Read machO sections section by section

This changes how ReflectionContext reads machO reflection sections by reading them individually instead of as one big memory block spanning from the first to the last section (and including whatever else is in between). This change will enable an optimization on LLDB's side where, if we're reading read-only data, we read from the file-cache instead of the child process, which should speed up debugging when working with remote processes.

(cherry picked from commit b54d426)

Author: augusto2112

include/swift/Reflection/ReflectionContext.h

@@ -205,36 +205,7 @@ class ReflectionContext
       return false;
 
     auto Slide = ImageStart.getAddressData() - Command->vmaddr;
-    std::string Prefix = "__swift5";
-    uint64_t RangeStart = UINT64_MAX;
-    uint64_t RangeEnd = UINT64_MAX;
     auto SectionsBuf = reinterpret_cast<const char *>(Sections.get());
-    for (unsigned I = 0; I < NumSect; ++I) {
-      auto S = reinterpret_cast<typename T::Section *>(
-          SectionsBuf + (I * sizeof(typename T::Section)));
-      if (strncmp(S->sectname, Prefix.c_str(), strlen(Prefix.c_str())) != 0)
-        continue;
-      if (RangeStart == UINT64_MAX && RangeEnd == UINT64_MAX) {
-        RangeStart = S->addr + Slide;
-        RangeEnd = S->addr + S->size + Slide;
-        continue;
-      }
-      RangeStart = std::min(RangeStart, (uint64_t)S->addr + Slide);
-      RangeEnd = std::max(RangeEnd, (uint64_t)(S->addr + S->size + Slide));
-      // Keep the range rounded to 8 byte alignment on both ends so we don't
-      // introduce misaligned pointers mapping between local and remote
-      // address space.
-      RangeStart = RangeStart & ~7;
-      RangeEnd = RangeEnd + 7 & ~7;      
-    }
- 
-    if (RangeStart == UINT64_MAX && RangeEnd == UINT64_MAX)
-      return false;
-
-    auto SectBuf = this->getReader().readBytes(RemoteAddress(RangeStart),
-                                               RangeEnd - RangeStart);
-    if (!SectBuf)
-      return false;
 
     auto findMachOSectionByName = [&](llvm::StringRef Name)
         -> std::pair<RemoteRef<void>, uint64_t> {
@@ -244,11 +215,13 @@ class ReflectionContext
         if (strncmp(S->sectname, Name.data(), strlen(Name.data())) != 0)
           continue;
         auto RemoteSecStart = S->addr + Slide;
-        auto SectBufData = reinterpret_cast<const char *>(SectBuf.get());
-        auto LocalSectStart =
-            reinterpret_cast<const char *>(SectBufData + RemoteSecStart - RangeStart);
-        
-        auto StartRef = RemoteRef<void>(RemoteSecStart, LocalSectStart);
+        auto LocalSectBuf =
+            this->getReader().readBytes(RemoteAddress(RemoteSecStart), S->size);
+        if (!LocalSectBuf)
+          return {nullptr, 0};
+
+        auto StartRef = RemoteRef<void>(RemoteSecStart, LocalSectBuf.get());
+        savedBuffers.push_back(std::move(LocalSectBuf));
         return {StartRef, S->size};
       }
       return {nullptr, 0};
@@ -307,7 +280,6 @@ class ReflectionContext
     }
 
     savedBuffers.push_back(std::move(Buf));
-    savedBuffers.push_back(std::move(SectBuf));
     savedBuffers.push_back(std::move(Sections));
     return true;
   }