[MachineBlockPlacement] Respect target limits on padding amount when aligning all blocks

[chill]

Summary

If the compiler (llc) is given --align-all-blocks=N or
--align-all-nofallthru-blocks=N it will emit alignment directives
without regard to the limit on the number of padding bytes inserted
for alignment.

With this patch, the compiler will respect the limits.

Previous behaviour of having no limits can be obtained by passing
-max-bytes-for-alignment=0.

llvm/lib/CodeGen/MachineBlockPlacement.cpp

bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
 
   optimizeBranches();
   alignBlocks();
 
   BlockToChain.clear();
   ComputedEdges.clear();
   ChainAllocator.DestroyAll();
 
-  bool HasMaxBytesOverride =
-      MaxBytesForAlignmentOverride.getNumOccurrences() > 0;
-
-  if (AlignAllBlock)
+  if (AlignAllBlock) {
     // Align all of the blocks in the function to a specific alignment.
-    for (MachineBasicBlock &MBB : MF) {
-      if (HasMaxBytesOverride)
-        MBB.setAlignment(Align(1ULL << AlignAllBlock),
-                         MaxBytesForAlignmentOverride);
-      else
-        MBB.setAlignment(Align(1ULL << AlignAllBlock));
-    }
-  else if (AlignAllNonFallThruBlocks) {
+    for (MachineBasicBlock &MBB : MF)
+      MBB.setAlignment(Align(1ULL << AlignAllBlock), MaxBytesForAlignment);
+
+  } 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)) {
-        if (HasMaxBytesOverride)
-          MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks),
-                            MaxBytesForAlignmentOverride);
-        else
-          MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks));
-      }
+      if (LayoutPred->isSuccessor(&*MBI))
+        continue;
+                        MaxBytesForAlignment);
     }
   }
+
   if (ViewBlockLayoutWithBFI != GVDT_None &&
       (ViewBlockFreqFuncName.empty() ||
        F->getFunction().getName().equals(ViewBlockFreqFuncName))) {
     if (RenumberBlocksBeforeView)
       MF.RenumberBlocks();
     MBFI->view("MBP." + MF.getName(), false);
   }
 

llvm/test/CodeGen/AArch64/align-all-blocks.ll

+; RUN: llc -align-all-blocks=5                            < %s | FileCheck %s --check-prefixes=CHECK,CHECK-ALL
+; RUN: llc -align-all-blocks=5 -max-bytes-for-alignment=0 < %s | FileCheck %s --check-prefixes=CHECK,CHECK-NOLIMIT
+; RUN: llc -align-all-nofallthru-blocks=5                 < %s | FileCheck %s --check-prefixes=CHECK,CHECK-NOFALLTHRU
+
+target triple = "aarch64-linux"
+
+declare void @g(...)
+
+define void @f(i1 %c) "tune-cpu"="neoverse-v1" {
+; CHECK-LABEL: f:
+
+; CHECK-ALL: .p2align 5, , 16
+; CHECK-NOLIMIT: .p2align 5{{$}}
+; CHECK-NOFALLTHRU-NOT: p2align
+; CHECK: // %entry
+entry:
+    br i1 %c, label %if.then, label %if.else
+
+; CHECK-ALL: .p2align 5, , 16
+; CHECK-NOLIMIT: .p2align 5
+; CHECK-NOFALLTHRU-NOT: p2align
+; CHECK: // %if.then
+if.then:
+    call void @g(i32 0)
+    br label %exit
+
+; CHECK-ALL: .p2align 5, , 16
+; CHECK-NOLIMIT: .p2align 5{{$}}
+; CHECK-NOFALLTHRU: .p2align 5, , 16
+; CHECK: // %if.else
+if.else:
+    call void @g(i32 1, i32 2)
+    br label %exit
+
+; CHECK-All: .p2align 5, , 16
+; CHECK-NOLIMIT: .p2align 5{{$}}
+; CHECK-NOFALLTHRU-NOT: p2align
+; CHECK: // %exit
+exit:
+    call void @f(i32 3)
+    ret void
+}