[llvm-commits] [llvm] r157521 - in /llvm/trunk: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp test/CodeGen/X86/switch-order-weight.ll

[Benjamin Kramer]

Author: d0k
Date: Sat May 26 15:01:32 2012
New Revision: 157521

URL: http://llvm.org/viewvc/llvm-project?rev=157521&view=rev
Log:
SelectionDAGBuilder: When emitting small compare chains for switches order them by using edge weights.

SimplifyCFG tends to form a lot of 2-3 case switches when merging branches. Move
the most likely condition to the front so it is checked first and the others can
be skipped. This is currently not as effective as it could be because SimplifyCFG
destroys profiling metadata when merging branches and switches. Merging branch
weight metadata is tricky though.

This code touches at most 3 cases so I didn't use a proper sorting algorithm.

Added:
    llvm/trunk/test/CodeGen/X86/switch-order-weight.ll
Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp?rev=157521&r1=157520&r2=157521&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp Sat May 26 15:01:32 2012
@@ -1903,8 +1903,6 @@
                                                  const Value* SV,
                                                  MachineBasicBlock *Default,
                                                  MachineBasicBlock *SwitchBB) {
-  Case& BackCase  = *(CR.Range.second-1);
-
   // Size is the number of Cases represented by this range.
   size_t Size = CR.Range.second - CR.Range.first;
   if (Size > 3)
@@ -1972,11 +1970,28 @@
     }
   }
 
+  // Order cases by weight so the most likely case will be checked first.
+  BranchProbabilityInfo *BPI = FuncInfo.BPI;
+  if (BPI) {
+    for (CaseItr I = CR.Range.first, IE = CR.Range.second; I != IE; ++I) {
+      uint32_t IWeight = BPI->getEdgeWeight(SwitchBB->getBasicBlock(),
+                                            I->BB->getBasicBlock());
+      for (CaseItr J = CR.Range.first; J < I; ++J) {
+        uint32_t JWeight = BPI->getEdgeWeight(SwitchBB->getBasicBlock(),
+                                              J->BB->getBasicBlock());
+        if (IWeight > JWeight)
+          std::swap(*I, *J);
+      }
+    }
+  }
+
   // Rearrange the case blocks so that the last one falls through if possible.
+  Case &BackCase = *(CR.Range.second-1);
   if (NextBlock && Default != NextBlock && BackCase.BB != NextBlock) {
     // The last case block won't fall through into 'NextBlock' if we emit the
     // branches in this order.  See if rearranging a case value would help.
-    for (CaseItr I = CR.Range.first, E = CR.Range.second-1; I != E; ++I) {
+    // We start at the bottom as it's the case with the least weight.
+    for (CaseItr I = CR.Range.second-2, E = CR.Range.first-1; I != E; --I) {
       if (I->BB == NextBlock) {
         std::swap(*I, BackCase);
         break;

Added: llvm/trunk/test/CodeGen/X86/switch-order-weight.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/switch-order-weight.ll?rev=157521&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/X86/switch-order-weight.ll (added)
+++ llvm/trunk/test/CodeGen/X86/switch-order-weight.ll Sat May 26 15:01:32 2012
@@ -0,0 +1,37 @@
+; RUN: llc -mtriple=x86_64-apple-darwin11 < %s | FileCheck %s
+
+; Check that the cases which lead to unreachable are checked after "10"
+
+define void @test1(i32 %x) nounwind uwtable ssp {
+entry:
+  switch i32 %x, label %if.end7 [
+    i32 0, label %if.then
+    i32 10, label %if.then2
+    i32 20, label %if.then5
+  ]
+
+; CHECK: test1:
+; CHECK-NOT: unr
+; CHECK: cmpl $10
+; CHECK: bar
+; CHECK: cmpl $20
+
+if.then:
+  tail call void @unr(i32 23) noreturn nounwind
+  unreachable
+
+if.then2:
+  tail call void @bar(i32 42) nounwind
+  br label %if.end7
+
+if.then5:
+  tail call void @unr(i32 5) noreturn nounwind
+  unreachable
+
+if.end7:
+  ret void
+}
+
+declare void @unr(i32) noreturn
+
+declare void @bar(i32)