Use logarithmic units for basic block alignment.

This was actually a bit of a mess. TLI.setPrefLoopAlignment was clearly
documented as taking log2(bytes) units, but the x86 target would still
set a preferred loop alignment of '16'.

CodePlacementOpt passed this number on to the basic block, and
AsmPrinter interpreted it as bytes.

Now both MachineFunction and MachineBasicBlock use logarithmic
alignments.

Obviously, MachineConstantPool still measures alignments in bytes, so we
can emulate the thrill of using as.

llvm-svn: 145889

include/llvm/CodeGen/MachineBasicBlock.h

@@ -84,8 +84,9 @@ class MachineBasicBlock : public ilist_node<MachineBasicBlock> {
 
   /// Alignment - Alignment of the basic block. Zero if the basic block does
   /// not need to be aligned.
+  /// The alignment is specified as log2(bytes).
   unsigned Alignment;
-  
+
   /// IsLandingPad - Indicate that this basic block is entered via an
   /// exception handler.
   bool IsLandingPad;
@@ -219,10 +220,12 @@ public:
   bool            livein_empty() const { return LiveIns.empty(); }
 
   /// getAlignment - Return alignment of the basic block.
+  /// The alignment is specified as log2(bytes).
   ///
   unsigned getAlignment() const { return Alignment; }
 
   /// setAlignment - Set alignment of the basic block.
+  /// The alignment is specified as log2(bytes).
   ///
   void setAlignment(unsigned Align) { Alignment = Align; }
 

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -1970,7 +1970,7 @@ static void EmitBasicBlockLoopComments(const MachineBasicBlock &MBB,
 void AsmPrinter::EmitBasicBlockStart(const MachineBasicBlock *MBB) const {
   // Emit an alignment directive for this block, if needed.
   if (unsigned Align = MBB->getAlignment())
-    EmitAlignment(Log2_32(Align));
+    EmitAlignment(Align);
 
   // If the block has its address taken, emit any labels that were used to
   // reference the block.  It is possible that there is more than one label

lib/Target/X86/X86ISelLowering.cpp

@@ -1211,10 +1211,10 @@ X86TargetLowering::X86TargetLowering(X86TargetMachine &TM)
   maxStoresPerMemcpyOptSize = Subtarget->isTargetDarwin() ? 8 : 4;
   maxStoresPerMemmove = 8; // For @llvm.memmove -> sequence of stores
   maxStoresPerMemmoveOptSize = Subtarget->isTargetDarwin() ? 8 : 4;
-  setPrefLoopAlignment(16);
+  setPrefLoopAlignment(4); // 2^4 bytes.
   benefitFromCodePlacementOpt = true;
 
-  setPrefFunctionAlignment(4);
+  setPrefFunctionAlignment(4); // 2^4 bytes.
 }