[LLVMdev] BasicBlock back()

[Duncan Sands]

Hi Alexandru,

On 17/12/12 17:34, 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();

the terminator is the last instruction in a basic block (by definition).

>              errs() << "\n LAST: "<<*current<<"\n";
>
>              Instruction* prev = &BB.back();

"back" returns you the last instruction in the basic block, thus it is the same
as the terminator.  I think you are looking for current->getPrev()

>              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>.)
>
>
>
> 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));

Probably this should be:

   const Instruction* prev = *prev_iter;

Ciao, Duncan.

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