[Memory] Add basic support for large/huge memory pages

This patch introduces Memory::MF_HUGE_HINT which indicates that allocateMappedMemory() shall return a pointer to a large memory page.
However the flag is a hint because we're not guaranteed in any way that we will get back a large memory page. There are several restrictions:

- Large/huge memory pages aren't enabled by default on modern OSes (Windows 10 and Linux at least), and should be manually enabled/reserved.
- Once enabled, it should be kept in mind that large pages are physical only, they can't be swapped.
- Memory fragmentation can affect the availability of large pages, especially after running the OS for a long time and/or running along many other applications.

Memory::allocateMappedMemory() will fallback to 4KB pages if it can't allocate 2MB large pages (if Memory::MF_HUGE_HINT is provided)

Currently, Memory::MF_HUGE_HINT only works on Windows. The hint will be ignored on Linux, 4KB pages will always be returned.

Differential Revision: https://reviews.llvm.org/D58718

llvm-svn: 355065

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/Support/Unix/Memory.inc b/llvm/lib/Support/Unix/Memory.inc
index 3c4d324a8bb..3473b488384 100644
--- a/llvm/lib/Support/Unix/Memory.inc
+++ b/llvm/lib/Support/Unix/Memory.inc
@@ -45,7 +45,7 @@ extern "C" void __clear_cache(void *, void*);
 namespace {
 
 int getPosixProtectionFlags(unsigned Flags) {
-  switch (Flags) {
+  switch (Flags & llvm::sys::Memory::MF_RWE_MASK) {
   case llvm::sys::Memory::MF_READ:
     return PROT_READ;
   case llvm::sys::Memory::MF_WRITE:
@@ -114,6 +114,7 @@ Memory::allocateMappedMemory(size_t NumBytes,
   if (Start && Start % PageSize)
     Start += PageSize - Start % PageSize;
 
+  // FIXME: Handle huge page requests (MF_HUGE_HINT).
   void *Addr = ::mmap(reinterpret_cast<void *>(Start), NumBytes, Protect,
                       MMFlags, fd, 0);
   if (Addr == MAP_FAILED) {