[LLVMdev] BasicBlock back()

[John Criswell]

On 12/17/12 10:34 AM, Alexandru Ionut Diaconescu wrote:
> Hello,
>
> I am a beginner of LLVM. I am trying to move among the instructions of 
> a BasicBlock and I cannot. In this particular example, I try to get 
> the previous instruction of the end instruction. I am trying 2 methods:
>
>
>
> 1. I have the following sequence of code:
>
> bool patternDC::runOnBasicBlock(BasicBlock &BB) {
> ...
> if (BB.getTerminator())
>   {
>             Instruction* current = BB.getTerminator();
>             errs() << "\n LAST: "<<*current<<"\n";
>
>             Instruction* prev = &BB.back();
>             errs() << "\n PENULTIMATE: "<<*prev<<"\n";
> ...
>
> The terminal prints the SAME instruction. I don't know how back() 
> works. (Definition at line 199 
> <http://llvm.org/doxygen/BasicBlock_8h_source.html#l00199> of file 
> BasicBlock.h <http://llvm.org/doxygen/BasicBlock_8h_source.html>.)

I believe BasicBlock::back() returns an iterator to the last instruction 
in the BasicBlock which should be its terminator instruction (basic 
blocks are required to have a TerminatorInst as their last instruction).


>
>
>
> 2. I also tried :
>
> bool patternDC::runOnBasicBlock(BasicBlock &BB) {
> ...
> BasicBlock::const_iterator I = BB.begin();
> BasicBlock::const_iterator E = BB.end();
> BasicBlock::const_iterator prev_iter,last_iter;
> prev_iter = NULL; last_iter = NULL;
> for(;I!=E;I++){
>     prev_iter = last_iter;
>     last_iter = I;
> }
> if(prev_iter){
>     errs() << "prev_iter: " << *(dyn_cast<Instruction>(prev_iter)) << 
> "\n";
> }
> if(last_iter){
>     errs() << "last_iter: " << *(dyn_cast<Instruction>(last_iter)) << 
> "\n";
> }
> // not related to the main question: uncomment the next line for an 
> unusual behavior: lastlast is DIFFERENT from last. lastlast is kind of 
> parts of the BasicBlock
> // errs() << "lastlast: " << *(dyn_cast<Instruction>(I)) << "\n";
> ...
> Instruction* prev = *(dyn_cast<Instruction*>(prev_iter));
> errs() << "\n prev: "<<*prev<<"\n";
>
> The terminal prints well prev and last, but I have compilation errors 
> when trying to assign to Instruction* prev
> The Clang error is:
> ".....
> /home/alex/llvm/include/llvm/Support/Casting.h:51:28: error: 'classof' 
> is not a member of 'llvm::Instruction*'"
>
>
> If someone knows a better way to use any element from the basic block 
> or knows why these are not working, please let me know :)

Sometimes you have to dereference an iterator to get the thing that it's 
pointing at.  Try using:

Instruction* prev = (dyn_cast<Instruction>(*prev_iter));

That might work.

-- John T.

>
> Thank you,
> Alex
>
> -- 
> Best regards,
> Alexandru Ionut Diaconescu
>
>
>
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