Created attachment 31902 [details]
PoC code

Modifications of global variables are sometimes not carried out properly when FLTO is enabled. Short PoC follows.

// FILE bug-flto.cpp //
#include <vector>
#include <stdio.h>

#include "shared.h"

std::vector<int> globalvec;

S1::S1() {
    printf("S1() called\n");
    globalvec.push_back(42);
    printf("S1() globalvec.size() = %d\n", (int) globalvec.size());
}

int main() {
    printf("main globalvec.size() = %d\n", (int) globalvec.size());
    return 0;
}

// FILE tu2.cpp
#include "shared.h"

S1 s1;

// FILE shared.h
#pragma once

struct S1 {
    S1();
};

// FILE Makefile
run: bug-flto
	./bug-flto

bug-flto: bug-flto.o tu2.o
	g++-4.8 -O -flto -obug-flto bug-flto.o tu2.o

bug-flto.o: bug-flto.cpp
	g++-4.8 -O -flto -obug-flto.o -c bug-flto.cpp
	
tu2.o: tu2.cpp
	g++-4.8 -O -flto -otu2.o -c tu2.cpp
	
clean:
	rm *.o
	rm