CODEWITHSUNDEEP
llvm
winepretzel
winepretzel

Reputation: 227

Using the address from LLVM store instruction to create another

I'm working with LLVM to take a store instruction and replace it with another so that I can take something like

store i64 %0, i64* %a

and replace it with

store i64 <value>, i64* %a

I've used

llvm::Value *str = i->getOperand(1);

to get the address that my old instruction is using, and then I create a new store via (i is the current instruction location, so this store will be created before the store I'm replacing)

StoreInstr *store = new StoreInst(value, str, i);

I then delete the store I've replaced with

i->eraseFromParent();

But I'm getting the error: While deleting: i64% Use still stuck around after Def is destroyed: store i64 , i64* %a and a failure message that Assertion "use empty" && uses remain when a value is destroyed fail.

How could I get around this? I'd love to create a store instruction and then use LLVM's ReplaceInstWithInst, but I can't find a way to create a store instruction without giving it a location to insert itself. I'm also not 100% that will solve my issue.

I'll add that prior to my store replacement, I'm matching an instruction i, then getting the value I need before performing i->eraseFromParent, so I'm not sure if that is part of my problem; I'm assuming that eraseFromParent moves i along to the following store instruction.

Upvotes: 0

Views: 1392

Answers (1)

Oak
Oak

Reputation: 26868

eraseFromParent removes an instruction from the enclosing basic block (and consequently, from the enclosing function). It doesn't move it anywhere. Erasing an instruction this way without taking care of its uses first will leave your IR malformed, which is why you're getting the error - it's as if you deleted line 1 from the following C snippet:

1   int x = 3;
2   int y = x + 1;

Obviously you'll get an error on the remaining line, the definition of x is now missing!

ReplaceInstWithInst is probably the best way to replace one instruction with another. You don't need to supply the new instruction with a location to insert it with: just leave the instruction as NULL (or better yet, omit the argument) and it will create a dangling instruction which you can then place wherever you want.

Because of the above, by the way, the key method that ReplaceInstWithInst invokes is Value::replaceAllUsesWith, this ensures that you won't be left with missing values in your IR.

Upvotes: 1

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