[llvm-dev] Why does an LLVM pass based on FunctionPass not get triggered for certain functions? (original) (raw)

Dipanjan Das via llvm-dev llvm-dev at lists.llvm.org
Sat Apr 15 15:42:00 PDT 2017

I am learning to write LLVM pass by trying to reproduce hello world example. The pass hello.cpp looks like:

#include "llvm/Pass.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

namespace {
  struct Hello : public FunctionPass {
    static char ID;
    Hello() : FunctionPass(ID) {}

    bool runOnFunction(Function &F) override {
      errs() << "Hello: ";
      errs().write_escaped(F.getName()) << '\n';
      return false;
    }
  };
}

char Hello::ID = 0;9
static RegisterPass<Hello> X("hello", "Hello World Pass", false, false);

The sample program world.c looks like:

#include <stdio.h>

int main() {
printf("Hello World\n");
return 0;
}

The program is compiled using the following command line: clang world.c -c -emit-llvm -O3 -o world.bc

The bitcode produced by llvm-dis looks like:

; ModuleID = 'src/hello.bc'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@str = private unnamed_addr constant [12 x i8] c"Hello World\00"

; Function Attrs: nounwind uwtable
define i32 @main() #0 {
  %puts = tail call i32 @puts(i8* getelementptr inbounds ([12 x i8],

[12 x i8]* @str, i64 0, i64 0)) ret i32 0 }

; Function Attrs: nounwind
declare i32 @puts(i8* nocapture) #1

attributes #0 = { nounwind uwtable "disable-tail-calls"="false"

"less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" } attributes #1 = { nounwind }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.8.0 (tags/RELEASE_380/final)"}

When I run the pass on the bitcode: opt -load hello/libhello.so -hello src/world.bc > /dev/null, the output is:

Hello: main

However, the tutorial claims that the output should have been:

Hello: __main
Hello: puts
Hello: main

Why does my pass not get triggered for the first two functions?

--

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