[llvm-dev] LTO early plugin is too late - or how to disable dead code optimization?

[Teresa Johnson via llvm-dev]

LTO itself has global dead code elimination. Try disabling with an internal
option: -Wl,-mllvm,-compute-dead=false.

Teresa

On Wed, Feb 24, 2021 at 8:46 AM Marc via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> I am writing an LTO plugin path that should be run before optimization,
> however before my pass is running functions have already been removed.
>
> Simple example below.
> b.c contains foo(), bar() and foobar(). a.c contains the main and runs
> either foo() or bar() but not foobar().
>
> The bitcode file that ld.lld receives still contains the empty foobar()
> function. I try to prevent any kind of optimization - but still the
> foobar() function is already eliminated before the plugin is running.
>
> How can I prevent the elimination of dead code?
> I already use -O0 -fno-inline -fno-inline-functions -Wl,--discard-none
> -Wl,--lto-O0 -femit-all-decls -fno-virtual-function-elimination
> -Wl,--no-gc-sections
> Thank you!
>
> # cat a.c
> #include <stdio.h>
> int main(int argc, char **argv) {
>   if (argc == 2)
>     foo();
>   else
>     bar();
>   return 0;
> }
>
> # cat b.c
> void foo() {}
> void bar() {}
> void foobar() {}
>
> # cat f.cpp
> #include "llvm/Support/Debug.h"
> #include "llvm/Support/raw_ostream.h"
> #include "llvm/IR/LegacyPassManager.h"
> #include "llvm/IR/Module.h"
> #include "llvm/Transforms/IPO/PassManagerBuilder.h"
> #include "llvm/Pass.h"
> using namespace llvm;
> namespace {
> class Unreachable : public ModulePass {
>  public:
>   static char ID;
>   Unreachable() : ModulePass(ID) {}
>   bool runOnModule(Module &M) override;
> };
> }
> char Unreachable::ID = 0;
> bool Unreachable::runOnModule(Module &M) {
>   errs() << "Running plugin ...\n";
>   for (auto &F : M)
>     errs() << F.getName() << "\n";
>   return true;
> }
> static void registerUnreachablePass(const PassManagerBuilder &,
> legacy::PassManagerBase &PM) {
>   auto p = new Unreachable();
>   PM.add(p);
> }
> static RegisterStandardPasses RegisterUnreachablePassLTO(
>     PassManagerBuilder::EP_FullLinkTimeOptimizationEarly,
>     registerUnreachablePass);
>
> # export CFLAGS="-O0 -g -flto=full"
> # clang $CFLAGS -c a.c
> # clang $CFLAGS -c b.c
> # clang++ `llvm-config --cxxflags` -fno-rtti -fPIC -std=c++14 -o f.so
> -shared f.cpp
>
> # clang -fno-inline -fno-inline-functions -Wl,--discard-none
> -Wl,--lto-O0 -femit-all-decls -fno-virtual-function-elimination
> -Wl,--no-gc-sections -fuse-ld=/usr/bin/ld.lld $CFLAGS
> -fno-experimental-new-pass-manager -Wl,-mllvm=-load=./f.so -o ab a.o b.o
>
> Running plugin ...
> main
> llvm.dbg.declare
> foo
> bar
>
> #^^^the foobar function is gone :-(
>
> Regards,
> Marc
>
> --
> Marc Heuse
> www.mh-sec.de
>
> PGP: AF3D 1D4C D810 F0BB 977D  3807 C7EE D0A0 6BE9 F573
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>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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