[llvm] r240072 - Avoid redundant select node in early if-conversion pass

[Yi Jiang]

Author: yjiang
Date: Thu Jun 18 17:34:09 2015
New Revision: 240072

URL: http://llvm.org/viewvc/llvm-project?rev=240072&view=rev
Log:
Avoid redundant select node in early if-conversion pass

Added:
    llvm/trunk/test/CodeGen/AArch64/ifcvt-select.ll
Modified:
    llvm/trunk/lib/CodeGen/EarlyIfConversion.cpp

Modified: llvm/trunk/lib/CodeGen/EarlyIfConversion.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/EarlyIfConversion.cpp?rev=240072&r1=240071&r2=240072&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/EarlyIfConversion.cpp (original)
+++ llvm/trunk/lib/CodeGen/EarlyIfConversion.cpp Thu Jun 18 17:34:09 2015
@@ -479,11 +479,20 @@ void SSAIfConv::rewritePHIOperands() {
   // Convert all PHIs to select instructions inserted before FirstTerm.
   for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
     PHIInfo &PI = PHIs[i];
+    unsigned DstReg = 0;
+    
     DEBUG(dbgs() << "If-converting " << *PI.PHI);
-    unsigned PHIDst = PI.PHI->getOperand(0).getReg();
-    unsigned DstReg = MRI->createVirtualRegister(MRI->getRegClass(PHIDst));
-    TII->insertSelect(*Head, FirstTerm, HeadDL, DstReg, Cond, PI.TReg, PI.FReg);
-    DEBUG(dbgs() << "          --> " << *std::prev(FirstTerm));
+    if (PI.TReg == PI.FReg) {
+      // We do not need the select instruction if both incoming values are
+      // equal.
+      DstReg = PI.TReg;
+    } else {
+      unsigned PHIDst = PI.PHI->getOperand(0).getReg();
+      DstReg = MRI->createVirtualRegister(MRI->getRegClass(PHIDst));
+      TII->insertSelect(*Head, FirstTerm, HeadDL,
+                         DstReg, Cond, PI.TReg, PI.FReg);
+      DEBUG(dbgs() << "          --> " << *std::prev(FirstTerm));
+    }
 
     // Rewrite PHI operands TPred -> (DstReg, Head), remove FPred.
     for (unsigned i = PI.PHI->getNumOperands(); i != 1; i -= 2) {

Added: llvm/trunk/test/CodeGen/AArch64/ifcvt-select.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/ifcvt-select.ll?rev=240072&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/AArch64/ifcvt-select.ll (added)
+++ llvm/trunk/test/CodeGen/AArch64/ifcvt-select.ll Thu Jun 18 17:34:09 2015
@@ -0,0 +1,41 @@
+; RUN: llc -mtriple=arm64-apple-ios -mcpu=cyclone < %s | FileCheck %s
+; Do not generate redundant select in early if-converstion pass. 
+
+define i32 @foo(i32 %a, i32 %b)  {
+entry:
+;CHECK-LABEL: foo:
+;CHECK: csinc
+;CHECK-NOT: csel
+  %sub = sub nsw i32 %b, %a
+  %cmp10 = icmp sgt i32 %a, 0
+  br i1 %cmp10, label %while.body.lr.ph, label %while.end
+
+while.body.lr.ph:
+  br label %while.body
+
+while.body:                                  
+  %j.012 = phi i32 [ %sub, %while.body.lr.ph ], [ %inc, %if.then ], [ %inc, %if.else ]
+  %i.011 = phi i32 [ %a, %while.body.lr.ph ], [ %inc2, %if.then ], [ %dec, %if.else ]
+  %cmp1 = icmp slt i32 %i.011, %j.012
+  br i1 %cmp1, label %while.end, label %while.cond
+
+while.cond:
+  %inc = add nsw i32 %j.012, 5
+  %cmp2 = icmp slt i32 %inc, %b
+  br i1 %cmp2, label %if.then, label %if.else
+
+if.then:
+  %inc2 = add nsw i32 %i.011, 1
+  br label %while.body
+
+if.else:
+  %dec = add nsw i32 %i.011, -1
+  br label %while.body
+
+while.end:
+  %j.0.lcssa = phi i32 [ %j.012, %while.body ], [ %sub, %entry ]
+  %i.0.lcssa = phi i32 [ %i.011, %while.body ], [ %a, %entry ]
+  %add = add nsw i32 %j.0.lcssa, %i.0.lcssa
+  ret i32 %add
+}
+