Don't specify -j option when parallel builds is set to 1

[MGraefe]

Summary

For CMake builds, the plugin defines the -j flag even if parallelism is set to 1 in the settings. This interferes with MSBuild builds, which behave different when -j is defined. Instead the -j option shouldn't be defined at all when parallelism is set to 1.

Elaboration

CMake disables the /MP option from MSVC builds when the -j flag is specified, to avoid overloading the CPU. For Visual Studio / MSBuild builds, the /MP option enables parallel building of source files, while the CMake -j option enables parallel compilation of projects. In fact, CMake effectively removes any manually defined /MP flag from the compiler options in case -j is defined.

But I want my project to be compiled with /MP because for me it's much faster then relying on CMake -j. With the CMake plugin I can't, because it keeps specifying -j 1 on the command line even if I set the parallelism to 1.
It all works fine from command line if I just do cmake --build . and I can reproduce the /MP problem when I do cmake --build . -j 16

Proposal:

Don't add the -j option at all if parallelism is set to 1. Alternatively add another settings which can be used to completely disable addition of the -j flag.

Platform and Versions

• Operating System: Windows 10
• CMake Version: 3.20.5
• VSCode Version: 1.57.1
• CMake Tools Extension Version: 1.7.3
• Compiler/Toolchain: Visual Studio 2017 AMD64

[andreeis]

@MGraefe, thank you for this suggestion. We will address this.

[andreeis]

@MGraefe, a fix for this problem was included in the CMake Tools 1.9.1 release. Upgrade your extension in VSCode and let us know if you encounter any other issues.

[MGraefe]

Awesome, thanks to everyone involved!

[mikedld]

Won't omitting it affect Ninja (and maybe other tools) then, where max parallelization is the default, i.e. omitting -j will not result in a single job?

[andreeis]

@mikedld, excellent question. I just double checked, the extension will only omit -j when parallel builds is set to 1. If not set, the extension defaults to number of cpus + 2.

[mikedld]

@andreeis, right, so if I'm using Ninja generator and setting cmake.parallelJobs to 1 now, it won't really be 1, so I can't limit ninja jobs this way? Or am I missing something?

[MGraefe]

These kinds of inconsistencies that come by overloading a single value with a special meaning is why I didn't immediately create a PR for this.

I think a separate toggle option "specify parallel builds" in addition to the existing option would be the way to go. But I don't feel comfortable adding this as I never worked with this codebase.