--force-lto gem5 scons生成选项是否可以显着加快仿真速度，并且与gem5.fast生成相比如何? [英] Does the --force-lto gem5 scons build option speed up simulation significantly and how does it compare to a gem5.fast build?

问题描述

在寻找加快仿真速度的方法时，我遇到了--force-lto选项.

While looking for ways to speed up my simulation, I came across the --force-lto option.

我以前听说过LTO(链接时间优化)，所以让我想知道为什么--force-lto在构建gem5时不是默认设置?

I've heard about LTO (Link Time Optimization) before, so that made me wonder why isn't --force-lto the default while building gem5?

与gem5.opt构建相比，使模拟进行的速度比gem5.fast构建快得多吗?

Would that make a simulation go much faster than a gem5.fast build compared to a gem5.opt build?

推荐答案

在gem5 fe15312aae8007967812350f8cdac9ad766dcff7(2019)中，默认情况下gem5.fast构建已启用LTO，因此您通常不希望显式使用该选项，而只希望gem5.opt.

In gem5 fe15312aae8007967812350f8cdac9ad766dcff7 (2019), the gem5.fast build already enables LTO by default, so you generally never want to use that option explicitly, but rather want just gem5.opt.

关于.fast的其他事项:

• 它也会删除-g，因此您没有调试符号.我不知道为什么，因为这不能使运行速度更快.
• 它还会打开NDEBUG，它具有完全禁用assert的标准库效果，但加上一些#ifndef NDEBUG检查
，在代码中散布了一些gem5特有的效果
• 它禁用了TRACING_ON，这使DPRINTF和family变为空语句，如src/base/trace.hh
所示.

• it also removes -g and so you get no debug symbols. I wonder why, since that does not make runs any faster.
• it also turns on NDEBUG, which has the standard library effect of disabling asserts entirely, but plus some gem5 specific effects spread throughout the code with #ifndef NDEBUG checks
• it disables TRACING_ON, which makes DPRINTF and family become empty statements as seen at: src/base/trace.hh

这些效果很容易看到在src/SConstruct .

Those effects can be seen easily at src/SConstruct.

存在该选项是因为更常见的gem5.opt构建也使用

That option exists because the more common gem5.opt build also uses partial linking, which in some versions of GCC was incompatible with LTO.

因此，顾名思义，--force-lto强制将LTO与部分链接一起使用，这可能是不稳定的.这就是为什么我建议您使用gem5.fast而不是触摸--force-lto的原因.

Therefore, as its the name suggests, --force-lto forces the use of LTO together with partial linking, which might not be stable. That's why I recommend that you use gem5.fast rather than touching --force-lto.

部分链接的目的大概是为了加快链接步骤，这很容易成为更改文件，重新构建，重新链接，测试"循环的瓶颈，尽管在我的实验中尚不清楚它是否有效在这样做.今天，它可能只是过去的遗物.

The goal of partial linking is presumably to speed up the link step, which can easily be the bottleneck in a "change on file, rebuild, relink, test" loop, although in my experiments it is not clear that it is efficient at doing that. Today it might just be a relic from the past.

要尝试加快链接速度，建议您改用scons --gold-linker，它

To try to speed up linking, I recommend that you try scons --gold-linker instead, which uses the GOLD linker instead of ld. Note that this option was more noticeably effective for gem5.debug however.

我发现对于原子CPU，gem5.fast通常比gem5.opt快20％.

I have found that gem5.fast is generally 20% faster than gem5.opt for Atomic CPUs.

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