What is the purpose of @preprocess_tester

[certik]

I tried this simple input sin2.ll:

; ModuleID = '<stdin>'
source_filename = "<stdin>"

; Function Attrs: norecurse nounwind readnone
define double @tester(double %x) #0 {
entry:
  %0 = fmul double %x, %x
  ret double %0
}

define double @test_derivative(double %x) local_unnamed_addr {
entry:
  %0 = tail call double (double (double)*, ...) @__enzyme_autodiff(double (double)* nonnull @tester, double %x)
  ret double %0
}

declare double @__enzyme_autodiff(double (double)*, ...) local_unnamed_addr

attributes #0 = { norecurse nounwind readnone }

and run it through like this:

opt-6.0 < sin2.ll -load ../../build/Enzyme/LLVMEnzyme-6.so  -enzyme -enzyme_preopt=false -O3 -S

with the result:

; ModuleID = '<stdin>'
source_filename = "<stdin>"

; Function Attrs: norecurse nounwind readnone
define double @tester(double %x) local_unnamed_addr #0 {
entry:
  %0 = fmul double %x, %x
  ret double %0
}

; Function Attrs: norecurse nounwind readnone
define double @test_derivative(double %x) local_unnamed_addr #0 {
entry:
  %factor.i = fmul fast double %x, 2.000000e+00
  ret double %factor.i
}

; Function Attrs: norecurse nounwind readnone
define double @preprocess_tester(double %x) local_unnamed_addr #0 {
entry:
  %0 = fmul double %x, %x
  ret double %0
}

attributes #0 = { norecurse nounwind readnone }

I can see that it left @tester intact that returns x^2. Then it optimized out the @__enzyme_autodiff to just return 2*x.

But why did it create the @preprocess_tester function?

[wsmoses]

For all functions Enzyme analyzes, it creates a copy called preprocess_ where it can do some preprocessing optimizations (useful if you have Enzyme take a derivative of its own output). In this case it didn't do anything (especially since the enzyme_preopt=false flag says only do the bare minimum preprocessing necessary).

All preprocessing functions can be ignored/deleted (they've only been left around so far for debugging purposes).

It's particularly useful to view the preprocessed function if you supply the optional flag enzyme_inline=1 where it tries to inline all functions when doing AD if possible (often positive for performance, and a good comparison to make).

[certik]

Cool, thanks for the info.