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

Yaron Keren via llvm-dev llvm-dev at lists.llvm.org
Sun Apr 16 09:23:57 PDT 2017

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/rawostream.h" using namespace llvm; namespace { struct Hello : public FunctionPass { static char ID; Hello() : FunctionPass(ID) {} bool runOnFunction(Function &F) override { errs() << "Hello: ";_ _errs().writeescaped(F.getName()) << '\n';_ _return false;_ _}_ _};_ _}_ _char Hello::ID = 0;9_ _static RegisterPass 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 = "x8664-unknown-linux-gnu" @str = private unnamedaddr 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/RELEASE380/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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