[SR-5109] Expression Too Complex Regression

[an0]

Previous ID	SR-5109
Radar	None
Original Reporter	@an0
Type	Bug

Environment

Xcode 9M136h

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, 4.0Regression, TypeChecker
Assignee	None
Priority	Medium

md5: 21eb6257d8147c142c6940d072a0b7a4

Issue Description:

let a = 0
let b = 0
let c = 0

let d = 0
let e = 0
let f = 0

// error: expression was too complex to be solved in reasonable time; consider breaking up the expression into distinct sub-expressions
let diff = abs(d - a) + abs(e + b) + abs(f - c)

It was not Too Complex for Swift 3.

[swift-ci]

Comment by Graydon Hoare (JIRA)

The code takes just as much time to solve the type-constraints of in swift 3 as in swift 4; the difference is that swift 4 is more precise and conservative about detecting when it's entered into an exponential-time constraint-solving task and giving up, rather than continuing to consume user time and memory.

In some cases, this means swift 4 will give up on something that it might have had enough time or memory to make it through solving in swift 3 (albeit at great cost – on my machine that case takes 15 seconds and almost 400mb resident before it gives up; whereas it takes about 26 seconds and 600mb to solve in -swift-version 3 mode).

This change was made as a stopgap, while work continues on eliminating causes of exponential behaviour in the type constraint solver. Many users indicated that they would greatly prefer to know which few expressions in their projects (it's usually just a few) were causing slow compiles, rather than suffering with them while waiting for a complete fix. Refactoring that final expression to:

let a1 = abs(d - a)
let a2 = abs(e + b)
let a3 = abs(f - c)
let diff = a1 + a2 + a3

Avoids the interaction between operator overloads and abs overloads, and causes type constraint solving to take only 8 milliseconds.

[slavapestov]

This is now fast.