[LLVMdev] A basicblock iterator bug in llvm

[hc2428 at columbia.edu]

Dear developers:
     When I am doing basicblock pass, I meet a bug: there is an  
iterator "I" in a basicblock, and it is not pointing to the first  
instruction in this basicblock. However, "I--;" will fail by an  
assertion.

The basic block ("I" is pointing to the second instruction) in test.ll:
bb:		; preds = %bb1
	%1 = call i32 (i8*, ...)* @printf(i8* noalias getelementptr ([42 x  
i8]* @.str1, i32 0, i32 0)) nounwind		; <i32> [#uses=0]
	%2 = call i32 (i8*, ...)* @printf(i8* noalias getelementptr ([40 x  
i8]* @.str2, i32 0, i32 0)) nounwind		; <i32> [#uses=0]
	%3 = call i32 (i8*, ...)* @printf(i8* noalias getelementptr ([40 x  
i8]* @.str3, i32 0, i32 0)) nounwind		; <i32> [#uses=0]
	%4 = call i32 (i8*, ...)* @printf(i8* noalias getelementptr ([42 x  
i8]* @.str, i32 0, i32 0)) nounwind		; <i32> [#uses=0]
	br label %bb1


The code slice in verify-fixrace.cpp in attachment:

cerr << "IsFirst Current Inst: " << *I;
cerr << "IsFirst First Inst: " << *BB->begin();
I--;



Output is:
Inst: 	%2 = call i32 (i8*, ...)* @printf(i8* noalias getelementptr  
([40 x i8]* @.str2, i32 0, i32 0)) nounwind		; <i32> [#uses=0]
Backward Visit BB: bb
IsFirst Current Inst: 	%2 = call i32 (i8*, ...)* @printf(i8* noalias  
getelementptr ([40 x i8]* @.str2, i32 0, i32 0)) nounwind		; <i32>  
[#uses=0]
IsFirst First Inst: 	%1 = call i32 (i8*, ...)* @printf(i8* noalias  
getelementptr ([42 x i8]* @.str1, i32 0, i32 0)) nounwind		; <i32>  
[#uses=0]
opt: /home/heming/defens/llvm-2.5/include/llvm/ADT/ilist.h:173:  
llvm::ilist_iterator<NodeTy>&  
llvm::ilist_iterator<NodeTy>::operator--() [with NodeTy =  
llvm::Instruction]: Assertion `Traits::getNext(NodePtr) && "--'d off  
the beginning of an ilist!"' failed.
0   opt                    0x08642be4
1   opt                    0x08642f26
2                          0xb7f7f420 __kernel_sigreturn + 0
3   libc.so.6              0xb7d0ca01 abort + 257
4   libc.so.6              0xb7d0410e __assert_fail + 238
5   opt                    0x08357997  
llvm::ilist_iterator<llvm::Instruction>::operator--() + 87
6   opt                    0x08367104  
llvm::ilist_iterator<llvm::Instruction>::operator--(int) + 28
7   verify-fixrace-stub.so 0xb7f781be  
defens::verify_fixrace_stub::DFSBackward(llvm::Instruction*,  
std::string&, int, bool) + 826
8   verify-fixrace-stub.so 0xb7f78311  
defens::verify_fixrace_stub::runOnBasicBlock(llvm::BasicBlock&) + 261
9   opt                    0x085cdaf4
10  opt                    0x085cee1c  
llvm::FPPassManager::runOnFunction(llvm::Function&) + 250
11  opt                    0x085cefb8  
llvm::FPPassManager::runOnModule(llvm::Module&) + 98
12  opt                    0x085cd8a3  
llvm::MPPassManager::runOnModule(llvm::Module&) + 197
13  opt                    0x085cf256  
llvm::PassManagerImpl::run(llvm::Module&) + 116
14  opt                    0x085cf2af  
llvm::PassManager::run(llvm::Module&) + 27
15  opt                    0x082f91d0 main + 4452
16  libc.so.6              0xb7cf6450 __libc_start_main + 224
17  opt                    0x082e9c11 __gxx_personality_v0 + 849