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hc2428 at columbia.edu

2009-Sep-27 03:45 UTC

[LLVMdev] A basicblock iterator bug in llvm

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

hc2428 at columbia.edu

2009-Sep-27 03:49 UTC

head link

[LLVMdev] A basicblock iterator bug in llvm

Quoting hc2428 at columbia.edu:

I am using LLVM-2.5. Here is the attachment.
> 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
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>

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Gabor Greif

2009-Sep-27 06:15 UTC

head link

[LLVMdev] A basicblock iterator bug in llvm

On 27 Sep., 05:45, hc2... at columbia.edu wrote:> 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.
Hi hc!

are you on SVN trunk?

Cheers,

    Gabor
>
> 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
>
> _______________________________________________
> LLVM Developers mailing list
> LLVM... at cs.uiuc.edu      
 http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

hc2428 at columbia.edu

2009-Sep-27 06:22 UTC

head link

[LLVMdev] A basicblock iterator bug in llvm

Quoting Gabor Greif <ggreif at gmail.com>:

Hi Gabor,
     I am using the release version of 2.5
> On 27 Sep., 05:45, hc2... at columbia.edu wrote:
>> 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.
>
> Hi hc!
>
> are you on SVN trunk?
>
> Cheers,
>
>     Gabor
>
>>
>> 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
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVM... at cs.uiuc.edu         
>>
 http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>

Gabor Greif

2009-Sep-27 06:25 UTC

head link

[LLVMdev] A basicblock iterator bug in llvm

AAAH!

I see you are still at 2.5. Then this patch (the fix)
is relevant for you:

<http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/
ilist.h?r1=66061&r2=68785&diff_format=h>

Cheers,

    Gabor



On 27 Sep., 05:45, hc2... at columbia.edu wrote:> 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
>
> _______________________________________________
> LLVM Developers mailing list
> LLVM... at cs.uiuc.edu      
 http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

hc2428 at columbia.edu

2009-Sep-27 06:35 UTC

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[LLVMdev] A basicblock iterator bug in llvm

Quoting Gabor Greif <ggreif at gmail.com>:

Hi Gabor,
     I can not open the link, can you send again?
> AAAH!
>
> I see you are still at 2.5. Then this patch (the fix)
> is relevant for you:
>
> <http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/
> ilist.h?r1=66061&r2=68785&diff_format=h>
>
> Cheers,
>
>     Gabor
>
>
>
> On 27 Sep., 05:45, hc2... at columbia.edu wrote:
>> 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
>>
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>>
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>
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