Clang hangs with -O1 on switches

[Quuxplusone]

Bugzilla Link	PR18898
Status	NEW
Importance	P normal
Reported by	skvadrik@gmail.com
Reported on	2014-02-19 06:57:26 -0800
Last modified on	2014-02-24 05:49:22 -0800
Version	3.4
Hardware	PC Linux
CC	geek4civic@gmail.com, hans@chromium.org, hfinkel@anl.gov, llvm-bugs@lists.llvm.org, llvm@sunfishcode.online, rnk@google.com
Fixed by commit(s)
Attachments	utf16_group_C.w.cpp (944465 bytes, text/x-c++src)
Blocks
Blocked by
See also

Created attachment 12088
re2c-generated C++ source file, contains big 'switch' statemants.

This hangs infinitely:

$ clang++ -O1 utf16_group_C.w.cpp

This .cpp file is autogenerated by RE2C scanner generator. RE2C can generate
either 'switch' statements or nested 'if' statements that express the same
logic.
Clang hangs on 'switch'es, but doesn't hang on 'if's. Clang hangs with -O1 or
higher level of optimisations, but not with -O0.

[Quuxplusone]

Attached utf16_group_C.w.cpp (944465 bytes, text/x-c++src): re2c-generated C++ source file, contains big 'switch' statemants.

[Quuxplusone]

Here's a backtrace. It seems we're stuck in the loop in
Loop::hasDedicatedExits().

#0  0x00000000010e499d in llvm::PredIterator<llvm::BasicBlock,
llvm::value_use_iterator<llvm::User> >::operator== (this=0x7fffffffb5a0, x=...)
    at /usr/local/google/work/llvm/include/llvm/Support/CFG.h:52
#1  0x00000000010e395f in llvm::PredIterator<llvm::BasicBlock,
llvm::value_use_iterator<llvm::User> >::operator!= (this=0x7fffffffb5a0, x=...)
    at /usr/local/google/work/llvm/include/llvm/Support/CFG.h:53
#2  0x000000000312e15a in llvm::Loop::hasDedicatedExits (this=0x63eb410)
    at /usr/local/google/work/llvm/lib/Analysis/LoopInfo.cpp:341
#3  0x0000000002efa681 in (anonymous namespace)::LICM::runOnLoop (
    this=0x653b3c0, L=0x63eb410, LPM=...)
    at /usr/local/google/work/llvm/lib/Transforms/Scalar/LICM.cpp:281
#4  0x000000000313a29f in llvm::LPPassManager::runOnFunction (this=0x653a7a0,
    F=...) at /usr/local/google/work/llvm/lib/Analysis/LoopPass.cpp:232
#5  0x00000000032d35b4 in llvm::FPPassManager::runOnFunction (this=0x6534a60,
    F=...) at /usr/local/google/work/llvm/lib/IR/LegacyPassManager.cpp:1537
#6  0x00000000030a4360 in (anonymous namespace)::CGPassManager::RunPassOnSCC (
    this=0x6532ad0, P=0x6534a60, CurSCC=..., CG=...,
    CallGraphUpToDate=@0x7fffffffbb0f: true,
    DevirtualizedCall=@0x7fffffffbc1f: false)
    at /usr/local/google/work/llvm/lib/Analysis/IPA/CallGraphSCCPass.cpp:148
#7  0x00000000030a515b in (anonymous
namespace)::CGPassManager::RunAllPassesOnSCC (this=0x6532ad0, CurSCC=...,
CG=...,
    DevirtualizedCall=@0x7fffffffbc1f: false)
    at /usr/local/google/work/llvm/lib/Analysis/IPA/CallGraphSCCPass.cpp:405
#8  0x00000000030a54af in (anonymous namespace)::CGPassManager::runOnModule (
    this=0x6532ad0, M=...)
    at /usr/local/google/work/llvm/lib/Analysis/IPA/CallGraphSCCPass.cpp:460
#9  0x00000000032d3a78 in (anonymous namespace)::MPPassManager::runOnModule (
    this=0x652c170, M=...)
    at /usr/local/google/work/llvm/lib/IR/LegacyPassManager.cpp:1615
#10 0x00000000032d4088 in llvm::legacy::PassManagerImpl::run (this=0x652be80,
    M=...) at /usr/local/google/work/llvm/lib/IR/LegacyPassManager.cpp:1710
#11 0x00000000032d4293 in llvm::legacy::PassManager::run (this=0x58ed510,
    M=...) at /usr/local/google/work/llvm/lib/IR/LegacyPassManager.cpp:1745
#12 0x0000000000f4a237 in (anonymous
namespace)::EmitAssemblyHelper::EmitAssembly (this=0x7fffffffbf40,
Action=clang::Backend_EmitObj, OS=0x5917b00)
    at /usr/local/google/work/llvm/tools/clang/lib/CodeGen/BackendUtil.cpp:599
#13 0x0000000000f4a35f in clang::EmitBackendOutput (Diags=..., CGOpts=...,
    TOpts=..., LOpts=..., TDesc=..., M=0x591f3d0,
    Action=clang::Backend_EmitObj, OS=0x5917b00)

[Quuxplusone]

It is reproducible with "clang -O0 -emit-llvm -S | opt -O1"

...and it is not infinite loop. I hit LICM at first.

===-------------------------------------------------------------------------===
                      ... Pass execution timing report ...
===-------------------------------------------------------------------------===
  Total Execution Time: 2516.2075 seconds (2516.6312 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
  685.5958 ( 27.2%)   0.0000 (  0.0%)  685.5958 ( 27.2%)  685.7107 ( 27.2%)  Delete dead loops
  454.4459 ( 18.1%)   0.0000 (  0.0%)  454.4459 ( 18.1%)  454.5218 ( 18.1%)  Block Frequency Analysis
  349.6658 ( 13.9%)   0.0000 (  0.0%)  349.6658 ( 13.9%)  349.7282 ( 13.9%)  Loop Invariant Code Motion
  345.2845 ( 13.7%)   0.0000 (  0.0%)  345.2845 ( 13.7%)  345.3420 ( 13.7%)  Unswitch loops
  340.6992 ( 13.5%)   0.0000 (  0.0%)  340.6992 ( 13.5%)  340.7560 ( 13.5%)  Loop Vectorization
  340.0083 ( 13.5%)   0.0000 (  0.0%)  340.0083 ( 13.5%)  340.0651 ( 13.5%)  Induction Variable Simplification
(snip)