如果我在代码块中构建代码，为什么我的代码运行得更快？

Question

I have created a small library (around 600 lines) in C++ in Codeblocks and I'm using OMP and O3 optimization to build it. When I try to build the same code through the terminal with a Makefile with exactly the same options ( -fopenmp -O3 ) it runs around 3 times slower. I need to build it in various machines so I need to do the process through a terminal and not through Codeblocks. Why is this happening? This is my Makefile if you're interested:

CC=g++ 
CFLAGS= 
LDFLAGS= -fopenmp -O3 -std=c++11 
SOURCES=main.cpp CNNFunctions.cpp     
OBJECTS=$(SOURCES:.cpp=.o) EXECUTABLE=cnn

all: $(SOURCES) $(EXECUTABLE) 

$(EXECUTABLE): $(OBJECTS) 
    $(CC) $(LDFLAGS) $(OBJECTS) -o $@

.cpp.o: 
    $(CC) $(CFLAGS) $< -o $@

Answer 1

Because, contrary to your claim, you're not building it with the exact same options.

Your CFLAGS are empty, and those are the flags you're using for the compilation. You cannot resolve that by the time you get to the linking step.

Answer 2

Your Makefile is wrong. The optimization flags are relevant mostly at compile time. Try at least:

CXX=g++ 
CXXFLAGS=  -Wall -fopenmp -O3 -std=c++11 -mtune=native
LDFLAGS= -fopenmp
SOURCES=main.cpp CNNFunctions.cpp     
OBJECTS=$(SOURCES:.cpp=.o) 
EXECUTABLE=cnn

all: $(SOURCES) $(EXECUTABLE) 

$(EXECUTABLE): $(OBJECTS) 
       $(LINK.cpp) $(OBJECTS) -o $@

Optimization might matter at link time for Link-Time Optimization , for that use CXX=g++ -flto and LDFLAGS=$(CXXFLAGS)