[docs] Sccache bug has been fixed

[philipturner]

In v2.15.0, the target duplication problem has supposedly been addressed. I have not verified it's actually fixed because I haven't built Swift from source yet. Here is the sequence of events surrounding the fix:

• [Dec 11, 2020] The following PR, which is currently linked in GettingStarted.md, is merged and creates the bug:
  • Reject caching when multiple -arch arguments are present mozilla/sccache#898
• [Feb 18, 2021] The first PR is mentioned in a second PR to mozilla/sccache:
  • Allow caching gcc calls if same -arch is passed twice mozilla/sccache#959
• [Mar 10, 2021] The second PR is rejected because the following comment shows it's already been fixed:
  • Allow caching gcc calls if same -arch is passed twice mozilla/sccache#959 (comment)
• [Jul 27, 2021] @mcc-devel recognizes the fix and makes the following PR:
  • Update GettingStarted.md #38659
• [Sep 5, 2021] @xwu states that the PR must be rebased. After that, no further action is taken and the warning in GettingStarted.md remains.
  • Update GettingStarted.md #38659 (comment)

---

I have one question though: the commit goes to the file gcc.rs, yet (based on my limited and likely flawed understanding) Clang is often used for Swift instead of GCC. Does that mean the patch fixes builds using GCC, but Clang builds are still broken? For reference, I plan to use Xcode instead of Ninja to build the Swift compiler from source, and am still reading the beginner documentation on how to do it.

[CodaFi]

We are still seeing instances of this in sccache 0.2.15 with clang builds. I believe this bug is fixed on their mainline branch as building from source appears to fix the issue. We would probably need another formal release from Mozilla to remove this caveat text.

[philipturner]

I just did an incremental build of the Swift compiler using Ninja and it was definitely faster the second time.

[CodaFi]

Try running sccache --show-stats. I have sccache 0.2.15 installed and this is what it says:

Compile requests                   5394
Compile requests executed            84
Cache hits                           84
Cache hits (C/C++)                   84
Cache misses                          0
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    0
Compilation failures                  0
Cache errors                          0
Non-cacheable compilations            0
Non-cacheable calls                5310
Non-compilation calls                 0
Unsupported compiler calls            0
Average cache write               0.000 s
Average cache read miss           0.000 s
Average cache read hit            0.002 s
Failed distributed compilations       0

Non-cacheable reasons:
multiple -arch                     5310

[philipturner]

I just compiled the Swift toolchain and it took a few minutes (it was an incremental build with no changes). I ran sccache --show-stats both before and after, and this is what I got:

Compile requests                      0
Compile requests executed             0
Cache hits                            0
Cache misses                          0
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    0
Compilation failures                  0
Cache errors                          0
Non-cacheable compilations            0
Non-cacheable calls                   0
Non-compilation calls                 0
Unsupported compiler calls            0
Average cache write               0.000 s
Average cache read miss           0.000 s
Average cache read hit            0.000 s
Failed distributed compilations       0
Cache location                  Local disk: "/Users/philipturner/Library/Caches/Mozilla.sccache"
Cache size                            3 MiB
Max cache size                       10 GiB

Next, I'll modify a source code file and see if anything pops up. I did NOT set the max cache size to 10 GiB when starting the server; I set it to 50 GiB.

[philipturner]

I got the following after slightly modifying one C++ file and rebuilding:

Compile requests                      1
Compile requests executed             0
Cache hits                            0
Cache misses                          0
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    0
Compilation failures                  0
Cache errors                          0
Non-cacheable compilations            0
Non-cacheable calls                   1
Non-compilation calls                 0
Unsupported compiler calls            0
Average cache write               0.000 s
Average cache read miss           0.000 s
Average cache read hit            0.000 s
Failed distributed compilations       0

Non-cacheable reasons:
multiple -arch                        1

Cache location                  Local disk: "/Users/philipturner/Library/Caches/Mozilla.sccache"
Cache size                            3 MiB
Max cache size                       10 GiB

[CodaFi]

Non-cacheable reasons:
multiple -arch 1

So then it seems that this isn't fixed. Looks like @ahoppen got some code in to improve the situation, but it missed the tag by two months 😞

[ahoppen]

The sccache issue actually still exists in 0.2.15. mozilla/sccache#898 is the PR that introduced the issue and I submitted mozilla/sccache#959 to fix the issue introduced by that PR. However, sccache hasn’t release a new version since.

Regarding gcc vs. clang: I’m no sccache expert but I believe that gcc and clang have sufficiently similar compiler arguments that sccache can use the same rules to determine whether a particular compiler invocation can be cached.

[Kyle-Ye]

sccache release 0.3.0 version which include the fix PR.
https://github.com/mozilla/sccache/releases/tag/v0.3.0

[philipturner]

If this is an external dependency that people download separately from the Swift compiler, then people might already be using the new Sccache with the bug fix. We need to have someone validate that the newer version is being used.

[ahoppen]

I would like to hold this until sccache 0.3.0 has been published on Homebrew because I think that’s the dominant way people install sccache.

[Kyle-Ye]

I would like to hold this until sccache 0.3.0 has been published on Homebrew because I think that’s the dominant way people install sccache.

Homebrew/homebrew-core#101009

[philipturner]

I have set up notifications so that I’ll be pinged when that PR is merged. Then we should be able to get this reviewed.

[philipturner]

@CodaFi please review

[ahoppen]

Verified that version 0.3.0 of sccache installed via homebrew correctly caches compilation commands again.

[ahoppen]

@swift-ci Please smoke test