Swift package compile times are 10-20 times worse on Windows than on Linux

[silverhammermba]

Description

Building the exact same Swift package on the same computer but using Windows takes orders of magnitude longer for both clean and incremental builds than doing it on Linux.

Reproduction

I've been trying to build this Swift package: https://github.com/migueldeicaza/SwiftGodot/. For future reference, I'm building tag 0.40.0

I reported this issue to the maintainer of that package and he insists the slowdown is purely up to the Swift compiler to fix: migueldeicaza/SwiftGodot#299 (comment)

Expected behavior

The above package build on Linux takes 83 seconds for a clean build and 3 seconds for an incremental build (after making trivial changes).

In Windows 10 it takes 19 minutes for a clean build and 13 minutes for an incremental build. This makes the package all but unusable on Windows. Comparing the incremental builds (which are the most important, in my opinion), that's over 240 times slower than on Linux.

Environment

Swift version 5.9.2 (swift-5.9.2-RELEASE)
Target: x86_64-unknown-windows-msvc

Additional information

I'm up for profiling this to figure out exactly where the slowdown is but I don't know where to get started with that.

[finagolfin]

Can you give more info on your environment, ie what hardware and OS versions are you running on and where do you get your toolchains from? Also, try it with Swift 5.10, as that is the latest release.

[silverhammermba]

AMD Ryzen 9 3900XT CPU, 40GB RAM

Samsung SSD 980 PRO 500GB, Arch Linux x86_64, kernel 6.7.9-arch1-1, installed Swift 5.10 binary via AUR

Building SwiftGodot commit db759c57

• clean build: 76 seconds
• incremental build (trivial change to a log statement): 39 seconds

Samsung SSD 970 EVO Plus 1TB, Windows 10 Home v22H2. Followed traditional installation steps and installed Swift 5.10 via installer.

• clean build: 916 seconds (12x longer than Linux)
• incremental build (trivial change to a log statement): 761 seconds (19x longer than Linux)

Pretty dang slow.

[silverhammermba]

I've heard of Windows Defender's real-time protection slowing down compilers before so I tried disabling it. I turned off real-time protection, cloud-delivered protection, and automatic sample submission. Maybe it helped but not a lot. The clean build time was 870 seconds, still 11x slower than Linux.

I also tried adding the swift package as an excluded folder for Microsoft Defender Antivirus scans but that made no difference. I'm not sure if there are other temporary files written by Swift elsewhere in Windows.

[finagolfin]

@compnerd or @hjyamauchi, I know you guys build Swift on Windows: any input here?

[compnerd]

Compile times are slower on Windows (at least subjectively), and is definitely something that I am interested in seeing improved. I don't have a good understanding of what is contributing to that, but having more details might be helpful to identify a plan of attack.

There are some temporary files written to %LocalAppData% IIRC. The initial compile is going to be extremely slow as it builds the initial WinSDK module which is extremely large. Subsequent builds are slightly better but still feels slow. Do you have specific files/modules which are greatly different? If we can find a starting point and start chipping away from the largest outliers, perhaps we can improve this.

[compnerd]

One thing that would be interesting to test if you have the motivation: try replacing the build with CMake + Ninja rather than SPM. The Browser Company noticed a lot of problems with SPM, particularly around incremental builds. With the latest CMake (3.29), it seems that most of those problems have been resolved.

[silverhammermba]

I can try CMake. SwiftGodot defines macros so is this a good starting point for the CMake config?

Also @compnerd, do you mind trying to build SwiftGodot on your Windows machine? It's pretty easy, just clone and swift build. I'm curious if it's significantly faster for you, especially for incremental builds.

I don't know how to narrow down which files/modules are causing the slowdown since even trivial changes (for example changing this print statement) take over 10 minutes to rebuild for me. Do you have any tips there?

[silverhammermba]

@compnerd I switched to building https://github.com/apple/swift-syntax as a benchmark since it already has cmake set up. I was easily able to build it with Swift build (81 seconds), but could not get cmake to work.

I installed cmake 3.29.0-rc4 and ninja 1.12.0 (from commit 86997c40) and ran

mkdir build
cd build
cmake .. -G Ninja
ninja

And the Ninja build fails at

[1/47] Linking Swift static library lib\swift\host\SwiftSyntax.lib
error: unableToFind(tool: "link")