[SR-8913] It takes much time to compile code containing "else if" statements in some situations.

[YOCKOW]

Previous ID	SR-8913
Radar	rdar://problem/45049364
Original Reporter	@YOCKOW
Type	Improvement

Attachment: Download

Environment

• OS: macOS Mojave

• Swift: 4.2

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement
Assignee	None
Priority	Medium

md5: f188e3d745da38e11c104ef33cce615e

Issue Description:

Please see the attached file named "conditions.swift"
There are 6 functions in the source, and each of them contains 2,000 "if", "else if", or "switch case" statements. (They will return the same result with the same argument, though that is not essential.)

Now, type the following command to compile the source:
swiftc -Xfrontend -warn-long-function-bodies=1 conditions.swift

Then, the messages like below will be viewed.

conditions.swift:1:13: warning: global function 'function_if' took 1179ms to type-check (limit: 1ms)
public func function_if(_ integer:Int) -> Int {
            ^
conditions.swift:2006:13: warning: global function 'function_if_without_print' took 738ms to type-check (limit: 1ms)
public func function_if_without_print(_ integer:Int) -> Int {
            ^
conditions.swift:4011:13: warning: global function 'function_else_if' took 23861ms to type-check (limit: 1ms)
public func function_else_if(_ integer:Int) -> Int {
            ^
conditions.swift:6016:13: warning: global function 'function_else_if_without_print' took 599ms to type-check (limit: 1ms)
public func function_else_if_without_print(_ integer:Int) -> Int {
            ^
conditions.swift:8021:13: warning: global function 'function_switch' took 916ms to type-check (limit: 1ms)
public func function_switch(_ integer:Int) -> Int {
            ^
conditions.swift:10028:13: warning: global function 'function_switch_without_print' took 612ms to type-check (limit: 1ms)
public func function_switch_without_print(_ integer:Int) -> Int {
            ^

It seems that it takes significantly much time to compile "function_else_if" compared with other functions.
Furthermore, the compilation time seems to depend on the contents of code block even if there are the same number of "else if"s.

[belkadan]

@swift-ci create

[rudkx]

We're spending all our time here in FindLocalVal::visitIfStmt. I'll take a look at fixing it.

[rudkx]

Okay, it looks like this is a very non-trivial issue with unqualified lookup so not something I'll be able to address quickly.