[TTI] Fix getGEPCost() for geps with a single operand.

Previously this would sporadically crash as TargetType
was never initialized. We special-case the single-operand
case returning earlier and trying to mimic the behaviour of
isLegalAddressingMode as closely as possible.

Differential Revision:  https://reviews.llvm.org/D37277

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312357 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/TargetTransformInfoImpl.h

@@ -675,6 +675,12 @@ public:
 
     auto GTI = gep_type_begin(PointeeType, Operands);
     Type *TargetType = nullptr;
+
+    // Handle the case where the GEP instruction has a single operand,
+    // the basis, therefore TargetType is a nullptr.
+    if (Operands.empty())
+      return !BaseGV ? TTI::TCC_Free : TTI::TCC_Basic;
+
     for (auto I = Operands.begin(); I != Operands.end(); ++I, ++GTI) {
       TargetType = GTI.getIndexedType();
       // We assume that the cost of Scalar GEP with constant index and the

test/Transforms/SimplifyCFG/gepcost.ll

@@ -0,0 +1,28 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S -simplifycfg | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "thumbv7m-none--eabi"
+
+@glob = external unnamed_addr constant [16 x i8]
+
+define void @f() {
+; CHECK-LABEL: @f(
+; CHECK-NEXT:  entr:
+; CHECK-NEXT:    br i1 undef, label [[NEXT:%.*]], label [[EXIT:%.*]]
+; CHECK:       next:
+; CHECK-NEXT:    [[PAT:%.*]] = getelementptr [16 x i8], [16 x i8]* @glob
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entr:
+  br i1 undef, label %next, label %exit
+
+next:
+  %pat = getelementptr [16 x i8], [16 x i8]* @glob
+  br label %exit
+
+exit:
+  ret void
+}