[llvm-dev] Placing an existing function's arguments in a call to another function in an LLVM Function Pass

[Joseph via llvm-dev]

Hey Craig. Thank you for answering my question. You're correct, I did
stupid and used the arguments from the Function*. The arguments of the
CallInst worked perfectly.

Many thanks!

On Thu, Apr 9, 2020, 04:19 Craig Topper <craig.topper at gmail.com> wrote:

> Is foo_func a Function*? If so that’s just the function declaration. You
> need to copy the arguments from the CallInst that calls foo.
>
> On Wed, Apr 8, 2020 at 6:21 PM Joseph via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hey,
>>
>> I'm writing an LLVM function pass. So far, the "Programmer's Manual" was
>> a tremendous help.
>>
>> I have a function `foo(int a, int b)`, where in some instances, I'll
>> need to replace it's call with `bar(int a, int b)`.
>>
>> The way I wanted to do it is to basically:
>> * Locate the `foo()` I need to replace
>> * Make a `CallInst` to `bar()`
>> * Populate `CallInst::Create` with the arguments of `foo()`
>> * Make a call to `ReplaceInstWithInst()` to have it work
>>
>> Everything is working just fine, but the arguments of `foo()` are not
>> getting copied to `bar()`. When the replacement call is executed, the
>> arguments of `bar()` are just null.
>>
>> Here's the relevant code:
>>
>> ```c
>> bool runOnFunction(Function& F) override
>> {
>>      CallInst* call_to_foo = 0;
>>      Function* foo_func = 0;
>>
>>      /*
>>      Loop over all calls in the function and populate foo_func when you
>> find it.
>>
>>      If we reached below, that means the current function we're in has a
>> call to
>>      foo() (inside call_to_foo) that we need to replace with bar().
>> Also, foo_func
>>      is pointing to a foo Function
>>      */
>>
>>      Function* bar_func = get_bar_func();
>>
>>      // Collect foo args
>>      // I believe here is the issue: the arguments are not copied
>>      //  properly or there must be a deep-copy of sorts for it to work
>>      std::vector<Value*> bar_func_args;
>>      for (size_t i = 0; i < foo_func->arg_size(); i++) {
>>          Argument* arg = foo_func->arg_begin() + i;
>>          bar_func_args.push_back(arg);
>>      }
>>
>>      auto* inst_to_replace = CallInst::Create(
>>          bar_func, ArrayRef<Value*>(bar_func_args),
>>          "bar_func");
>>
>>      ReplaceInstWithInst(
>>      call_inst->getParent()->getInstList(),
>>      BBI, inst_to_replace);
>>
>>      return true;
>> }
>> ```
>>
>> Any help would be tremendously appreciated.
>>
>> --
>> Joseph
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
> --
> ~Craig
>
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