diff --git a/include/llvm/Support/Compiler.h b/include/llvm/Support/Compiler.h
index 1a49956e225..8534316600a 100644
--- a/include/llvm/Support/Compiler.h
+++ b/include/llvm/Support/Compiler.h
@@ -283,7 +283,8 @@
 
 /// \macro LLVM_ALIGNAS
 /// \brief Used to specify a minimum alignment for a structure or variable. The
-/// alignment must be a constant integer.
+/// alignment must be a constant integer. Use LLVM_PTR_SIZE to compute
+/// alignments in terms of the size of a pointer.
 ///
 /// Note that __declspec(align) has special quirks, it's not legal to pass a
 /// structure with __declspec(align) as a formal parameter.
@@ -295,6 +296,22 @@
 # define LLVM_ALIGNAS(x) alignas(x)
 #endif
 
+/// \macro LLVM_PTR_SIZE
+/// \brief A constant integer equivalent to the value of sizeof(void*).
+/// Generally used in combination with LLVM_ALIGNAS or when doing computation in
+/// the preprocessor.
+#ifdef __SIZEOF_POINTER__
+# define LLVM_PTR_SIZE __SIZEOF_POINTER__
+#elif defined(_WIN64)
+# define LLVM_PTR_SIZE 8
+#elif defined(_WIN32)
+# define LLVM_PTR_SIZE 4
+#elif defined(_MSC_VER)
+# error "could not determine LLVM_PTR_SIZE as a constant int for MSVC"
+#else
+# define LLVM_PTR_SIZE sizeof(void *)
+#endif
+
 /// \macro LLVM_FUNCTION_NAME
 /// \brief Expands to __func__ on compilers which support it.  Otherwise,
 /// expands to a compiler-dependent replacement.