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

Wed Jan 23 06:29:53 PST 2019

Hi,

We have also found cases that crashes with the enabled term folding, 
that are not fixed with D57095.

E.g.

opt -S -o - bbi-23408.ll -loop-simplifycfg

which yields

opt: ../lib/Analysis/LoopInfo.cpp:460: void (anonymous 
namespace)::UnloopUpdater::updateBlockParents(): Assertion `(NL != 
&Unloop && (!NL || NL->contains(&Unloop))) && "uninitialized successor"' 
failed.
Stack dump:
0.      Program arguments: build-all/bin/opt -S -o - bbi-23408.ll 
-loop-simplifycfg
1.      Running pass 'Function Pass Manager' on module 'bbi-23408.ll'.
2.      Running pass 'Loop Pass Manager' on function '@f1'
3.      Running pass 'Simplify loop CFG' on basic block '%bb3'
  #0 0x000000000229af54 PrintStackTraceSignalHandler(void*) 
(build-all/bin/opt+0x229af54)
  #1 0x0000000002298f60 llvm::sys::RunSignalHandlers() 
(build-all/bin/opt+0x2298f60)
  #2 0x000000000229b2b8 SignalHandler(int) (build-all/bin/opt+0x229b2b8)
  #3 0x00007fe1fe4cb330 __restore_rt 
(/lib/x86_64-linux-gnu/libpthread.so.0+0x10330)
  #4 0x00007fe1fd0bac37 gsignal 
/build/eglibc-ripdx6/eglibc-2.19/signal/../nptl/sysdeps/unix/sysv/linux/raise.c:56:0
  #5 0x00007fe1fd0be028 abort 
/build/eglibc-ripdx6/eglibc-2.19/stdlib/abort.c:91:0
  #6 0x00007fe1fd0b3bf6 __assert_fail_base 
/build/eglibc-ripdx6/eglibc-2.19/assert/assert.c:92:0
  #7 0x00007fe1fd0b3ca2 (/lib/x86_64-linux-gnu/libc.so.6+0x2fca2)
  #8 0x00000000016f3b4d llvm::LoopInfo::erase(llvm::Loop*) 
(build-all/bin/opt+0x16f3b4d)
  #9 0x00000000020e0319 (anonymous 
namespace)::ConstantTerminatorFoldingImpl::run() 
(build-all/bin/opt+0x20e0319)
#10 0x00000000020dc38e simplifyLoopCFG(llvm::Loop&, 
llvm::DominatorTree&, llvm::LoopInfo&, llvm::ScalarEvolution&, 
llvm::MemorySSAUpdater*) (build-all/bin/opt+0x20dc38e)
#11 0x00000000020dce53 (anonymous 
namespace)::LoopSimplifyCFGLegacyPass::runOnLoop(llvm::Loop*, 
llvm::LPPassManager&) (build-all/bin/opt+0x20dce53)
#12 0x00000000016fb08c 
llvm::LPPassManager::runOnFunction(llvm::Function&) 
(build-all/bin/opt+0x16fb08c)
#13 0x0000000001ca18fd 
llvm::FPPassManager::runOnFunction(llvm::Function&) 
(build-all/bin/opt+0x1ca18fd)
#14 0x0000000001ca1bb8 llvm::FPPassManager::runOnModule(llvm::Module&) 
(build-all/bin/opt+0x1ca1bb8)
#15 0x0000000001ca201a llvm::legacy::PassManagerImpl::run(llvm::Module&) 
(build-all/bin/opt+0x1ca201a)
#16 0x00000000007a073b main (build-all/bin/opt+0x7a073b)
#17 0x00007fe1fd0a5f45 __libc_start_main 
/build/eglibc-ripdx6/eglibc-2.19/csu/libc-start.c:321:0
#18 0x000000000078618d _start (build-all/bin/opt+0x78618d)
Abort

/Mikael

On 1/23/19 1:59 PM, Maxim Kazantsev via llvm-commits wrote:
> 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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> 
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