Clang hangs with -O1 on switches

[llvmbot]

Bugzilla Link	18898
Version	3.4
OS	Linux
Attachments	re2c-generated C++ source file, contains big 'switch' statemants.
Reporter	LLVM Bugzilla Contributor
CC	@zmodem,@hfinkel,@sunfishcode,@rnk

Extended Description

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.

[llvmbot]

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)

[zmodem]

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 >::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 >::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)