[llvm] r215344 - In Machine CSE pass, the source register of a COPY machine instruction can

[Jiangning Liu]

Author: jiangning
Date: Mon Aug 11 00:17:19 2014
New Revision: 215344

URL: http://llvm.org/viewvc/llvm-project?rev=215344&view=rev
Log:
In Machine CSE pass, the source register of a COPY machine instruction can
be propagated to all its users, and this propagation could increase the 
probability of finding common subexpressions. If the COPY has only one user,
the COPY itself can be removed.


Added:
    llvm/trunk/test/CodeGen/AArch64/machine_cse.ll
Modified:
    llvm/trunk/lib/CodeGen/MachineCSE.cpp

Modified: llvm/trunk/lib/CodeGen/MachineCSE.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineCSE.cpp?rev=215344&r1=215343&r2=215344&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineCSE.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineCSE.cpp Mon Aug 11 00:17:19 2014
@@ -79,7 +79,8 @@ namespace {
     SmallVector<MachineInstr*, 64> Exps;
     unsigned CurrVN;
 
-    bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
+    bool PerformTrivialCopyPropagation(MachineInstr *MI,
+                                       MachineBasicBlock *MBB);
     bool isPhysDefTriviallyDead(unsigned Reg,
                                 MachineBasicBlock::const_iterator I,
                                 MachineBasicBlock::const_iterator E) const;
@@ -113,8 +114,12 @@ INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
 INITIALIZE_PASS_END(MachineCSE, "machine-cse",
                 "Machine Common Subexpression Elimination", false, false)
 
-bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
-                                          MachineBasicBlock *MBB) {
+/// The source register of a COPY machine instruction can be propagated to all
+/// its users, and this propagation could increase the probability of finding
+/// common subexpressions. If the COPY has only one user, the COPY itself can
+/// be removed.
+bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
+                                               MachineBasicBlock *MBB) {
   bool Changed = false;
   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
     MachineOperand &MO = MI->getOperand(i);
@@ -123,10 +128,7 @@ bool MachineCSE::PerformTrivialCoalescin
     unsigned Reg = MO.getReg();
     if (!TargetRegisterInfo::isVirtualRegister(Reg))
       continue;
-    if (!MRI->hasOneNonDBGUse(Reg))
-      // Only coalesce single use copies. This ensure the copy will be
-      // deleted.
-      continue;
+    bool OnlyOneUse = MRI->hasOneNonDBGUse(Reg);
     MachineInstr *DefMI = MRI->getVRegDef(Reg);
     if (!DefMI->isCopy())
       continue;
@@ -154,10 +156,14 @@ bool MachineCSE::PerformTrivialCoalescin
       continue;
     DEBUG(dbgs() << "Coalescing: " << *DefMI);
     DEBUG(dbgs() << "***     to: " << *MI);
+    // Propagate SrcReg of copies to MI.
     MO.setReg(SrcReg);
     MRI->clearKillFlags(SrcReg);
-    DefMI->eraseFromParent();
-    ++NumCoalesces;
+    // Coalesce single use copies.
+    if (OnlyOneUse) {
+      DefMI->eraseFromParent();
+      ++NumCoalesces;
+    }
     Changed = true;
   }
 
@@ -454,13 +460,15 @@ bool MachineCSE::ProcessBlock(MachineBas
 
     bool FoundCSE = VNT.count(MI);
     if (!FoundCSE) {
-      // Look for trivial copy coalescing opportunities.
-      if (PerformTrivialCoalescing(MI, MBB)) {
+      // Using trivial copy propagation to find more CSE opportunities.
+      if (PerformTrivialCopyPropagation(MI, MBB)) {
         Changed = true;
 
         // After coalescing MI itself may become a copy.
         if (MI->isCopyLike())
           continue;
+
+        // Try again to see if CSE is possible.
         FoundCSE = VNT.count(MI);
       }
     }

Added: llvm/trunk/test/CodeGen/AArch64/machine_cse.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/machine_cse.ll?rev=215344&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/AArch64/machine_cse.ll (added)
+++ llvm/trunk/test/CodeGen/AArch64/machine_cse.ll Mon Aug 11 00:17:19 2014
@@ -0,0 +1,45 @@
+; RUN: llc < %s -mtriple=aarch64-linux-gnuabi -O2 | FileCheck %s
+
+; marked as external to prevent possible optimizations
+ at a = external global i32
+ at b = external global i32
+ at c = external global i32
+ at d = external global i32
+ at e = external global i32
+
+define void @combine-sign-comparisons-by-cse(i32 *%arg) {
+; CHECK: cmp
+; CHECK: b.ge
+; CHECK-NOT: cmp
+; CHECK: b.le
+
+entry:
+  %a = load i32* @a, align 4
+  %b = load i32* @b, align 4
+  %c = load i32* @c, align 4
+  %d = load i32* @d, align 4
+  %e = load i32* @e, align 4
+
+  %cmp = icmp slt i32 %a, %e
+  br i1 %cmp, label %land.lhs.true, label %lor.lhs.false
+
+land.lhs.true:
+  %cmp1 = icmp eq i32 %b, %c
+  br i1 %cmp1, label %return, label %if.end
+
+lor.lhs.false:
+  %cmp2 = icmp sgt i32 %a, %e
+  br i1 %cmp2, label %land.lhs.true3, label %if.end
+
+land.lhs.true3:
+  %cmp4 = icmp eq i32 %b, %d
+  br i1 %cmp4, label %return, label %if.end
+
+if.end:
+  br label %return
+
+return:
+  %retval.0 = phi i32 [ 0, %if.end ], [ 1, %land.lhs.true3 ], [ 1, %land.lhs.true ]
+  store i32 %a, i32 *%arg
+  ret void
+}