c++ - llvm function pass ReplaceInstWithInst malloc -

#include "llvm/pass.h" #include "llvm/ir/module.h" #include "llvm/ir/function.h" #include "llvm/support/raw_ostream.h" #include "llvm/ir/legacypassmanager.h" #include "llvm/ir/instrtypes.h" #include "llvm/transforms/ipo/passmanagerbuilder.h" #include "llvm/ir/irbuilder.h" #include "llvm/transforms/utils/basicblockutils.h"  using namespace llvm;  namespace {   struct replacepass : public functionpass {     static char id;     replacepass() : functionpass(id) {}      virtual bool runonfunction(function &f) {       allocainst* insttoreplace = ???        basicblock::iterator ii(insttoreplace);       replaceinstwithinst(insttoreplace->getparent()->getinstlist(), ii, new allocainst(type::int32ty, 0, insttoreplace));       return true;     }   }; }  char replacepass::id = 0;  static void registerreplacepass(const passmanagerbuilder &, legacy::passmanagerbase &pm) {     pm.add(new replacepass()); } static registerstandardpasses     registermypass(passmanagerbuilder::ep_earlyaspossible, registerreplacepass); 

hi, i'm trying understand llvm passes i'm kinda stuck.
i wanted start simple thing, replacing stack-based memory allocation alloca heap-based memory allocation malloc.
using gdb saw alloca instruction doesn't have "a name" so, think, can't use getfunction utility.
something came mind searching alloca opcode , replacing malloc opcode.
but llvm docs not best doc i've ever seen don't know if can such thing.
can give me suggestions, please?

edit:

#include <iostream> #include <stdio.h> #include <stdlib.h> #include <alloca.h>  using namespace ::std;  void func() {  int *p = (int *)alloca(20);  char *s = (char *)alloca(150); }  int main (void) {     cout << "hello, llvm\n";     char *arr = (char *)malloc(10);     func();     free (arr)     return 0; } 

edit 2:

#include "llvm/pass.h" #include "llvm/ir/module.h" #include "llvm/ir/function.h" #include "llvm/support/raw_ostream.h" #include "llvm/ir/legacypassmanager.h" #include "llvm/ir/instrtypes.h" #include "llvm/transforms/ipo/passmanagerbuilder.h" #include "llvm/ir/irbuilder.h" #include "llvm/transforms/utils/basicblockutils.h" #include <string.h>  using namespace llvm;  namespace {     struct replacepass : public functionpass {         static char id;         replacepass() : functionpass(id) {}          virtual bool runonfunction(function &f) {             // iterate on basic blocks of function             (auto &bb : f) {                 // iterate on instructions of basic block                 (auto &i : bb) {                     if (allocainst *ci = dyn_cast<allocainst>(&i)) {                         if (!((i.getname()).empty()))                         {                             const char *s = i.getopcodename();                             if(strcmp(s, "alloca") == 0){                                 errs().write_escaped(i.getname()) << " " << i.getopcodename();                                 errs() << "\n";                             }                             if(strcmp(s, "malloc") == 0){                                 errs().write_escaped(i.getname()) << " " << i.getopcodename();                                 errs() << "\n";                             }                         }                     }                 }             }             return false;         }     }; }  char replacepass::id = 0;  static void registerreplacepass(const passmanagerbuilder &, legacy::passmanagerbase &pm) {     pm.add(new replacepass()); } static registerstandardpasses     registermypass(passmanagerbuilder::ep_earlyaspossible, registerreplacepass); 

i wrote above code doesn't work because 1) malloc , alloca both called "alloca" in llvm , 2) have same opcode (26)

you can iterate on function's basicblocks begin() , on bb's instructions in same way. alternatively, can make pass work on basicblock level. when analysing instruction use isa<insttype> find out type.

i find strange didn't liked llvm docs. this manual, example, super useful me.

edit:

to see llvm representation of code working on use cppbackend. compile code clang -march=cpp , produce c++ source, make code using llvm api. there see how every instruction created and, therefore, how differ.

another useful tool -emit-llvm flag conjuction of -s flag. running clang -emit-llvm -s produce llvm ir assembly makes code.

regarding problem, think misunderstood llvm's alloca sematics. allocainst corresponds to, int a=1 in c code. looking callinst "alloca" , "malloc" functions.