diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -132,7 +132,8 @@
 
 std::string getMainExecutable(const char *argv0, void *MainExecAddr) {
   SmallVector<wchar_t, MAX_PATH> PathName;
-  DWORD Size = ::GetModuleFileNameW(NULL, PathName.data(), PathName.capacity());
+  PathName.resize_for_overwrite(PathName.capacity());
+  DWORD Size = ::GetModuleFileNameW(NULL, PathName.data(), PathName.size());
 
   // A zero return value indicates a failure other than insufficient space.
   if (Size == 0)
@@ -145,7 +146,7 @@
 
   // On success, GetModuleFileNameW returns the number of characters written to
   // the buffer not including the NULL terminator.
-  PathName.set_size(Size);
+  PathName.truncate(Size);
 
   // Convert the result from UTF-16 to UTF-8.
   SmallVector<char, MAX_PATH> PathNameUTF8;
@@ -201,8 +202,8 @@
   DWORD len = MAX_PATH;
 
   do {
-    cur_path.reserve(len);
-    len = ::GetCurrentDirectoryW(cur_path.capacity(), cur_path.data());
+    cur_path.resize_for_overwrite(len);
+    len = ::GetCurrentDirectoryW(cur_path.size(), cur_path.data());
 
     // A zero return value indicates a failure other than insufficient space.
     if (len == 0)
@@ -210,11 +211,11 @@
 
     // If there's insufficient space, the len returned is larger than the len
     // given.
-  } while (len > cur_path.capacity());
+  } while (len > cur_path.size());
 
   // On success, GetCurrentDirectoryW returns the number of characters not
   // including the null-terminator.
-  cur_path.set_size(len);
+  cur_path.truncate(len);
 
   if (std::error_code EC =
           UTF16ToUTF8(cur_path.begin(), cur_path.size(), result))
@@ -328,7 +329,7 @@
   // the null terminator, it will leave the output unterminated.  Push a null
   // terminator onto the end to ensure that this never happens.
   VolumePath.push_back(L'\0');
-  VolumePath.set_size(wcslen(VolumePath.data()));
+  VolumePath.truncate(wcslen(VolumePath.data()));
   const wchar_t *P = VolumePath.data();
 
   UINT Type = ::GetDriveTypeW(P);
@@ -364,18 +365,19 @@
 
 static std::error_code realPathFromHandle(HANDLE H,
                                           SmallVectorImpl<wchar_t> &Buffer) {
+  Buffer.resize_for_overwrite(Buffer.capacity());
   DWORD CountChars = ::GetFinalPathNameByHandleW(
       H, Buffer.begin(), Buffer.capacity(), FILE_NAME_NORMALIZED);
   if (CountChars && CountChars >= Buffer.capacity()) {
     // The buffer wasn't big enough, try again.  In this case the return value
     // *does* indicate the size of the null terminator.
-    Buffer.reserve(CountChars);
+    Buffer.resize_for_overwrite(CountChars);
     CountChars = ::GetFinalPathNameByHandleW(
-        H, Buffer.begin(), Buffer.capacity(), FILE_NAME_NORMALIZED);
+        H, Buffer.begin(), Buffer.size(), FILE_NAME_NORMALIZED);
   }
+  Buffer.truncate(CountChars);
   if (CountChars == 0)
     return mapWindowsError(GetLastError());
-  Buffer.set_size(CountChars);
   return std::error_code();
 }
 
@@ -1450,14 +1452,14 @@
   SmallVector<wchar_t, 1024> Buf;
   size_t Size = 1024;
   do {
-    Buf.reserve(Size);
-    Size = GetEnvironmentVariableW(Var, Buf.data(), Buf.capacity());
+    Buf.resize_for_overwrite(Size);
+    Size = GetEnvironmentVariableW(Var, Buf.data(), Buf.size());
     if (Size == 0)
       return false;
 
     // Try again with larger buffer.
-  } while (Size > Buf.capacity());
-  Buf.set_size(Size);
+  } while (Size > Buf.size());
+  Buf.truncate(Size);
 
   return !windows::UTF16ToUTF8(Buf.data(), Size, Res);
 }
@@ -1506,7 +1508,7 @@
     }
 
     utf16.reserve(len + 1);
-    utf16.set_size(len);
+    utf16.resize_for_overwrite(len);
 
     len = ::MultiByteToWideChar(codepage, MB_ERR_INVALID_CHARS, original.begin(),
                                 original.size(), utf16.begin(), utf16.size());
@@ -1546,8 +1548,8 @@
       return mapWindowsError(::GetLastError());
     }
 
-    converted.reserve(len);
-    converted.set_size(len);
+    converted.reserve(len + 1);
+    converted.resize_for_overwrite(len);
 
     // Now do the actual conversion.
     len = ::WideCharToMultiByte(codepage, 0, utf16, utf16_len, converted.data(),
diff --git a/llvm/lib/Support/Windows/Process.inc b/llvm/lib/Support/Windows/Process.inc
--- a/llvm/lib/Support/Windows/Process.inc
+++ b/llvm/lib/Support/Windows/Process.inc
@@ -129,16 +129,16 @@
   SmallVector<wchar_t, MAX_PATH> Buf;
   size_t Size = MAX_PATH;
   do {
-    Buf.reserve(Size);
+    Buf.resize_for_overwrite(Size);
     SetLastError(NO_ERROR);
     Size =
-      GetEnvironmentVariableW(NameUTF16.data(), Buf.data(), Buf.capacity());
+      GetEnvironmentVariableW(NameUTF16.data(), Buf.data(), Buf.size());
     if (Size == 0 && GetLastError() == ERROR_ENVVAR_NOT_FOUND)
       return None;
 
     // Try again with larger buffer.
-  } while (Size > Buf.capacity());
-  Buf.set_size(Size);
+  } while (Size > Buf.size());
+  Buf.truncate(Size);
 
   // Convert the result from UTF-16 to UTF-8.
   SmallVector<char, MAX_PATH> Res;
diff --git a/llvm/lib/Support/Windows/Program.inc b/llvm/lib/Support/Windows/Program.inc
--- a/llvm/lib/Support/Windows/Program.inc
+++ b/llvm/lib/Support/Windows/Program.inc
@@ -72,7 +72,7 @@
     SmallVector<wchar_t, MAX_PATH> U16Result;
     DWORD Len = MAX_PATH;
     do {
-      U16Result.reserve(Len);
+      U16Result.resize_for_overwrite(Len);
       // Lets attach the extension manually. That is needed for files
       // with a point in name like aaa.bbb. SearchPathW will not add extension
       // from its argument to such files because it thinks they already had one.
@@ -82,13 +82,13 @@
         return EC;
 
       Len = ::SearchPathW(Path, c_str(U16NameExt), nullptr,
-                          U16Result.capacity(), U16Result.data(), nullptr);
-    } while (Len > U16Result.capacity());
+                          U16Result.size(), U16Result.data(), nullptr);
+    } while (Len > U16Result.size());
 
     if (Len == 0)
       continue;
 
-    U16Result.set_size(Len);
+    U16Result.truncate(Len);
 
     if (std::error_code EC =
         windows::UTF16ToUTF8(U16Result.data(), U16Result.size(), U8Result))