SelectionDAG switch lowering: Replace unreachable default with most popular case.

This can significantly reduce the size of the switch, allowing for more
efficient lowering.

I also worked with the idea of exploiting unreachable defaults by
omitting the range check for jump tables, but always ended up with a
non-neglible binary size increase. It might be worth looking into some more.

SimplifyCFG currently does this transformation, but I'm working towards changing
that so we can optimize harder based on unreachable defaults.

Differential Revision: http://reviews.llvm.org/D6510

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223566 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index c3b197e97eb5836be7bbf9937e4c7e825b8b03a5..a5fdfc55487763274d2d393d67d0e6c106665043 100644 (file)
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2698,32 +2698,55 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
   if (SwitchMBB + 1 != FuncInfo.MF->end())
     NextBlock = SwitchMBB + 1;
 
+
+  // Create a vector of Cases, sorted so that we can efficiently create a binary
+  // search tree from them.
+  CaseVector Cases;
+  Clusterify(Cases, SI);
+
+  // Get the default destination MBB.
   MachineBasicBlock *Default = FuncInfo.MBBMap[SI.getDefaultDest()];
 
-  // If there is only the default destination, branch to it if it is not the
-  // next basic block.  Otherwise, just fall through.
-  if (!SI.getNumCases()) {
+  if (isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg()) &&
+      !Cases.empty()) {
+    // Replace an unreachable default destination with the most popular case
+    // destination.
+    DenseMap<const BasicBlock *, uint64_t> Popularity;
+    uint64_t MaxPop = 0;
+    const BasicBlock *MaxBB = nullptr;
+    for (auto I : SI.cases()) {
+      const BasicBlock *BB = I.getCaseSuccessor();
+      if (++Popularity[BB] > MaxPop) {
+        MaxPop = Popularity[BB];
+        MaxBB = BB;
+      }
+    }
+
+    // Set new default.
+    assert(MaxPop > 0);
+    assert(MaxBB);
+    Default = FuncInfo.MBBMap[MaxBB];
+
+    // Remove cases that were pointing to the destination that is now the default.
+    Cases.erase(std::remove_if(Cases.begin(), Cases.end(),
+                               [&](const Case &C) { return C.BB == Default; }),
+                Cases.end());
+  }
+
+  // If there is only the default destination, go there directly.
+  if (Cases.empty()) {
     // Update machine-CFG edges.
     SwitchMBB->addSuccessor(Default);
 
     // If this is not a fall-through branch, emit the branch.
-    if (Default != NextBlock)
-      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(),
-                              MVT::Other, getControlRoot(),
-                              DAG.getBasicBlock(Default)));
-
+    if (Default != NextBlock) {
+      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(), MVT::Other,
+                              getControlRoot(), DAG.getBasicBlock(Default)));
+    }
     return;
   }
 
-  // If there are any non-default case statements, create a vector of Cases
-  // representing each one, and sort the vector so that we can efficiently
-  // create a binary search tree from them.
-  CaseVector Cases;
-  Clusterify(Cases, SI);
-
-  // Get the Value to be switched on and default basic blocks, which will be
-  // inserted into CaseBlock records, representing basic blocks in the binary
-  // search tree.
+  // Get the Value to be switched on.
   const Value *SV = SI.getCondition();
 
   // Push the initial CaseRec onto the worklist

diff --git a/test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll b/test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll
index 10dc927200b629d9fe571a8d9ee5916cefb59c8b..9cd150a2f56db4ff11284727956cf3f93124a5f7 100644 (file)
--- a/test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll
+++ b/test/CodeGen/X86/2013-10-14-FastISel-incorrect-vreg.ll
@@ -41,7 +41,7 @@ entry:
     i1 false, label %label_end
   ]
 default:
-  unreachable
+  br label %label_end
 
 label_true:
   br label %label_end
@@ -80,7 +80,7 @@ entry:
     i1 false, label %label_end
   ]
 default:
-  unreachable
+  br label %label_end
 
 label_true:
   br label %label_end
@@ -119,7 +119,7 @@ entry:
     i1 false, label %label_end
   ]
 default:
-  unreachable
+  br label %label_end
 
 label_true:
   br label %label_end

diff --git a/test/CodeGen/X86/asm-label.ll b/test/CodeGen/X86/asm-label.ll
index 1fc6e2eaf2b73364df674c4e7ad928c110759a7f..1da66e74d34f5055db61a803706644ff31dc72ba 100644 (file)
--- a/test/CodeGen/X86/asm-label.ll
+++ b/test/CodeGen/X86/asm-label.ll
@@ -24,7 +24,7 @@ if.end:                                           ; preds = %if.then
   br label %cleanup
 
 cleanup:                                          ; preds = %if.end, %if.then9
-  switch i32 undef, label %unreachable [
+  switch i32 undef, label %default [
     i32 0, label %cleanup.cont
     i32 1, label %if.end11
   ]
@@ -35,6 +35,6 @@ cleanup.cont:                                     ; preds = %cleanup
 if.end11:                                         ; preds = %cleanup.cont, %cleanup, %land.lhs.true, %entry
   ret void
 
-unreachable:                                      ; preds = %cleanup
-  unreachable
+default:                                          ; preds = %cleanup
+  br label %if.end11
 }

diff --git a/test/CodeGen/X86/switch-jump-table.ll b/test/CodeGen/X86/switch-jump-table.ll
new file mode 100644 (file)
index 0000000..7ae45b5
--- /dev/null
+++ b/test/CodeGen/X86/switch-jump-table.ll
@@ -0,0 +1,52 @@
+; RUN: llc -march=x86 < %s | FileCheck %s
+
+
+; An unreachable default destination is replaced with the most popular case label.
+
+define void @sum2(i32 %x, i32* %to) {
+; CHECK-LABEL: sum2:
+; CHECK: movl 4(%esp), [[REG:%e[a-z]{2}]]
+; cmpl $3, [[REG]]
+; CHECK: jbe .LBB0_1
+; CHECK: movl $4
+; CHECK: retl
+; CHECK-LABEL: .LBB0_1:
+; CHECK-NEXT: jmpl *.LJTI0_0(,[[REG]],4)
+
+entry:
+  switch i32 %x, label %default [
+    i32 0, label %bb0
+    i32 1, label %bb1
+    i32 2, label %bb2
+    i32 3, label %bb3
+    i32 4, label %bb4
+    i32 5, label %bb4
+  ]
+bb0:
+  store i32 0, i32* %to
+  br label %exit
+bb1:
+  store i32 1, i32* %to
+  br label %exit
+bb2:
+  store i32 2, i32* %to
+  br label %exit
+bb3:
+  store i32 3, i32* %to
+  br label %exit
+bb4:
+  store i32 4, i32* %to
+  br label %exit
+exit:
+  ret void
+default:
+  unreachable
+
+; The jump table has four entries.
+; CHECK-LABEL: .LJTI0_0:
+; CHECK-NEXT: .long  .LBB0_2
+; CHECK-NEXT: .long  .LBB0_3
+; CHECK-NEXT: .long  .LBB0_4
+; CHECK-NEXT: .long  .LBB0_5
+; CHECK-NOT: .long
+}