[AST] Add a fast path to ConstantArrayType::getNumAddressingBits().

 - This fast path is almost 100% effective on real code, and lets us avoid
   multiple allocations of 128-bit APSInt objects in the common case.

 - As with any overflow-check-skipping-code, I'd appreciate someone double
   checking my logic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177183 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 22 insertions, 3 deletions

diff --git a/lib/AST/Type.cpp b/lib/AST/Type.cpp
index cdd67fda4f..0c5636d840 100644
--- a/lib/AST/Type.cpp
+++ b/lib/AST/Type.cpp
@@ -76,16 +76,35 @@ bool QualType::isConstant(QualType T, ASTContext &Ctx) {
 unsigned ConstantArrayType::getNumAddressingBits(ASTContext &Context,
                                                  QualType ElementType,
                                                const llvm::APInt &NumElements) {
+  uint64_t ElementSize = Context.getTypeSizeInChars(ElementType).getQuantity();
+
+  // Fast path the common cases so we can avoid the conservative computation
+  // below, which in common cases allocates "large" APSInt values, which are
+  // slow.
+
+  // If the element size is a power of 2, we can directly compute the additional
+  // number of addressing bits beyond those required for the element count.
+  if (llvm::isPowerOf2_64(ElementSize)) {
+    return NumElements.getActiveBits() + llvm::Log2_64(ElementSize);
+  }
+
+  // If both the element count and element size fit in 32-bits, we can do the
+  // computation directly in 64-bits.
+  if ((ElementSize >> 32) == 0 && NumElements.getBitWidth() <= 64 &&
+      (NumElements.getZExtValue() >> 32) == 0) {
+    uint64_t TotalSize = NumElements.getZExtValue() * ElementSize;
+    return 64 - llvm::CountLeadingZeros_64(TotalSize);
+  }
+
+  // Otherwise, use APSInt to handle arbitrary sized values.
   llvm::APSInt SizeExtended(NumElements, true);
   unsigned SizeTypeBits = Context.getTypeSize(Context.getSizeType());
   SizeExtended = SizeExtended.extend(std::max(SizeTypeBits,
                                               SizeExtended.getBitWidth()) * 2);
 
-  uint64_t ElementSize
-    = Context.getTypeSizeInChars(ElementType).getQuantity();
   llvm::APSInt TotalSize(llvm::APInt(SizeExtended.getBitWidth(), ElementSize));
   TotalSize *= SizeExtended;  
-  
+
   return TotalSize.getActiveBits();
 }