Add loop aligning to MachineBlockPlacement based on review discussion so

it's a bit more plausible to use this instead of CodePlacementOpt. The
code for this was shamelessly stolen from CodePlacementOpt, and then
trimmed down a bit. There doesn't seem to be much utility in returning
true/false from this pass as we may or may not have rewritten all of the
blocks. Also, the statistic of counting how many loops were aligned
doesn't seem terribly important so I removed it. If folks would like it
to be included, I'm happy to add it back.

This was probably the most egregious of the missing features, and now
I'm going to start gathering some performance numbers and looking at
specific loop structures that have different layout between the two.

Test is updated to include both basic loop alignment and nested loop
alignment.

llvm-svn: 142645

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -20,13 +20,14 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "block-placement2"
-#include "llvm/CodeGen/Passes.h"
-#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
-#include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/Passes.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/ADT/DenseMap.h"
@@ -35,6 +36,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetLowering.h"
 #include <algorithm>
 using namespace llvm;
 
@@ -259,9 +261,15 @@ class MachineBlockPlacement : public MachineFunctionPass {
   /// \brief A handle to the function-wide block frequency pass.
   const MachineBlockFrequencyInfo *MBFI;
 
+  /// \brief A handle to the loop info.
+  const MachineLoopInfo *MLI;
+
   /// \brief A handle to the target's instruction info.
   const TargetInstrInfo *TII;
 
+  /// \brief A handle to the target's lowering info.
+  const TargetLowering *TLI;
+
   /// \brief A prioritized list of edges in the BB-graph.
   ///
   /// For each function, we insert all control flow edges between BBs, along
@@ -307,6 +315,7 @@ class MachineBlockPlacement : public MachineFunctionPass {
   void BuildBlockChains();
   void PrioritizeChains(MachineFunction &F);
   void PlaceBlockChains(MachineFunction &F);
+  void AlignLoops(MachineFunction &F);
 
 public:
   static char ID; // Pass identification, replacement for typeid
@@ -319,6 +328,7 @@ public:
   void getAnalysisUsage(AnalysisUsage &AU) const {
     AU.addRequired<MachineBranchProbabilityInfo>();
     AU.addRequired<MachineBlockFrequencyInfo>();
+    AU.addRequired<MachineLoopInfo>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 
@@ -331,6 +341,7 @@ INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
                       "Branch Probability Basic Block Placement", false, false)
 INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
 INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
+INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
                     "Branch Probability Basic Block Placement", false, false)
 
@@ -595,6 +606,28 @@ void MachineBlockPlacement::PlaceBlockChains(MachineFunction &F) {
   }
 }
 
+/// \brief Recursive helper to align a loop and any nested loops.
+static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
+  // Recurse through nested loops.
+  for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
+    AlignLoop(F, *I, Align);
+
+  L->getTopBlock()->setAlignment(Align);
+}
+
+/// \brief Align loop headers to target preferred alignments.
+void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
+  if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
+    return;
+
+  unsigned Align = TLI->getPrefLoopAlignment();
+  if (!Align)
+    return;  // Don't care about loop alignment.
+
+  for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
+    AlignLoop(F, *I, Align);
+}
+
 bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
   // Check for single-block functions and skip them.
   if (llvm::next(F.begin()) == F.end())
@@ -602,7 +635,9 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
 
   MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
+  MLI = &getAnalysis<MachineLoopInfo>();
   TII = F.getTarget().getInstrInfo();
+  TLI = F.getTarget().getTargetLowering();
   assert(Edges.empty());
   assert(BlockToChain.empty());
   assert(PChains.empty());
@@ -612,6 +647,7 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
   BuildBlockChains();
   PrioritizeChains(F);
   PlaceBlockChains(F);
+  AlignLoops(F);
 
   Edges.clear();
   BlockToChain.clear();

llvm/test/CodeGen/X86/block-placement.ll

@@ -2,10 +2,10 @@
 
 declare void @error(i32 %i, i32 %a, i32 %b)
 
-define i32 @test1(i32 %i, i32* %a, i32 %b) {
+define i32 @test_ifchains(i32 %i, i32* %a, i32 %b) {
 ; Test a chain of ifs, where the block guarded by the if is error handling code
 ; that is not expected to run.
-; CHECK: test1:
+; CHECK: test_ifchains:
 ; CHECK: %entry
 ; CHECK: %else1
 ; CHECK: %else2
@@ -73,3 +73,70 @@ exit:
 }
 
 !0 = metadata !{metadata !"branch_weights", i32 4, i32 64}
+
+define i32 @test_loop_align(i32 %i, i32* %a) {
+; Check that we provide basic loop body alignment with the block placement
+; pass.
+; CHECK: test_loop_align:
+; CHECK: %entry
+; CHECK: .align 16,
+; CHECK-NEXT: %body
+; CHECK: %exit
+
+entry:
+  br label %body
+
+body:
+  %iv = phi i32 [ 0, %entry ], [ %next, %body ]
+  %base = phi i32 [ 0, %entry ], [ %sum, %body ]
+  %arrayidx = getelementptr inbounds i32* %a, i32 %iv
+  %0 = load i32* %arrayidx
+  %sum = add nsw i32 %0, %base
+  %next = add i32 %iv, 1
+  %exitcond = icmp eq i32 %next, %i
+  br i1 %exitcond, label %exit, label %body
+
+exit:
+  ret i32 %sum
+}
+
+define i32 @test_nested_loop_align(i32 %i, i32* %a, i32* %b) {
+; Check that we provide nested loop body alignment.
+; CHECK: test_nested_loop_align:
+; CHECK: %entry
+; CHECK: .align 16,
+; CHECK-NEXT: %loop.body.1
+; CHECK: .align 16,
+; CHECK-NEXT: %inner.loop.body
+; CHECK-NOT: .align
+; CHECK: %loop.body.2
+; CHECK: %exit
+
+entry:
+  br label %loop.body.1
+
+loop.body.1:
+  %iv = phi i32 [ 0, %entry ], [ %next, %loop.body.2 ]
+  %arrayidx = getelementptr inbounds i32* %a, i32 %iv
+  %bidx = load i32* %arrayidx
+  br label %inner.loop.body
+
+inner.loop.body:
+  %inner.iv = phi i32 [ 0, %loop.body.1 ], [ %inner.next, %inner.loop.body ]
+  %base = phi i32 [ 0, %loop.body.1 ], [ %sum, %inner.loop.body ]
+  %scaled_idx = mul i32 %bidx, %iv
+  %inner.arrayidx = getelementptr inbounds i32* %b, i32 %scaled_idx
+  %0 = load i32* %inner.arrayidx
+  %sum = add nsw i32 %0, %base
+  %inner.next = add i32 %iv, 1
+  %inner.exitcond = icmp eq i32 %inner.next, %i
+  br i1 %inner.exitcond, label %loop.body.2, label %inner.loop.body
+
+loop.body.2:
+  %next = add i32 %iv, 1
+  %exitcond = icmp eq i32 %next, %i
+  br i1 %exitcond, label %exit, label %loop.body.1
+
+exit:
+  ret i32 %sum
+}