JIT: Disallow 0-sized block ops (#83224)

Avoid creating 0-sized block ops in the importer (but keep the side effects of the addresses). Add asserts that we don't create these nodes and get rid of some downstream checks for the case.

Fix #12807

Also merge CEE_INITBLK and CEE_CPBLK importer cases. As part of this fix a bug where we weren't setting GTF_EXCEPT on the destination in the CEE_INITBLK case.

src/coreclr/jit/gentree.h

@@ -7340,6 +7340,7 @@ public:
     void Initialize(ClassLayout* layout)
     {
         assert(OperIsBlk(OperGet()) && ((layout != nullptr) || OperIs(GT_STORE_DYN_BLK)));
+        assert((layout == nullptr) || (layout->GetSize() != 0));
 
         m_layout    = layout;
         gtBlkOpKind = BlkOpKindInvalid;

src/coreclr/jit/importer.cpp

@@ -10641,56 +10641,47 @@ void Compiler::impImportBlockCode(BasicBlock* block)
             }
 
             case CEE_INITBLK:
+            case CEE_CPBLK:
 
                 op3 = impPopStack().val; // Size
-                op2 = impPopStack().val; // Value
+                op2 = impPopStack().val; // Value / Src addr
                 op1 = impPopStack().val; // Dst addr
 
                 if (op3->IsCnsIntOrI())
                 {
-                    size = (unsigned)op3->AsIntConCommon()->IconValue();
-                    op1  = new (this, GT_BLK) GenTreeBlk(GT_BLK, TYP_STRUCT, op1, typGetBlkLayout(size));
-                    op1  = gtNewBlkOpNode(op1, op2, (prefixFlags & PREFIX_VOLATILE) != 0);
-                }
-                else
-                {
-                    if (!op2->IsIntegralConst(0))
+                    if (op3->IsIntegralConst(0))
                     {
-                        op2 = gtNewOperNode(GT_INIT_VAL, TYP_INT, op2);
-                    }
-
-#ifdef TARGET_64BIT
-                    // STORE_DYN_BLK takes a native uint size as it turns into call to memset.
-                    op3 = gtNewCastNode(TYP_I_IMPL, op3, /* fromUnsigned */ true, TYP_U_IMPL);
-#endif
+                        if ((op1->gtFlags & GTF_SIDE_EFFECT) != 0)
+                        {
+                            impAppendTree(gtUnusedValNode(op1), CHECK_SPILL_ALL, impCurStmtDI);
+                        }
 
-                    op1  = new (this, GT_STORE_DYN_BLK) GenTreeStoreDynBlk(op1, op2, op3);
-                    size = 0;
+                        if ((op2->gtFlags & GTF_SIDE_EFFECT) != 0)
+                        {
+                            impAppendTree(gtUnusedValNode(op2), CHECK_SPILL_ALL, impCurStmtDI);
+                        }
 
-                    if ((prefixFlags & PREFIX_VOLATILE) != 0)
-                    {
-                        op1->gtFlags |= GTF_BLK_VOLATILE;
+                        break;
                     }
-                }
-                goto SPILL_APPEND;
-
-            case CEE_CPBLK:
-
-                op3 = impPopStack().val; // Size
-                op2 = impPopStack().val; // Src addr
-                op1 = impPopStack().val; // Dst addr
 
-                if (op3->IsCnsIntOrI())
-                {
                     size = static_cast<unsigned>(op3->AsIntConCommon()->IconValue());
-
-                    op1 = gtNewBlockVal(op1, size);
-                    op2 = gtNewBlockVal(op2, size);
-                    op1 = gtNewBlkOpNode(op1, op2, (prefixFlags & PREFIX_VOLATILE) != 0);
+                    op1  = gtNewBlockVal(op1, size);
+                    op2  = opcode == CEE_INITBLK ? op2 : gtNewBlockVal(op2, size);
+                    op1  = gtNewBlkOpNode(op1, op2, (prefixFlags & PREFIX_VOLATILE) != 0);
                 }
                 else
                 {
-                    op2 = gtNewOperNode(GT_IND, TYP_STRUCT, op2);
+                    if (opcode == CEE_INITBLK)
+                    {
+                        if (!op2->IsIntegralConst(0))
+                        {
+                            op2 = gtNewOperNode(GT_INIT_VAL, TYP_INT, op2);
+                        }
+                    }
+                    else
+                    {
+                        op2 = gtNewOperNode(GT_IND, TYP_STRUCT, op2);
+                    }
 
 #ifdef TARGET_64BIT
                     // STORE_DYN_BLK takes a native uint size as it turns into call to memcpy.

src/coreclr/jit/morphblock.cpp

@@ -211,6 +211,8 @@ void MorphInitBlockHelper::PrepareDst()
         m_blockSize = genTypeSize(m_dst);
     }
 
+    assert(m_blockSize != 0);
+
 #if defined(DEBUG)
     if (m_comp->verbose)
     {
@@ -481,12 +483,6 @@ void MorphInitBlockHelper::TryInitFieldByField()
     LclVarDsc* destLclVar = m_dstVarDsc;
     unsigned   blockSize  = m_blockSize;
 
-    if (blockSize == 0)
-    {
-        JITDUMP(" size is zero.\n");
-        return;
-    }
-
     if (destLclVar->IsAddressExposed() && destLclVar->lvContainsHoles)
     {
         JITDUMP(" dest is address exposed and contains holes.\n");
@@ -647,11 +643,6 @@ void MorphInitBlockHelper::TryInitFieldByField()
 //
 void MorphInitBlockHelper::TryPrimitiveInit()
 {
-    if (m_blockSize == 0)
-    {
-        return;
-    }
-
     if (m_src->IsIntegralConst(0) && (m_dstVarDsc != nullptr) && (genTypeSize(m_dstVarDsc) == m_blockSize))
     {
         var_types lclVarType = m_dstVarDsc->TypeGet();
@@ -1101,7 +1092,7 @@ void MorphCopyBlockHelper::MorphStructCases()
 //
 void MorphCopyBlockHelper::TryPrimitiveCopy()
 {
-    if (!m_dst->TypeIs(TYP_STRUCT) || (m_blockSize == 0))
+    if (!m_dst->TypeIs(TYP_STRUCT))
     {
         return;
     }