How obtain an Data Dependence Graph (DDG) with LLVM?

Question

It seems LLVM can now emit DDGs for each loop in a function (https://github.com/llvm/llvm-project/blob/llvmorg-12.0.1/llvm/lib/Analysis/DDGPrinter.cpp).

I am able to generate CFG and Callgraph with opt --dot-cfg foo.bc and with opt --dot-callgraph foo.bc, but the similar opt --dot-ddg-only foo.bc executes but generates no .dot file. I also tried opt -passes=dot-ddg foo.bc without success.

Is there another opt possible call? Or someone has suggestions for other similar tools?

C code used (foo.bc obtained with clang -c -emit-llvm foo.c -o foo.bc):

void foo(int b[], int c[], int n){
  for (int i = 1; i < n; i++) {
    b[i] = c[i] + b[i-1];
  }
}

int main(){
  int b[] = {1, 2, 3, 4, 5};
  int c[] = {1, 1, 1, 1, 1};
    
  foo(b, c, 5);
}

user_1551605 · Answer

My answer is probably naive as i am not an expert. I came across the same problem and found out this. First i compile your program to a readable IR:

clang -c -S -emit-llvm foo.c -o foo.ll . Notice this is equivalent to a O0 optimisation level.
the -debug-pass-manager option has been key to me to understand why nothing was neither printed nor written out. Call
```
% opt foo.ll -passes=dot-ddg -debug-pass-manager
```
(Note: this is using the new pass manager syntax) I noticed in the output, things like:
```
Skipping pass DDGDotPrinterPass on foo due to optnone attribute
```

Edit the IR foo.ll and indeed, we can spot the optnone attribute, e.g:

 ; Function Attrs: noinline nounwind optnone uwtable
 define dso_local void @foo(i32* noundef %b, i32* noundef %c, i32 noundef %n) #0 {

You can disable this attribute. I regenerate the IR with the proper option -disable-O0-optnone

% clang -O0 -Xclang -disable-O0-optnone foo.c -emit-llvm -S -o foo.ll

and now the attribute is gone :

; Function Attrs: noinline nounwind uwtable
define dso_local void @foo(i32* noundef %b, i32* noundef %c, i32 noundef %n) #0 {

This time the pass will take effect:

% opt foo.ll -disable-output  -passes=dot-ddg
Writing 'ddg.foo.for.cond.dot'

Final result, that i can't decipher yet, is:

How obtain an Data Dependence Graph (DDG) with LLVM?

1 Answers