Getting the value stored from register in llvm IR - c++

Question

I have a simple C program.
int
1.main(int argc, char **argv) {
2. unsigned buffer[4] = { 0, 0, 0, 0 };
3. return buffer[argc];
4. }
And the IR code is as below
; Function Attrs: norecurse nounwind readnone uwtable
define i32 #main(i32 %argc, i8** nocapture readnone %argv) #0 !dbg !6 {
%buffer = alloca [4 x i32], align 16
tail call void #llvm.dbg.value(metadata i32 %argc, i64 0, metadata !14, metadata !21), !dbg !22
tail call void #llvm.dbg.value(metadata i8** %argv, i64 0, metadata !15, metadata !21), !dbg !23
%1 = bitcast [4 x i32]* %buffer to i8*, !dbg !24
call void #llvm.lifetime.start(i64 16, i8* %1) #3, !dbg !24
tail call void #llvm.dbg.declare(metadata [4 x i32]* %buffer, metadata !16, metadata !21), !dbg !25
call void #llvm.memset.p0i8.i64(i8* %1, i8 0, i64 16, i32 16, i1 false), !dbg !26
%2 = sext i32 %argc to i64, !dbg !28
%3 = getelementptr inbounds [4 x i32], [4 x i32]* %buffer, i64 0, i64 %2, !dbg !28
%4 = load i32, i32* %3, align 4, !dbg !28, !tbaa !29
call void #llvm.lifetime.end(i64 16, i8* %1) #3, !dbg !33
ret i32 %4, !dbg !34
}
I want to compare whether the accessing index at line 3 is a valid index. For this comparison I need to extract the value stored for the argc. Below is piece of code I have written to obain the vvalue of argc
auto gep = llvm::dyn_cast<llvm::GetElementPtrInst>(inst);
auto operand2 = gep->getOperand(2);
outs() << "operand 2 "<<*operand2<<"\n";
auto newOperand =operand2.getOperand(0);
outs()<<"New operand "<<*newOperand<<"\n";
Output :-
operand 2 %2 = sext i32 %argc to i64, !dbg !28
New operand i32 %argc
How can I get the value of %argc?

Answer 1

The return value of getOperand function is Value* object of the argc variable (variable newOperand in your code). You can pass that value to any new instruction that you might want to inject (for example CreateICmpEQ) in the IR to compare value of argc with some constant value.