[llvm] r351520 - Re-enable terminator folding in LoopSimplifyCFG: underlying bugs fixed

[Maxim Kazantsev via llvm-commits]

Hi Jordan,

I’ve prepared a fix that helps this failure, it is https://reviews.llvm.org/D57095
It appeared to be surprisingly hard to create an IR repro of this situation in my case. The bug is sensitive to the order in which the pass manager processes loops. I am still struggling creating such. If you have an IR repro, could you please share it?

Also I’d like to ask you to check, if you still have any problems with LoopSimplifyCFG’s term folding with this fix? I want to be sure that there is no more failures on your side before I re-enable it back.

Thanks,
Max


From: Maxim Kazantsev
Sent: Wednesday, January 23, 2019 1:00 PM
To: 'Jordan Rupprecht' <rupprecht at google.com>
Cc: llvm-commits at lists.llvm.org
Subject: RE: [llvm] r351520 - Re-enable terminator folding in LoopSimplifyCFG: underlying bugs fixed

Thanks for reverting it Jordan! I’ll take a look.

--Max

From: Jordan Rupprecht <rupprecht at google.com<mailto:rupprecht at google.com>>
Sent: Wednesday, January 23, 2019 8:15 AM
To: Maxim Kazantsev <max.kazantsev at azul.com<mailto:max.kazantsev at azul.com>>
Cc: llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>
Subject: Re: [llvm] r351520 - Re-enable terminator folding in LoopSimplifyCFG: underlying bugs fixed

Here's the creduce + stack trace w/ a debug build:

$ cat /tmp/repro.c
int a, b, *c, d, e;
void f() {
  int g;
  for (;;) {
    for (; e; e = b) {
      c = g;
      for (; c; c = d)
        if (a) break;
      if (c) break;
    }
  }
}

$ clang -O1 -fexperimental-new-pass-manager -mllvm -enable-loop-simplifycfg-term-folding -c /tmp/repro.c
clang-9: ~/src/llvm-project/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp:63: void llvm::DeleteDeadBlocks(SmallVectorImpl<llvm::BasicBlock *> &, llvm::DomTreeUpdater *): Assertion
`Dead.count(Pred) && "All predecessors must be dead!"' failed.
...
 #9 0x00007f448da1723b llvm::DeleteDeadBlocks(llvm::SmallVectorImpl<llvm::BasicBlock*>&, llvm::DomTreeUpdater*) ~/src/llvm-project/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp:62:
5
#10 0x00007f448e5c67ab (anonymous namespace)::ConstantTerminatorFoldingImpl::deleteDeadLoopBlocks() ~/src/llvm-project/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp:415:5

On Tue, Jan 22, 2019 at 9:43 AM Jordan Rupprecht <rupprecht at google.com<mailto:rupprecht at google.com>> wrote:
It looks like this is still causing crashes, so I've temporarily reverted this as https://reviews.llvm.org/rL351845. I hope to have a repro later today.

On Thu, Jan 17, 2019 at 9:01 PM Max Kazantsev via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:
Author: mkazantsev
Date: Thu Jan 17 20:57:32 2019
New Revision: 351520

URL: http://llvm.org/viewvc/llvm-project?rev=351520&view=rev
Log:
Re-enable terminator folding in LoopSimplifyCFG: underlying bugs fixed

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=351520&r1=351519&r2=351520&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp Thu Jan 17 20:57:32 2019
@@ -42,7 +42,7 @@ 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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