Implement global merge optimization for global variables.

This commit implements two command line switches -global-merge-on-external
and -global-merge-aligned, and both of them are false by default, so this
optimization is disabled by default for all targets.

For ARM64, some back-end behaviors need to be tuned to get this optimization
further enabled.

llvm-svn: 208934

2 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM64/ARM64ISelLowering.cpp b/llvm/lib/Target/ARM64/ARM64ISelLowering.cpp
index 118007fa70c..880df0c1a81 100644
--- a/llvm/lib/Target/ARM64/ARM64ISelLowering.cpp
+++ b/llvm/lib/Target/ARM64/ARM64ISelLowering.cpp
@@ -630,6 +630,20 @@ unsigned ARM64TargetLowering::getMaximalGlobalOffset() const {
   return 4095;
 }
 
+/// getGlobalMergeAlignment - Set alignment to be the max size of merged
+/// global variable data structure, and make it aligned up to power of 2.
+/// This way, we could guarantee the merged global variable data structure
+/// doesn't cross page boundary, because usually OS always allocates page at
+/// 4096-byte aligned boundary.
+unsigned ARM64TargetLowering::getGlobalMergeAlignment(
+                                StructType *MergedTy) const {
+  unsigned Align = getDataLayout()->getTypeAllocSize(MergedTy);
+  if (Align & (Align - 1))
+    Align = llvm::NextPowerOf2(Align);
+
+  return Align;
+}
+
 FastISel *
 ARM64TargetLowering::createFastISel(FunctionLoweringInfo &funcInfo,
                                     const TargetLibraryInfo *libInfo) const {
diff --git a/llvm/lib/Target/ARM64/ARM64ISelLowering.h b/llvm/lib/Target/ARM64/ARM64ISelLowering.h
index 55792317dba..00b2710a35c 100644
--- a/llvm/lib/Target/ARM64/ARM64ISelLowering.h
+++ b/llvm/lib/Target/ARM64/ARM64ISelLowering.h
@@ -236,6 +236,10 @@ public:
   /// be used for loads / stores from the global.
   unsigned getMaximalGlobalOffset() const override;
 
+  /// getGlobalMergeAlignment - Set alignment to be the max size of merged
+  /// global variable data structure, and make it aligned up to power of 2.
+  unsigned getGlobalMergeAlignment(StructType *MergedTy) const override;
+
   /// Returns true if a cast between SrcAS and DestAS is a noop.
   bool isNoopAddrSpaceCast(unsigned SrcAS, unsigned DestAS) const override {
     // Addrspacecasts are always noops.