[SR-5322] Slow type checking of MemoryLayout<T>.size + MemoryLayout<U>.size + ...

[rudkx]

Previous ID	SR-5322
Radar	rdar://problem/32999041
Original Reporter	@rudkx
Type	Bug

Additional Detail from JIRA

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

md5: c72cfc034a2bc02949fcd0bdb36cd2a7

Issue Description:

I'm not sure if this is anything more specific than other slow type checking cases we've seen, but I noticed that two expressions in swift-corelibs-foundation were accounting for quite a bit of type checking time (something like 7s and 63s respectively in an unoptimized asserts-enabled build).

diff --git a/Foundation/NSCFString.swift b/Foundation/NSCFString.swift
index 6d0d8a1..7cbb2b4 100644
--- a/Foundation/NSCFString.swift
+++ b/Foundation/NSCFString.swift
@@ -59,13 +59,19 @@ internal class _NSCFString : NSMutableString {
 
 internal final class _NSCFConstantString : _NSCFString {
     internal var _ptr : UnsafePointer<UInt8> {
-        let offset = MemoryLayout<OpaquePointer>.size + MemoryLayout<Int32>.size + MemoryLayout<Int32>.size + MemoryLayout<_CFInfo>.size
+        // FIXME: Split expression as a work-around for slow type checking.
+        let offTemp1 = MemoryLayout<OpaquePointer>.size + MemoryLayout<Int32>.size
+        let offTemp2 = MemoryLayout<Int32>.size + MemoryLayout<_CFInfo>.size
+        let offset = offTemp1 + offTemp2
         let ptr = Unmanaged.passUnretained(self).toOpaque()
         return ptr.load(fromByteOffset: offset, as: UnsafePointer<UInt8>.self)
     }
 
     private var _lenOffset : Int {
-        return MemoryLayout<OpaquePointer>.size + MemoryLayout<Int32>.size + MemoryLayout<Int32>.size + MemoryLayout<_CFInfo>.size + MemoryLayout<UnsafePointer<UInt8>>.size
+        // FIXME: Split expression as a work-around for slow type checking.
+        let offTemp1 = MemoryLayout<OpaquePointer>.size + MemoryLayout<Int32>.size
+        let offTemp2 = MemoryLayout<Int32>.size + MemoryLayout<_CFInfo>.size
+        return offTemp1 + offTemp2 + MemoryLayout<UnsafePointer<UInt8>>.size
     }
 
     private var _lenPtr :  UnsafeMutableRawPointer {

[rudkx]

@swift-ci create

[rudkx]

This is the PR for working around the slow type checking issue: swiftlang/swift-corelibs-foundation#1077

[slavapestov]

This is now fast.