How to use replaceUsesWithIf

Beginners
1

I am very new to LLVM. I illustrate what I want to achieve visually below.

Initially, Inst A is used by other Inst (B, C, D, etc). Now, I want to create a new Inst (A’), which uses A still. However, all other Inst now will use Inst A’ instead. After going through documentations, I find replaceUsesWithIf() function, but I can hardly find any example about how to use it, especially on how to insert the conditional as function_ref.

   A
  /|\
 / | \
B  C  D ...  // Before


   A
   |
   |
   A'
  /|\
 / | \
B  C  D ... // After

(Inst A).replaceUsesWithIf((Inst A'), ...);
2

Hello,

For your case it would be something like:

A1 = /* ... create insn and pass A as operand ... */
A->replaceUsesWithIf(A1, [=](Use *U) { return U.getUser() != A1; })

In general for such utility functions I recommend to grep through LLVM code base to get a feeling about usage patterns.

Disclaimer: I’m a beginner myself, so there might be some utility function I’m not aware of.

3

Also, be careful not to modify uses or instruction lists while iterating over those lists as it might invalidate the iterator. And there are no asserts that would tell you about it… I collect instructions I need to modify in some kind of a container and apply modifications afterwards.