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

[Jonathan Roelofs via llvm-dev]

On 4/16/17 10:23 AM, Yaron Keren via llvm-dev wrote:
> The legacy pass manager skips declarations, such as puts.
>
> bool FPPassManager::runOnFunction(Function &F) {
>   if (F.isDeclaration())
>     return false;
>
> maybe the documentation needs to be updated?


<wild speculation>Could be that the libc used for the tutorial had these 
other functions as inline.</wild speculation> Since there is a bit of 
variability when it comes to how libc is implemented, the tutorial 
probably ought to be modified to make it less platform dependent.


Jon


>
>
> ‫בתאריך יום א׳, 16 באפר׳ 2017 ב-19:20 מאת ‪Dipanjan Das via llvm-dev‬‏
> <‪llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>‬‏>:‬
>
>
>     This example is taken straight out of official documentation, yet it
>     does not quite work. Did anyone else face such issue with LLVM
>     function pass? Basically, the pass does not get triggered for
>     external library function calls, e.g. malloc(), puts(); however
>      behaves correctly for user defined ones.
>
>     On 15 April 2017 at 15:42, Dipanjan Das <mail.dipanjan.das at gmail.com
>     <mailto:mail.dipanjan.das at gmail.com>> wrote:
>
>
>         I am learning to write LLVM pass by trying to reproduce [hello
>         world][1] 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][2] 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?
>
>           [1]:
>         http://releases.llvm.org/3.8.0/docs/WritingAnLLVMPass.html#quick-start-writing-hello-world
>           [2]:
>         http://releases.llvm.org/3.8.0/docs/WritingAnLLVMPass.html#running-a-pass-with-opt
>
>         --
>
>         Thanks & Regards,
>
>         Dipanjan
>
>
>
>
>     --
>
>     Thanks & Regards,
>
>     Dipanjan
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>
>
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-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded / Siemens