[SR-13205] Slow compilation with GuaranteedARCOpts

[lxbndr]

Previous ID	SR-13205
Radar	None
Original Reporter	@lxbndr
Type	Bug

Attachment: Download

Environment

Swift version 5.3-dev (LLVM 4510748, Swift bf47403)
Target: x86_64-unknown-windows-msvc

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CompileTime, Optimizer
Assignee	None
Priority	Medium

md5: 16837e10c0c582e8cd7c30b39a7c799b

Issue Description:

Noticed heavy slowdown in compilation of specific code after #32390.

Code contains huge switch with ~1000 cases:

switch mime.lowercased() {     
    case "application/andrew-inset":
        return "ez"
    case "application/applixware":
        return "aw"
    case "application/atom+xml":
        return "atom"
    case "application/atomcat+xml":
        return "atomcat"
...

Normally it compiles in seconds, but now compilation takes about 20 minutes. Looks like removing the .lowercased() call fixes the issue. Compiling with -Xllvm -sil-disable-pass=GuaranteedARCOpts also helps.

Compiling on Windows:

swiftc.exe -emit-library -module-name sr13205 -sdk %SDKROOT% -I %SDKROOT%\usr\lib\swift -L %SDKROOT%\usr\lib\swift\windows sr13205.swift -o sr13205.dll

But I think it might be crossplatform. Full code sample attached.

[lxbndr]

cc @gottesmm

[gottesmm]

@lxbndr thanks for the small test case!

Is it happening with debug or release?

[lxbndr]

@gottesmm Did I understand correctly that Debug and Release most significant difference is optimization flags?

The issue reproduces on non-optimized runs: `-Onone` flag or no flag at all.

With `-O` flag the compilation is fast as expected.