update MemorySSA properly, but I had to make MemorySSA::createMemoryPhi
and MemorySSA::getWritableBlockDefs public for that for a time being (locally).
Not sure if I’m missing something, or there is a valid problem with APIs here. Maybe Alina could give us some insight about that?
Thanks,
Roman
On Feb 21, 2019, at 12:23 AM, Maxim Kazantsev <max.kazantsev at azul.com<mailto:max.kazantsev at azul.com>> wrote:
Hi Roman,
As Alina has pointed out, we do MSSA update wrong in general in this patch. I’m going to prepare a fix in ~3 hours from now. I’ll send it to you for review when it’s ready.
--Max
From: rtereshin at apple.com<mailto:rtereshin at apple.com> <rtereshin at apple.com<mailto:rtereshin at apple.com>>
Sent: Thursday, February 21, 2019 2:41 PM
To: Maxim Kazantsev <max.kazantsev at azul.com<mailto:max.kazantsev at azul.com>>
Cc: Alina Sbirlea <alina.sbirlea at gmail.com<mailto:alina.sbirlea at gmail.com>>; dberlin at dberlin.org<mailto:dberlin at dberlin.org>; llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>>; asbirlea at google.com<mailto:asbirlea at google.com>
Subject: Re: [llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default
Hi Max,
Thank you for replying so quickly!
I’ve applied the change locally and the original IR module I used to bugpoint the reproducer I’ve sent you earlier failed anyway, this time for a different reason.
I can’t send the module as it is for IP reasons, but here’s another bugpoint from the same source (with some manual tidying up):
define void @main(i32* %a, i32* %b) {
entry:
br label %for.body
for.body:
%i = phi i32 [ 0, %entry ], [ %i.inc, %latch ]
br label %switch.bb
switch.bb:
switch i2 1, label %default [
i2 1, label %case
]
case:
br label %latch
default:
unreachable
latch:
store i32 %i, i32* %a
store i32 %i, i32* %b
%i.inc = add nsw i32 %i, 1
%exitcond = icmp eq i32 %i.inc, 4
br i1 %exitcond, label %exit, label %for.body
exit:
ret void
}
./bin/opt -loop-simplifycfg loop-simplifycfg-term-folding-AND-mssa-loop-dependency-2.ll -enable-mssa-loop-dependency=true -verify-memoryssa=true -enable-loop-simplifycfg-term-folding=true -S -o -
Assertion failed: (dominates(MD, U) && "Memory Def does not dominate it's uses"), function verifyDomination, file ../lib/Analysis/MemorySSA.cpp, line 1912.
If I don’t apply https://reviews.llvm.org/rL354547 it fails the same way the previous reproducer did, so I guess this test is strictly better. Sorry for overbugpointing the first time around!
Thanks,
Roman
On Feb 20, 2019, at 10:12 PM, Alina Sbirlea <alina.sbirlea at gmail.com<mailto:alina.sbirlea at gmail.com>> wrote:
Are you sure your update is complete?
It should theoretically match the DT update, i.e. include insert edges not just the delete. All updates should be done together using the same list of updates used for DT, and an up to date DT (see MSSAU->applyUpdates(DTUpdates, DT)).
On Wed, Feb 20, 2019 at 9:53 PM Maxim Kazantsev via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:
Should be fixed by https://reviews.llvm.org/rL354547
From: Maxim Kazantsev
Sent: Thursday, February 21, 2019 11:49 AM
To: 'rtereshin at apple.com<mailto:rtereshin at apple.com>' <rtereshin at apple.com<mailto:rtereshin at apple.com>>
Cc: llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>>; dberlin at dberlin.org<mailto:dberlin at dberlin.org>; asbirlea at google.com<mailto:asbirlea at google.com>
Subject: RE: [llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default
Hi Roman,
Thanks for notification! I’ll fix it asap.
--Max
From: rtereshin at apple.com<mailto:rtereshin at apple.com> <rtereshin at apple.com<mailto:rtereshin at apple.com>>
Sent: Thursday, February 21, 2019 10:25 AM
To: Maxim Kazantsev <max.kazantsev at azul.com<mailto:max.kazantsev at azul.com>>
Cc: llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>>; dberlin at dberlin.org<mailto:dberlin at dberlin.org>; asbirlea at google.com<mailto:asbirlea at google.com>
Subject: Re: [llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default
Hi Max,
Looks like this doesn’t collaborate with MemorySSA very well:
./bin/opt -loop-simplifycfg loop-simplifycfg-term-folding-AND-mssa-loop-dependency.ll -enable-mssa-loop-dependency=true -verify-memoryssa=true -enable-loop-simplifycfg-term-folding=true -S -o -
Assertion failed: (find(predecessors(&B), Phi->getIncomingBlock(I)) != pred_end(&B) && "Incoming phi block not a block predecessor"), function verifyDefUses, file ../lib/Analysis/MemorySSA.cpp, line 1947.
The IR is:
define void @main() {
entry:
br label %for.body
for.body: ; preds = %exit, %entry
br label %switch.bb<http://switch.bb/>
switch.bb<http://switch.bb/>: ; preds = %for.body
switch i2 1, label %default.bb<http://default.bb/> [
i2 1, label %case.bb<http://case.bb/>
]
case.bb<http://case.bb/>: ; preds = %switch
br label %exit
default.bb<http://default.bb/>: ; preds = %switch
unreachable
exit: ; preds = %case.bb<http://case.bb/>
call void @foo()
br label %for.body
}
declare void @foo()
Thanks,
Roman
> On Feb 12, 2019, at 10:12 PM, Max Kazantsev via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:
>
> Author: mkazantsev
> Date: Tue Feb 12 22:12:48 2019
> New Revision: 353911
>
> URL: http://llvm.org/viewvc/llvm-project?rev=353911&view=rev
> Log:
> [LoopSimplifyCFG] Re-enable const branch folding by default
>
> Known underlying bugs have been fixed, intensive fuzz testing did not
> find any new problems. Re-enabling by default. Feel free to revert if
> it causes any functional failures.
>
> Modified:
> llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp?rev=353911&r1=353910&r2=353911&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp Tue Feb 12 22:12:48 2019
> @@ -41,7 +41,7 @@ using namespace llvm;
> #define DEBUG_TYPE "loop-simplifycfg"
>
> static cl::opt<bool> EnableTermFolding("enable-loop-simplifycfg-term-folding",
> - cl::init(false));
> + cl::init(true));
>
> STATISTIC(NumTerminatorsFolded,
> "Number of terminators folded to unconditional branches");
>
>
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