[PowerPC] Use 16-byte alignment for modern cores for functions/loops

Most modern PowerPC cores prefer that functions and loops start on
16-byte-aligned boundaries (*), so instruct block placement, etc. to make this
happen. The branch selector has also been adjusted so account for the extra
nops that might now be inserted before loop headers.

(*) Some cores actually prefer other alignments for small loops, but that will
    be addressed in a follow-up commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225115 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCBranchSelector.cpp

@@ -70,12 +70,37 @@ bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
   Fn.RenumberBlocks();
   BlockSizes.resize(Fn.getNumBlockIDs());
 
+  auto GetAlignmentAdjustment =
+    [TII](MachineBasicBlock &MBB, unsigned Offset) -> unsigned {
+    unsigned Align = MBB.getAlignment();
+    if (!Align)
+      return 0;
+
+    unsigned AlignAmt = 1 << Align;
+    unsigned ParentAlign = MBB.getParent()->getAlignment();
+
+    if (Align <= ParentAlign)
+      return OffsetToAlignment(Offset, AlignAmt);
+
+    // The alignment of this MBB is larger than the function's alignment, so we
+    // can't tell whether or not it will insert nops. Assume that it will.
+    return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
+  };
+
   // Measure each MBB and compute a size for the entire function.
   unsigned FuncSize = 0;
   for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
        ++MFI) {
     MachineBasicBlock *MBB = MFI;
 
+    // The end of the previous block may have extra nops if this block has an
+    // alignment requirement.
+    if (MBB->getNumber() > 0) {
+      unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize);
+      BlockSizes[MBB->getNumber()-1] += AlignExtra;
+      FuncSize += AlignExtra;
+    }
+
     unsigned BlockSize = 0;
     for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
          MBBI != EE; ++MBBI)