[llvm-dev] Replacing a function from one module into another one

[Philip Pfaffe via llvm-dev]

Hi Daniel,

[CloneFunctionInto](
http://llvm.org/doxygen/namespacellvm.html#adea5a1faf61727d5a3c00684a22d8df6)
helps you copy entire functions. It only takes care of the copy though,
you'll have to empty out the replaced function manually.

Cheers,
Philip

On Tue, Aug 28, 2018 at 5:53 PM Daniel Moya via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello LLVM Developers,
>
> I'm trying to replace a function defined in one module into another module
> (different files). The first issue I ran into was that llvm::Function does
> not have a method "moveBefore" or "moveAfter" as the llvm::BasicBlock or
> llvm::Instruction do, so I figured I would just move the BasicBlocks of the
> replacing function into the function that was being replaced, and then
> eliminate the original BasicBlocks. So far I had only one issue while
> eliminating the original BasicBlocks, I can only removeFromParent but not
> eraseFromParent, but the first function works fine anyways. For example,
> the original function is:
>
> define i32 @foo2(i32 %a, i32 %b) #0 {
> entry:
>   %a.addr = alloca i32, align 4
>   %b.addr = alloca i32, align 4
>   store i32 %a, i32* %a.addr, align 4
>   store i32 %b, i32* %b.addr, align 4
>
> And the function that is going to replace that is:
>
> define i32 @foo3(i32 %a, i32 %b) #0 {
> entry:
>   %a.addr = alloca i32, align 4
>   %b.addr = alloca i32, align 4
>   store i32 %a, i32* %a.addr, align 4
>   store i32 %b, i32* %b.addr, align 4
>
> So they are pretty much the same except for the name (the functions differ
> later in the code, but the error happens at the beginning). So what I do is
> move the entry block in foo3 before the entry block in foo2 and then
> "removeFromParent" the original entry block in the foo2. When I try to dump
> the Module, an error says:
>
> Use still stuck around after Def is destroyed:  store i32 %0, i32*
> %a.addr1, align 4
>
> Which I understand as that the new instruction "store" inserted in the new
> block in foo2 is still referring to the foo3 function's argument, which
> confused me at the beginning since I named both arguments the same to
> facilitate the process of substitution. What I thought as a solution was to
> then change the arguments of the function foo2 (the one that is being
> replaced) with the ones in the function foo3, to "move" the references, but
> I don't understand the difference between the functions of llvm::Function
> "addAttribute" and "addParamAttr", neither why they require to specify the
> attribute (or the  Attribute::AttrKind
> <http://llvm.org/doxygen/classllvm_1_1Attribute.html#aab7ee4b8fd1d3e7e4cea87868855e60e>
>  ) if the function already requires the argument position. When I did a
> similar process to change the calling instruction in the main function, I
> used the functions "setArgOperand" and "getArgOperand" which worked just
> fine, and also the "replaceAllUsesWith", I'm looking for something similar
> in the case of the function's arguments and their use in the function's
> body.
>
> So the question is: How can I properly replace a function (in my case only
> the function's body, since I'm limiting already that both functions have
> the same header) defined in one module intro another module.
>
> Regards,
> Daniel Moya
> _______________________________________________
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> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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