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

[Yaron Keren via llvm-dev]

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?


‫בתאריך יום א׳, 16 באפר׳ 2017 ב-19:20 מאת ‪Dipanjan Das via llvm-dev‬‏ <‪
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>
> 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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170416/19442b03/attachment.html>