How to change the operand type in the instruction from 32bit to 64bit in LLVM? - c++

Question

I am a newbie to LLVM. And I am trying to change the type of the loop variable (PHINode).
For example, I have an IR as follows:
for.cond1.preheader: ; preds = %entry, %for.inc15
%k.03 = phi i32 [ 0, %entry ], [ %inc16, %for.inc15 ]
br label %for.cond4.preheader
for.cond4.preheader: ; preds = %for.cond1.preheader, %for.inc12
%indvars.iv5 = phi i64 [ 0, %for.cond1.preheader ], [ %indvars.iv.next6, %for.inc12 ]
br label %for.body6
for.body6: ; preds = %for.cond4.preheader, %for.body6
......
%add7 = add nsw i32 %2, 1
......
for.inc12: ; preds = %for.body6
......
for.inc15: ; preds = %for.inc12
%inc16 = add nuw nsw i32 %k.03, 1
%exitcond9 = icmp ne i32 %inc16, 100
br i1 %exitcond9, label %for.cond1.preheader, label %for.end17, !llvm.loop !7 ​
I want to change the type of the variable %k.03, hoping it will change from 32bit to 64bit. And recursively change all its references (Uses) to 64bit. The effect is as follows:
for.cond1.preheader: ; preds = %entry, %for.inc15
%k.03 = phi i64 [ 0, %entry ], [ %inc16, %for.inc15 ]
br label %for.cond4.preheader
for.cond4.preheader: ; preds = %for.cond1.preheader, %for.inc12
%indvars.iv5 = phi i64 [ 0, %for.cond1.preheader ], [ %indvars.iv.next6, %for.inc12 ]
br label %for.body6
for.body6: ; preds = %for.cond4.preheader, %for.body6
......
%add7 = add nsw i32 %2, 1
......
for.inc12: ; preds = %for.body6
......
for.inc15: ; preds = %for.inc12
%inc16 = add nuw nsw i64 %k.03, 1
%exitcond9 = icmp ne i64 %inc16, 100
br i1 %exitcond9, label %for.cond1.preheader, label %for.end17, !llvm.loop !7 ​
Then my approach is as follows:
static void __ChangeInsTo64Bit(User *inst) {
if (!std::strcmp(cast<Instruction>(inst)->getOpcodeName(), "br")) {
return;
}
if (std::strcmp(cast<Instruction>(inst)->getOpcodeName(), "icmp")) {
inst->mutateType(inst->getType()->getWithNewBitWidth(64));
}
for (auto OI = inst->op_begin(), OE = inst->op_end(); OI != OE; ++OI) {
Value *val = *OI;
val->mutateType(val->getType()->getWithNewBitWidth(64));
LLVM_DEBUG(dbgs() << "The Operand is: " << val->getName()
<< "; The bitwidth is: " << val->getType()->getIntegerBitWidth() << "\n");
}
}
static void __ChangePHINodeWidthTo64(Loop *OuterLoop, ScalarEvolution *SE) {
PHINode *OuterPHINode = OuterLoop->getInductionVariable(*SE);
if (!OuterPHINode) {
return ;
}
unsigned int outerPHIWidth = OuterPHINode->getType()->getIntegerBitWidth();
if (outerPHIWidth == 32) {
__ChangeInsTo64Bit(OuterPHINode);
for (User* user : OuterPHINode->users()) {
__ChangeInsTo64Bit(user);
for (User* u : user->users()) {
__ChangeInsTo64Bit(u);
}
}
}
}
Through the above code, I can achieve my goal, but weirdly modify the data type in the basic block for.body6:
%add7 = add nsw i32 %2, i64 1
This will cause program errors. Can someone help me point out the wrong point in my approach or provide the correct approach?

Answer 1

if (!std::strcmp(cast<Instruction>(inst)->getOpcodeName(), "br")) {
return;
}
if (std::strcmp(cast<Instruction>(inst)->getOpcodeName(), "icmp")) {
inst->mutateType(inst->getType()->getWithNewBitWidth(64));
}
That seems like a very janky and error-prone way to do RTTI, especially since LLVM provides the isa<> template for that very purpose.
Case in point, the "icmp" check will match everything that is not icmp (since there is no ! in front of the condition), and I would expect that this is not what you intented.
At the very least, the following is equivalent to your code and a lot more legible:
if (isa<BranchInst>(inst)) {
return;
}
if (!isa<ICmpInst>(inst)) {
inst->mutateType(inst->getType()->getWithNewBitWidth(64));
}
Removing the negation in front of the second test should address at least part of your issue.