Possible `-whole-module-optimization` bug

[dabrahams]

Description

Tests fail in release mode unless -no-whole-module-optimization is passed.

Reproduction

Sorry, it's not a small repro, but then a WMO bug would tend not to be, right?

• Check out the possible-swift-wmo-bug tag of https://github.com/hylo-lang/hylo
• git submodule update --init
• Satisfy the prerequisites for building
• swift test --filter Execution to see a test pass
• swift test -Xswiftc -enable-testing -c release --filter Execution to see the failure (a fatalError call).

Apply this patch:

diff --git a/Package.swift b/Package.swift
index 36a041409..69f77b569 100644
--- a/Package.swift
+++ b/Package.swift
@@ -10,7 +10,7 @@ import PackageDescription
 
 /// Settings to be passed to swiftc for all targets.
 let allTargetsSwiftSettings: [SwiftSetting] = [
-  .unsafeFlags(["-warnings-as-errors", "-enable-testing"])
+  .unsafeFlags(["-warnings-as-errors", "-enable-testing", "-no-whole-module-optimization"])
 ]
 
 /// Dependencies for documentation extraction.

Repeat the command to see it not fail.

Alternatively, apply this patch

1 file changed, 1 insertion(+), 2 deletions(-)
StandardLibrary/StandardLibrary.swift | 3 +--

modified   StandardLibrary/StandardLibrary.swift
@@ -24,7 +24,6 @@ extension Utils.Host {
   /// platform.
   public static let hostedLibraryAST = Result {
     try AST(libraryRoot: hostedLibrarySourceRoot, for: CompilerConfiguration([]))
-      .roundTripSerialized()
   }
 
   /// An AST representing the freestanding core of standard library, conditionally compiled for
@@ -32,7 +31,7 @@ extension Utils.Host {
   public static let freestandingLibraryAST = Result {
     try AST(
       libraryRoot: freestandingLibrarySourceRoot, for: CompilerConfiguration(["freestanding"])
-    ).roundTripSerialized()
+    )
   }
 
 }

and repeat the command to see it not fail.

Expected behavior

Same behavior in debug and release builds; no observable change from turning on/off WMO; round-trip serialization makes no difference, or at least has the same effect in debug and release builds.

Environment

swift-driver version: 1.87.3 Apple Swift version 5.9.2 (swiftlang-5.9.2.2.56 clang-1500.1.0.2.5)
Target: arm64-apple-macosx14.0

Additional information

No response

[dabrahams]

And of course, you can't do a clean build with the -no-whole-module-optimization patch applied, because of swiftlang/swift-package-manager#7274

[dabrahams]

Applying -no-whole-module-optimization just to the Core module is enough to suppress the fatalError.