SimplifyCFG: GEPs with constant indices are cheap enough to be executed unconditionally.

llvm-svn: 126445

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -247,6 +247,11 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB,
     if (PBB->getFirstNonPHIOrDbg() != I)
       return false;
     break;
+  case Instruction::GetElementPtr:
+    // GEPs are cheap if all indices are constant.
+    if (!cast<GetElementPtrInst>(I)->hasAllConstantIndices())
+      return false;
+    break;
   case Instruction::Add:
   case Instruction::Sub:
   case Instruction::And:

test/Transforms/SimplifyCFG/select-gep.ll

@@ -0,0 +1,20 @@
+; RUN: opt -S -simplifycfg %s | FileCheck %s
+
+define i8* @test1(i8* %x) nounwind {
+entry:
+  %tmp1 = load i8* %x, align 1
+  %cmp = icmp eq i8 %tmp1, 47
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+  %incdec.ptr = getelementptr inbounds i8* %x, i64 1
+  br label %if.end
+
+if.end:
+  %x.addr = phi i8* [ %incdec.ptr, %if.then ], [ %x, %entry ]
+  ret i8* %x.addr
+
+; CHECK: @test1
+; CHECK: %x.addr = select i1 %cmp, i8* %incdec.ptr, i8* %x
+; CHECK: ret i8* %x.addr
+}