JIT: Fold null checks against initialized static readonly fields (#50000

)

Co-authored-by: Andy Ayers <andya@microsoft.com>
Co-authored-by: Sergey Andreenko <seandree@microsoft.com>

src/coreclr/jit/compiler.cpp

@@ -9069,6 +9069,10 @@ void cTreeFlags(Compiler* comp, GenTree* tree)
                 {
                     chars += printf("[IND_INVARIANT]");
                 }
+                if (tree->gtFlags & GTF_IND_NONNULL)
+                {
+                    chars += printf("[IND_NONNULL]");
+                }
                 break;
 
             case GT_CLS_VAR:

src/coreclr/jit/fgdiagnostic.cpp

@@ -2608,6 +2608,7 @@ void Compiler::fgDebugCheckDispFlags(GenTree* tree, unsigned dispFlags, unsigned
     {
         printf("%c", (dispFlags & GTF_IND_INVARIANT) ? '#' : '-');
         printf("%c", (dispFlags & GTF_IND_NONFAULTING) ? 'n' : '-');
+        printf("%c", (dispFlags & GTF_IND_NONNULL) ? '@' : '-');
     }
     GenTree::gtDispFlags(dispFlags, debugFlags);
 }

src/coreclr/jit/gentree.cpp

@@ -6107,6 +6107,12 @@ GenTree* Compiler::gtNewIndOfIconHandleNode(var_types indType, size_t addr, unsi
 
         // This indirection also is invariant.
         indNode->gtFlags |= GTF_IND_INVARIANT;
+
+        if (iconFlags == GTF_ICON_STR_HDL)
+        {
+            // String literals are never null
+            indNode->gtFlags |= GTF_IND_NONNULL;
+        }
     }
     return indNode;
 }
@@ -10252,6 +10258,12 @@ void Compiler::gtDispNode(GenTree* tree, IndentStack* indentStack, __in __in_z _
                         --msgLength;
                         break;
                     }
+                    if (tree->gtFlags & GTF_IND_NONNULL)
+                    {
+                        printf("@");
+                        --msgLength;
+                        break;
+                    }
                 }
                 FALLTHROUGH;
 

src/coreclr/jit/gentree.h

@@ -866,10 +866,11 @@ struct GenTree
                                                //             alignment of 1 byte)
 #define GTF_IND_INVARIANT           0x01000000 // GT_IND   -- the target is invariant (a prejit indirection)
 #define GTF_IND_ARR_INDEX           0x00800000 // GT_IND   -- the indirection represents an (SZ) array index
+#define GTF_IND_NONNULL             0x00400000 // GT_IND   -- the indirection never returns null (zero)
 
 #define GTF_IND_FLAGS \
     (GTF_IND_VOLATILE | GTF_IND_TGTANYWHERE | GTF_IND_NONFAULTING | GTF_IND_TLS_REF |          \
-     GTF_IND_UNALIGNED | GTF_IND_INVARIANT | GTF_IND_ARR_INDEX | GTF_IND_TGT_NOT_HEAP)
+     GTF_IND_UNALIGNED | GTF_IND_INVARIANT | GTF_IND_NONNULL | GTF_IND_ARR_INDEX | GTF_IND_TGT_NOT_HEAP)
 
 #define GTF_CLS_VAR_VOLATILE        0x40000000 // GT_FIELD/GT_CLS_VAR -- same as GTF_IND_VOLATILE
 #define GTF_CLS_VAR_INITCLASS       0x20000000 // GT_FIELD/GT_CLS_VAR -- same as GTF_FLD_INITCLASS

src/coreclr/jit/morph.cpp

@@ -6520,7 +6520,9 @@ GenTree* Compiler::fgMorphField(GenTree* tree, MorphAddrContext* mac)
                 if (isStaticReadOnlyInited)
                 {
                     JITDUMP("Marking initialized static read-only field '%s' as invariant.\n", eeGetFieldName(symHnd));
-                    tree->gtFlags |= GTF_IND_INVARIANT;
+
+                    // Static readonly field is not null at this point (see getStaticFieldCurrentClass impl).
+                    tree->gtFlags |= (GTF_IND_INVARIANT | GTF_IND_NONFAULTING | GTF_IND_NONNULL);
                     tree->gtFlags &= ~GTF_ICON_INITCLASS;
                     addr->gtFlags = GTF_ICON_CONST_PTR;
                 }

src/coreclr/jit/valuenum.cpp

@@ -8139,11 +8139,20 @@ void Compiler::fgValueNumberTree(GenTree* tree)
             {
                 assert(!isVolatile); // We don't expect both volatile and invariant
 
-                // Is it a string literal? (it's always non-null)
-                if (addr->IsCnsIntOrI() && addr->IsIconHandle(GTF_ICON_STR_HDL))
+                // Is this invariant indirect expected to always return a non-null value?
+                if ((tree->gtFlags & GTF_IND_NONNULL) != 0)
                 {
-                    tree->gtVNPair = vnStore->VNPairForFunc(tree->TypeGet(), VNF_StrCns, addrNvnp);
-                    assert(addrXvnp.BothEqual() && (addrXvnp.GetLiberal() == ValueNumStore::VNForEmptyExcSet()));
+                    assert(tree->gtFlags & GTF_IND_NONFAULTING);
+                    tree->gtVNPair = vnStore->VNPairForFunc(tree->TypeGet(), VNF_NonNullIndirect, addrNvnp);
+                    if (addr->IsCnsIntOrI())
+                    {
+                        assert(addrXvnp.BothEqual() && (addrXvnp.GetLiberal() == ValueNumStore::VNForEmptyExcSet()));
+                    }
+                    else
+                    {
+                        assert(false && "it's not expected to be hit at the moment, but can be allowed.");
+                        // tree->gtVNPair = vnStore->VNPWithExc(tree->gtVNPair, addrXvnp);
+                    }
                 }
                 else
                 {

src/coreclr/jit/valuenumfuncs.h

@@ -144,7 +144,7 @@ ValueNumFuncDef(Box, 3, false, false, false)
 ValueNumFuncDef(BoxNullable, 3, false, false, false)
 
 ValueNumFuncDef(LazyStrCns, 2, false, true, false)  // lazy-initialized string literal (helper)
-ValueNumFuncDef(StrCns, 1, false, true, false)      // indirect for a string literal
+ValueNumFuncDef(NonNullIndirect, 1, false, true, false)  // this indirect is expected to always return a non-null value
 ValueNumFuncDef(Unbox, 2, false, true, false)
 
 ValueNumFuncDef(LT_UN, 2, false, false, false)      // unsigned or unordered comparisons