Make HelloWorld pass required

[caojoshua]

HelloWorld pass was not printing anything for me. This PR makes the pass required, as I see many other example passes are required. Not really sure why HelloWorld pass is skipped, but I do know this change forces the pass to be run.

archlinux [~/src/llvm-tutor]$ llvm-config --version
13.0.1
archlinux [~/src/llvm-tutor]$ uname -a
Linux archlinux 5.16.12-arch1-1 #1 SMP PREEMPT Wed, 02 Mar 2022 12:22:51 +0000 x86_64 GNU/Linux

[banach-space]

Hi @caojoshua , many thanks for sending this!

How did you generate the input .ll file? Did you use -O1? Just trying to better to understand the case in which it didn’t work for you.

-Andrzej

[caojoshua]

I did not use -O1. I can see now, that with -O0, the hello-world pass does not do anything. But with -O1, I see the expected output. I can now see in the README example, -O1 is included.

I suppose this change is not necessary, so feel free to close. But maybe we want this change anyway?

---

Can you also help me understand why opt only runs the pass on -O1? Here is my input:

#include "stdio.h"

void foo() { printf("foobar\n"); }

int main() {
  int x = 1234;
  foo();

  return 0;
}

With -O0:

@.str = private unnamed_addr constant [8 x i8] c"foobar\0A\00", align 1

; Function Attrs: noinline nounwind optnone sspstrong uwtable
define dso_local void @foo() #0 {
  %1 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @.str, i64 0, i64 0))
  ret void
}

declare i32 @printf(i8*, ...) #1

; Function Attrs: noinline nounwind optnone sspstrong uwtable
define dso_local i32 @main() #0 {
  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  store i32 1234, i32* %2, align 4
  call void @foo()
  ret i32 0
}

With -O1:

; Function Attrs: nofree nounwind sspstrong uwtable
define dso_local void @foo() local_unnamed_addr #0 {
  %1 = call i32 @puts(i8* nonnull dereferenceable(1) getelementptr inbounds ([7 x i8], [7 x i8]* @str, i64 0, i64 0))
  ret void
}

; Function Attrs: nofree nounwind sspstrong uwtable
define dso_local i32 @main() local_unnamed_addr #0 {
  %1 = call i32 @puts(i8* nonnull dereferenceable(1) getelementptr inbounds ([7 x i8], [7 x i8]* @str, i64 0, i64 0)) #2
  ret i32 0
}

[banach-space]

Thanks for the info!

Can you also help me understand why opt only runs the pass on -O1? Here is my input:

Note the presence of the optnone function attribute when using -O0. That's the key thing here and means that no pass should/will be run for the corresponding function.

But maybe we want this change anyway?

I do :) think that right HelloWorld might be a bit confusing and it should just work ™️ . Would you mind adding a bit more comments to justify the need for isRequired? Perhaps something like (feel free to use this or suggested something else):

Without isRequired returning true, this pass will be skipped for functions decorated with the optnone LLVM attribute. Note that clang -O0 decorates all functions with optnone.

-Andrzej

[caojoshua]

Went with your comment. Note that I only changed HelloWorld.cpp, and not the other instances of isRequired in other passes. I don't think this comment needs to be copy pasted everywhere, as this information is clearly in the README.

[banach-space]

Your contribution is very much appreciated :)