[CodeGen] Emit alignment "Max Skip" operand

The current AsmPrinter has support to emit the "Max Skip" operand
(the 3rd of .p2align), however has no support for it to actually be specified.
Adding MaxBytesForAlignment to MachineBasicBlock provides this capability on a
per-block basis. Leaving the value as default (0) causes no observable differences
in behaviour.

Differential Revision: https://reviews.llvm.org/D114590

1 files changed, 37 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 692587c..c93ffaa 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -96,6 +96,12 @@ static cl::opt<unsigned> AlignAllNonFallThruBlocks(
              "format (e.g 4 means align on 16B boundaries)."),
     cl::init(0), cl::Hidden);
 
+static cl::opt<unsigned> MaxBytesForAlignmentOverride(
+    "max-bytes-for-alignment",
+    cl::desc("Forces the maximum bytes allowed to be emitted when padding for "
+             "alignment"),
+    cl::init(0), cl::Hidden);
+
 // FIXME: Find a good default for this flag and remove the flag.
 static cl::opt<unsigned> ExitBlockBias(
     "block-placement-exit-block-bias",
@@ -2929,10 +2935,21 @@ void MachineBlockPlacement::alignBlocks() {
     MachineBasicBlock *LayoutPred =
         &*std::prev(MachineFunction::iterator(ChainBB));
 
+    auto DetermineMaxAlignmentPadding = [&]() {
+      // Set the maximum bytes allowed to be emitted for alignment.
+      unsigned MaxBytes;
+      if (MaxBytesForAlignmentOverride.getNumOccurrences() > 0)
+        MaxBytes = MaxBytesForAlignmentOverride;
+      else
+        MaxBytes = TLI->getMaxPermittedBytesForAlignment(ChainBB);
+      ChainBB->setMaxBytesForAlignment(MaxBytes);
+    };
+
     // Force alignment if all the predecessors are jumps. We already checked
     // that the block isn't cold above.
     if (!LayoutPred->isSuccessor(ChainBB)) {
       ChainBB->setAlignment(Align);
+      DetermineMaxAlignmentPadding();
       continue;
     }
 
@@ -2943,8 +2960,10 @@ void MachineBlockPlacement::alignBlocks() {
     BranchProbability LayoutProb =
         MBPI->getEdgeProbability(LayoutPred, ChainBB);
     BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
-    if (LayoutEdgeFreq <= (Freq * ColdProb))
+    if (LayoutEdgeFreq <= (Freq * ColdProb)) {
       ChainBB->setAlignment(Align);
+      DetermineMaxAlignmentPadding();
+    }
   }
 }
 
@@ -3418,17 +3437,30 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
   ComputedEdges.clear();
   ChainAllocator.DestroyAll();
 
+  bool HasMaxBytesOverride =
+      MaxBytesForAlignmentOverride.getNumOccurrences() > 0;
+
   if (AlignAllBlock)
     // Align all of the blocks in the function to a specific alignment.
-    for (MachineBasicBlock &MBB : MF)
-      MBB.setAlignment(Align(1ULL << AlignAllBlock));
+    for (MachineBasicBlock &MBB : MF) {
+      if (HasMaxBytesOverride)
+        MBB.setAlignment(Align(1ULL << AlignAllBlock),
+                         MaxBytesForAlignmentOverride);
+      else
+        MBB.setAlignment(Align(1ULL << AlignAllBlock));
+    }
   else if (AlignAllNonFallThruBlocks) {
     // Align all of the blocks that have no fall-through predecessors to a
     // specific alignment.
     for (auto MBI = std::next(MF.begin()), MBE = MF.end(); MBI != MBE; ++MBI) {
       auto LayoutPred = std::prev(MBI);
-      if (!LayoutPred->isSuccessor(&*MBI))
-        MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks));
+      if (!LayoutPred->isSuccessor(&*MBI)) {
+        if (HasMaxBytesOverride)
+          MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks),
+                            MaxBytesForAlignmentOverride);
+        else
+          MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks));
+      }
     }
   }
   if (ViewBlockLayoutWithBFI != GVDT_None &&