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:
I have the following sequence of code:
bool patternDC::runOnBasicBlock(BasicBlock &BB) {
…
if (BB.getTerminator())
{
Instruction* current = BB.getTerminator();
errs() << “\n LAST: “<<*current<<”\n”;
The terminal prints the SAME instruction. I don’t know how back() works. (Definition at line 199 of file BasicBlock.h.)
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(prev_iter)) << “\n”;
}
if(last_iter){
errs() << "last_iter: " << *(dyn_cast(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(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
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). Sometimes you have to dereference an iterator to get the thing that it’s pointing at. Try using: Instruction* prev = (dyn_cast(*prev_iter)); That might work. – John T.
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()
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));
Hi, thank you for your response ! Please find below my comments:
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:
I have the following sequence of code:
bool patternDC::runOnBasicBlock(BasicBlock &BB) {
…
if (BB.getTerminator())
{
Instruction* current = BB.getTerminator();
errs() << “\n LAST: “<<*current<<”\n”;
The terminal prints the SAME instruction. I don’t know how back() works. (Definition at line 199 of file BasicBlock.h.)
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).
Here is the definition of back(): return *this->getPrev(getTail()), so basically it should get the previous function of the terminator. or maybe the tail is considered the head because it is a circular list…
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(prev_iter)) << “\n”;
}
if(last_iter){
errs() << "last_iter: " << *(dyn_cast(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(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(*prev_iter));
error: cannot convert ‘llvm::Instruction’ to ‘llvm::Instruction*’ in initialization , and if I make dyn_cast<Instruction*> I have the previous problem…do you know other method or a pass example from the LLVM site that is passing through the instructions of a BB? And not using complicated dependencies as http://llvm.org/doxygen/EdgeBundles_8cpp_source.html#l00078 ?
