Remove an instruction through Dead Code Elimination pass of llvm

Question

My pass in LLVM generates an IR like this:

%5 = icmp eq i32 %4, 0
%7 = or i1 %5, %5
...

Since the or instruction is actually not needed(dead code), I replaced all occurrences of %7 with %5. Now, the or instruction should get deleted. Can I call Dead Code Elimination pass of LLVM from my pass, or is there any method to remove that or instruction?

Answer 1

A solution that is more aligned with LLVM's design philosophy is, instead of doing the substitution in your pass, let InstCombine do the job. Then you will not need to worry about running DCE.

For example:

>cat foo.ll
define i32 @foo(i32 %a, i32 %b) #0 {
entry:
  %or = or i32 %a, %a
  ret i32 %or
}
> opt -S -instcombine < foo.ll
define i32 @foo(i32 %a, i32 %b) #0 {
entry:
  ret i32 %a
}

Answer 2

Why don't you just schedule DCE to run after your pass in the pass manager. Let it do its analysis and decide what it wants to throw away.