AArch64: add experimental support for address tagging.

AArch64 has the ability to use the top 8-bits of an "address" for extra
information, with the memory subsystem automatically masking them off for loads
and stores. When that's happening, we can sometimes skip masks on memory
operations in the compiler.

However, this requires the host OS and support stack to preserve those bits so
it can't be enabled everywhere. In principle iOS 8.0 and above do take the
required precautions and but we'll put it under a flag for now.

llvm-svn: 252573

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64Subtarget.cpp b/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
index e6ef6dc80e2..88af9602322 100644
--- a/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
+++ b/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
@@ -31,6 +31,11 @@ static cl::opt<bool>
 EnableEarlyIfConvert("aarch64-early-ifcvt", cl::desc("Enable the early if "
                      "converter pass"), cl::init(true), cl::Hidden);
 
+// If OS supports TBI, use this flag to enable it.
+static cl::opt<bool>
+UseAddressTopByteIgnored("aarch64-use-tbi", cl::desc("Assume that top byte of "
+                         "an address is ignored"), cl::init(false), cl::Hidden);
+
 AArch64Subtarget &
 AArch64Subtarget::initializeSubtargetDependencies(StringRef FS) {
   // Determine default and user-specified characteristics
@@ -125,6 +130,19 @@ bool AArch64Subtarget::enableEarlyIfConversion() const {
   return EnableEarlyIfConvert;
 }
 
+bool AArch64Subtarget::supportsAddressTopByteIgnored() const {
+  if (!UseAddressTopByteIgnored)
+    return false;
+
+  if (TargetTriple.isiOS()) {
+    unsigned Major, Minor, Micro;
+    TargetTriple.getiOSVersion(Major, Minor, Micro);
+    return Major >= 8;
+  }
+
+  return false;
+}
+
 std::unique_ptr<PBQPRAConstraint>
 AArch64Subtarget::getCustomPBQPConstraints() const {
   if (!isCortexA57())