g++ O1 不等于 O0 与所有相关的优化标志 [英] g++ O1 is not equal to O0 with all related optimization flags

问题描述

我知道标题有点混乱.让我用一点背景来澄清我的问题:

I know the title is a bit confusing. Let me clarify my problem with a little background:

就执行时间而言，当我使用 -O1 标志与 -O0 标志进行编译时，我的程序的行为很奇怪.我知道 -O1 标志做了许多优化，例如 fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments (根据手册页超过 40).为了弄清楚是哪些优化导致了这种行为，我计划一次删除一个标志，然后编译和测试看看是否有变化.

My program behaves strangely when I compile it with -O1 flag vs -O0 flag in terms of execution time. I know -O1 flag does many optimizations such as fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments (More than 40 according to the man page). To figure out which optimization(s) cause this behavior, I plan to remove flags one at a time then compile and test to see if something changes.

在做这个实验之前，我想确保用 -O1 编译的程序和用 -O0 编译的程序加上 -O1 的所有标志 启用(让我们调用 -O0+)行为类似.实际上，我希望这两种方法都应该生成相同的二进制文件，因为启用了相同的优化标志.

Before doing this experiment, I want to make sure that the program compiled with -O1 and the program compiled with -O0 plus all flags which -O1 enables (Lets call -O0+) behave similarly. Actually, I expect both method should produce the same binary file since the same optimization flags are enabled.

用 O1

CC = g++

CFLAGS = -std=c++11 -Wall -fopenmp
SOURCE = a_count_f.cpp
EXEC = run
INC = inc

all: $(EXEC)
.PHONY: all

$(EXEC): $(SOURCE)
    $(CC) $(CFLAGS) -O1 -o $(EXEC) -I$(INC) $^

用O0+

CC = g++

CFLAGS = -std=c++11 -Wall -fopenmp
SOURCE = a_count_f.cpp
EXEC = run
INC = inc

OPT_FLAGS = -fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim -fcprop-registers -fdce -fdefer-pop -ftree-builtin-call-dce -fdse -fforward-propagate -fguess-branch-probability -fif-conversion2 -fif-conversion -finline-functions-called-once -fipa-pure-const -fipa-profile -fipa-reference -fmerge-constants -fmove-loop-invariants -fomit-frame-pointer -freorder-blocks -fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-pta -ftree-ter -funit-at-a-time

all: $(EXEC)
.PHONY: all

$(EXEC): $(SOURCE)
    $(CC) $(CFLAGS) -O0 $(OPT_FLAGS) -o $(EXEC) -I$(INC) $^

然而，结果发现 -O1 和 -O0+ 给出了完全不同的结果.尽管存在所有优化差异，-O0 和 -O0+ 给出了非常相似的结果.(通过结果，我的意思是执行时间)

However, It turned out -O1 and -O0+ give quite different result. Despite of all optimizations differences, -O0 and -O0+ give very similar results. (By results, I mean execution time)

我已经使用 -Q --help=optimizers 检查了两个编译，并且输出确认两者都启用了相同的标志.

I have checked both compilation with -Q --help=optimizers and the output confirmed that both enables the same flags.

接下来对我来说是比较汇编代码.在此之前，我想在这里问一下是否有人知道为什么会发生这种情况.我没有包含源代码，因为似乎问题与源代码无关.但是，如果需要，我可以附上它.

The next for me is to compare assembly codes. Before doing that, I want to ask it here if anyone has an idea why this happens. I didn't include the source code as it seems the problem is not related to source code. But, I can attach it if needed.

g++ 版本:g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

推荐答案

-O1 应用的优化标志只在优化器开启时应用.您需要使用 n > 指定 -On0 以便优化标志实际执行任何操作.

The optimizations flags applied by -O1 only apply when the optimizer is turned on. You need to specify -On with n > 0 in order for the optimization flags to actually do anything.

换句话说，-O0 不会开启优化器，所以优化标志不会做任何事情.

To put it another way, -O0 doesn't turn on the optimizer, so the optimization flags don't do anything.

您可以使用标志的 -fno 形式关闭优化标志.比如

You can turn off optimzations flags by using the -fno form of the flag. For instance the

-fcompare-elim 

标志由 -O1 打开，您可以使用

flag is turned on by -O1 and you can turn it back off using

-fno-compare-elim 

<小时>

另一件需要注意的事情，正如 TC 所指出的，并非所有优化都有标志 所以没有任何方法可以关闭这些特定的优化.

---

Another thing to note, as pointed out by T.C., is that not all optimizations have a flag so there isn't any way to turn those particular optimizations off.

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