Fix maybe-uninitialized warning in vector_reduce handling

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@afa1f0b96f772ad

Author: svenvh

llvm-spirv/lib/SPIRV/SPIRVWriter.cpp

@@ -4416,24 +4416,28 @@ SPIRVValue *LLVMToSPIRVBase::transIntrinsicInst(IntrinsicInst *II,
         BM->addIntegerType(VecTy->getElementType()->getIntegerBitWidth());
     SPIRVTypeInt *I32STy = BM->addIntegerType(32);
     unsigned VecSize = VecTy->getElementCount().getFixedValue();
-    SmallVector<SPIRVValue *, 16> Extracts(VecSize);
-    for (unsigned Idx = 0; Idx < VecSize; ++Idx) {
-      Extracts[Idx] = BM->addVectorExtractDynamicInst(
-          VecSVal, BM->addIntegerConstant(I32STy, Idx), BB);
-    }
-    unsigned Counter = VecSize >> 1;
-    while (Counter != 0) {
-      for (unsigned Idx = 0; Idx < Counter; ++Idx) {
-        Extracts[Idx] = BM->addBinaryInst(Op, ResultSType, Extracts[Idx << 1],
-                                          Extracts[(Idx << 1) + 1], BB);
+    if (VecSize > 0) {
+      SmallVector<SPIRVValue *, 16> Extracts(VecSize);
+      for (unsigned Idx = 0; Idx < VecSize; ++Idx) {
+        Extracts[Idx] = BM->addVectorExtractDynamicInst(
+            VecSVal, BM->addIntegerConstant(I32STy, Idx), BB);
       }
-      Counter >>= 1;
-    }
-    if ((VecSize & 1) != 0) {
-      Extracts[0] = BM->addBinaryInst(Op, ResultSType, Extracts[0],
-                                      Extracts[VecSize - 1], BB);
+      unsigned Counter = VecSize >> 1;
+      while (Counter != 0) {
+        for (unsigned Idx = 0; Idx < Counter; ++Idx) {
+          Extracts[Idx] = BM->addBinaryInst(Op, ResultSType, Extracts[Idx << 1],
+                                            Extracts[(Idx << 1) + 1], BB);
+        }
+        Counter >>= 1;
+      }
+      if ((VecSize & 1) != 0) {
+        Extracts[0] = BM->addBinaryInst(Op, ResultSType, Extracts[0],
+                                        Extracts[VecSize - 1], BB);
+      }
+      return Extracts[0];
     }
-    return Extracts[0];
+    assert(VecSize && "Zero Extracts size for vector reduce lowering");
+    return nullptr;
   }
   case Intrinsic::vector_reduce_fadd:
   case Intrinsic::vector_reduce_fmul: {
@@ -4492,27 +4496,31 @@ SPIRVValue *LLVMToSPIRVBase::transIntrinsicInst(IntrinsicInst *II,
     SPIRVTypeInt *I32STy = BM->addIntegerType(32);
     unsigned VecSize = VecTy->getElementCount().getFixedValue();
     SmallVector<SPIRVValue *, 16> Extracts(VecSize);
-    for (unsigned Idx = 0; Idx < VecSize; ++Idx) {
-      Extracts[Idx] = BM->addVectorExtractDynamicInst(
-          VecSVal, BM->addIntegerConstant(I32STy, Idx), BB);
-    }
-    unsigned Counter = VecSize >> 1;
-    while (Counter != 0) {
-      for (unsigned Idx = 0; Idx < Counter; ++Idx) {
-        SPIRVValue *Cond = BM->addBinaryInst(Op, BoolSTy, Extracts[Idx << 1],
-                                             Extracts[(Idx << 1) + 1], BB);
-        Extracts[Idx] = BM->addSelectInst(Cond, Extracts[Idx << 1],
-                                          Extracts[(Idx << 1) + 1], BB);
+    if (VecSize > 0) {
+      for (unsigned Idx = 0; Idx < VecSize; ++Idx) {
+        Extracts[Idx] = BM->addVectorExtractDynamicInst(
+            VecSVal, BM->addIntegerConstant(I32STy, Idx), BB);
       }
-      Counter >>= 1;
-    }
-    if ((VecSize & 1) != 0) {
-      SPIRVValue *Cond = BM->addBinaryInst(Op, BoolSTy, Extracts[0],
-                                           Extracts[VecSize - 1], BB);
-      Extracts[0] =
-          BM->addSelectInst(Cond, Extracts[0], Extracts[VecSize - 1], BB);
+      unsigned Counter = VecSize >> 1;
+      while (Counter != 0) {
+        for (unsigned Idx = 0; Idx < Counter; ++Idx) {
+          SPIRVValue *Cond = BM->addBinaryInst(Op, BoolSTy, Extracts[Idx << 1],
+                                               Extracts[(Idx << 1) + 1], BB);
+          Extracts[Idx] = BM->addSelectInst(Cond, Extracts[Idx << 1],
+                                            Extracts[(Idx << 1) + 1], BB);
+        }
+        Counter >>= 1;
+      }
+      if ((VecSize & 1) != 0) {
+        SPIRVValue *Cond = BM->addBinaryInst(Op, BoolSTy, Extracts[0],
+                                             Extracts[VecSize - 1], BB);
+        Extracts[0] =
+            BM->addSelectInst(Cond, Extracts[0], Extracts[VecSize - 1], BB);
+      }
+      return Extracts[0];
     }
-    return Extracts[0];
+    assert(VecSize && "Zero Extracts size for vector reduce lowering");
+    return nullptr;
   }
   case Intrinsic::memset: {
     // Generally there is no direct mapping of memset to SPIR-V.  But it turns