[Backtracing] Add a caching wrapper for MemoryReader.

stdlib/public/Backtracing/ArrayImageSource.swift

@@ -16,8 +16,6 @@
 
 import Swift
 
-@_implementationOnly import OS.Libc
-
 enum ArrayImageSourceError: Error {
   case outOfBoundsRead(UInt64, UInt64)
 }
@@ -35,16 +33,16 @@ struct ArrayImageSource<T>: ImageSource {
     return Bounds(base: 0, size: Size(array.count * MemoryLayout<T>.stride))
   }
 
-  public func fetch<U>(from addr: Address,
-                       into buffer: UnsafeMutableBufferPointer<U>) throws {
+  public func fetch(from addr: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
     try array.withUnsafeBytes{
       let size = Size($0.count)
-      let requested = Size(buffer.count * MemoryLayout<U>.stride)
+      let requested = Size(buffer.count)
       if addr > size || requested > size - addr {
         throw ArrayImageSourceError.outOfBoundsRead(addr, requested)
       }
 
-      memcpy(buffer.baseAddress!, $0.baseAddress! + Int(addr), Int(requested))
+      buffer.copyBytes(from: $0[Int(addr)..<Int(addr+requested)])
     }
   }
 }

stdlib/public/Backtracing/CMakeLists.txt

@@ -25,6 +25,7 @@ set(BACKTRACING_SOURCES
   Backtrace.swift
   BacktraceFormatter.swift
   ByteSwapping.swift
+  CachingMemoryReader.swift
   Context.swift
   Compression.swift
   CoreSymbolication.swift

stdlib/public/Backtracing/CachingMemoryReader.swift

@@ -0,0 +1,86 @@
+//===--- CachingMemoryReader.swift ----------------------------*- swift -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+//  Wraps a MemoryReader in a layer that caches memory pages.
+//
+//===----------------------------------------------------------------------===//
+
+import Swift
+
+// The size of the pages in the page cache (must be a power of 2)
+fileprivate let pageSize = 4096
+
+fileprivate let pageMask = pageSize - 1
+
+// The largest chunk we will try to cache data for
+fileprivate let maxCachedSize = pageSize * 8
+
+@_spi(MemoryReaders)
+public class CachingMemoryReader<T: MemoryReader>: MemoryReader {
+  private var reader: T
+  private var cache: [Address:UnsafeRawBufferPointer]
+
+  public init(for reader: T) {
+    self.reader = reader
+    self.cache = [:]
+  }
+
+  deinit {
+    for (_, page) in cache {
+      page.deallocate()
+    }
+  }
+
+  private func getPage(at address: Address) throws -> UnsafeRawBufferPointer {
+    precondition((address & Address(pageMask)) == 0)
+
+    if let page = cache[address] {
+      return page
+    }
+
+    let page = UnsafeMutableRawBufferPointer.allocate(byteCount: pageSize,
+                                                      alignment: pageSize)
+    try reader.fetch(from: address, into: page)
+
+    let result = UnsafeRawBufferPointer(page)
+
+    cache[address] = result
+
+    return result
+  }
+
+  public func fetch(from address: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
+    guard buffer.count <= maxCachedSize else {
+      try reader.fetch(from: address, into: buffer)
+      return
+    }
+
+    var pageAddress = address & ~Address(pageMask)
+    var done = 0
+    var offset = Int(address - pageAddress)
+    var remaining = buffer.count
+
+    while remaining > 0 {
+      let page = try getPage(at: pageAddress)
+      let maxBytes = pageSize - offset
+      let chunk = min(remaining, maxBytes)
+
+      buffer[done..<done+chunk].copyBytes(from: page[offset..<offset+chunk])
+
+      offset = 0
+      done += chunk
+      remaining -= chunk
+      pageAddress += Address(pageSize)
+    }
+  }
+}

stdlib/public/Backtracing/Compression.swift

@@ -427,9 +427,9 @@ internal struct ElfCompressedImageSource<Traits: ElfTraits>: ImageSource {
     }
   }
 
-  public func fetch<T>(from addr: Address,
-                       into buffer: UnsafeMutableBufferPointer<T>) throws {
-    let toFetch = buffer.count * MemoryLayout<T>.stride
+  public func fetch(from addr: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
+    let toFetch = buffer.count
     if addr < 0 || addr > data.count || data.count - Int(addr) < toFetch {
       throw CompressedImageSourceError.outOfRangeFetch(addr, toFetch)
     }
@@ -474,9 +474,9 @@ internal struct ElfGNUCompressedImageSource: ImageSource {
                           uncompressedSize: uncompressedSize)
   }
 
-  public func fetch<T>(from addr: Address,
-                       into buffer: UnsafeMutableBufferPointer<T>) throws {
-    let toFetch = buffer.count * MemoryLayout<T>.stride
+  public func fetch(from addr: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
+    let toFetch = buffer.count
     if addr < 0 || addr > data.count || data.count - Int(addr) < toFetch {
       throw CompressedImageSourceError.outOfRangeFetch(addr, toFetch)
     }
@@ -509,9 +509,9 @@ internal struct LZMACompressedImageSource: ImageSource {
                           dataBounds: bounds)
   }
 
-  public func fetch<T>(from addr: Address,
-                       into buffer: UnsafeMutableBufferPointer<T>) throws {
-    let toFetch = buffer.count * MemoryLayout<T>.stride
+  public func fetch(from addr: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
+    let toFetch = buffer.count
     if addr < 0 || addr > data.count || data.count - Int(addr) < toFetch {
       throw CompressedImageSourceError.outOfRangeFetch(addr, toFetch)
     }

stdlib/public/Backtracing/FileImageSource.swift

@@ -20,51 +20,54 @@ import Swift
 
 enum FileImageSourceError: Error {
   case posixError(Int32)
-  case truncatedRead
+  case outOfRangeRead
 }
 
 class FileImageSource: ImageSource {
-  private var fd: Int32
+  private var _mapping: UnsafeRawBufferPointer
 
   public var isMappedImage: Bool { return false }
 
   private var _path: String
   public var path: String? { return _path }
 
-  public lazy var bounds: Bounds? = {
-    let size = lseek(fd, 0, SEEK_END)
-    if size < 0 {
-      return nil
-    }
-    return Bounds(base: 0, size: Size(size))
-  }()
+  public var bounds: Bounds? {
+    return Bounds(base: 0, size: Size(_mapping.count))
+  }
 
   public init(path: String) throws {
     _path = path
-    fd = _swift_open(path, O_RDONLY, 0)
+    let fd = _swift_open(path, O_RDONLY, 0)
     if fd < 0 {
       throw FileImageSourceError.posixError(_swift_get_errno())
     }
+    defer { close(fd) }
+    let size = lseek(fd, 0, SEEK_END)
+    if size < 0 {
+      throw FileImageSourceError.posixError(_swift_get_errno())
+    }
+    let base = mmap(nil, Int(size), PROT_READ, MAP_FILE|MAP_PRIVATE, fd, 0)
+    if base == nil || base! == UnsafeRawPointer(bitPattern: -1)! {
+      throw FileImageSourceError.posixError(_swift_get_errno())
+    }
+    _mapping = UnsafeRawBufferPointer(start: base, count: Int(size))
   }
 
   deinit {
-    close(fd)
+    munmap(UnsafeMutableRawPointer(mutating: _mapping.baseAddress),
+           _mapping.count)
   }
 
-  public func fetch<T>(from addr: Address,
-                       into buffer: UnsafeMutableBufferPointer<T>) throws {
-    while true {
-      let size = MemoryLayout<T>.stride * buffer.count
-      let result = pread(fd, buffer.baseAddress, size, off_t(addr))
-
-      if result < 0 {
-        throw FileImageSourceError.posixError(_swift_get_errno())
-      }
-
-      if result != size {
-        throw FileImageSourceError.truncatedRead
-      }
-      break
+  public func fetch(from addr: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
+    let start = Int(addr)
+    guard _mapping.indices.contains(start) else {
+      throw FileImageSourceError.outOfRangeRead
+    }
+    let slice = _mapping[start...]
+    guard slice.count >= buffer.count else {
+      throw FileImageSourceError.outOfRangeRead
     }
+    buffer.copyBytes(from: slice[start..<start+buffer.count])
   }
 }

stdlib/public/Backtracing/ImageSource.swift

@@ -60,6 +60,11 @@ struct ImageSourceCursor {
     self.pos = offset
   }
 
+  public mutating func read(into buffer: UnsafeMutableRawBufferPointer) throws {
+    try source.fetch(from: pos, into: buffer)
+    pos += UInt64(buffer.count)
+  }
+
   public mutating func read<T>(into buffer: UnsafeMutableBufferPointer<T>) throws {
     try source.fetch(from: pos, into: buffer)
     pos += UInt64(MemoryLayout<T>.stride * buffer.count)
@@ -138,9 +143,9 @@ struct SubImageSource<S: ImageSource>: ImageSource {
     return parent.isMappedImage
   }
 
-  public func fetch<T>(from addr: Address,
-                       into buffer: UnsafeMutableBufferPointer<T>) throws {
-    let toFetch = buffer.count * MemoryLayout<T>.stride
+  public func fetch(from addr: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
+    let toFetch = buffer.count
     if addr < 0 || addr > length {
       throw SubImageSourceError.outOfRangeFetch(UInt64(addr), toFetch)
     }

stdlib/public/Backtracing/MemoryImageSource.swift

@@ -28,8 +28,8 @@ class MemoryImageSource<M: MemoryReader>: ImageSource {
     self.reader = reader
   }
 
-  public func fetch<T>(from addr: Address,
-                       into buffer: UnsafeMutableBufferPointer<T>) throws {
+  public func fetch(from addr: Address,
+                    into buffer: UnsafeMutableRawBufferPointer) throws {
     try reader.fetch(from: addr, into: buffer)
   }
 }