JIT: Disable old promotion for structs with significant padding

src/coreclr/jit/compiler.h

@@ -604,12 +604,6 @@ class LclVarDsc
     unsigned char lvIsStructField : 1; // Is this local var a field of a promoted struct local?
     unsigned char lvContainsHoles : 1; // Is this a promoted struct whose fields do not cover the struct local?
 
-    // True for a promoted struct that has significant padding in it.
-    // Significant padding is any data in the struct that is not covered by a
-    // promoted field and that the EE told us we need to preserve on block
-    // copies/inits.
-    unsigned char lvAnySignificantPadding : 1;
-
     unsigned char lvIsMultiRegArg : 1; // true if this is a multireg LclVar struct used in an argument context
     unsigned char lvIsMultiRegRet : 1; // true if this is a multireg LclVar struct assigned from a multireg call
 
@@ -4209,7 +4203,6 @@ class Compiler
         CORINFO_CLASS_HANDLE typeHnd;
         bool                 canPromote;
         bool                 containsHoles;
-        bool                 anySignificantPadding;
         bool                 fieldsSorted;
         unsigned char        fieldCnt;
         lvaStructFieldInfo   fields[MAX_NumOfFieldsInPromotableStruct];
@@ -4218,7 +4211,6 @@ class Compiler
             : typeHnd(typeHnd)
             , canPromote(false)
             , containsHoles(false)
-            , anySignificantPadding(false)
             , fieldsSorted(false)
             , fieldCnt(0)
         {

src/coreclr/jit/lclvars.cpp

@@ -2371,9 +2371,15 @@ bool Compiler::StructPromotionHelper::CanPromoteStructType(CORINFO_CLASS_HANDLE
     if (fieldsSize != treeNodes[0].size)
     {
         structPromotionInfo.containsHoles = true;
-    }
 
-    structPromotionInfo.anySignificantPadding = treeNodes[0].hasSignificantPadding && structPromotionInfo.containsHoles;
+        if (treeNodes[0].hasSignificantPadding)
+        {
+            // Struct has significant data not covered by fields we would promote;
+            // this would typically result in dependent promotion, so leave this
+            // struct to physical promotion.
+            return false;
+        }
+    }
 
     // Cool, this struct is promotable.
 
@@ -2716,11 +2722,6 @@ bool Compiler::StructPromotionHelper::ShouldPromoteStructVar(unsigned lclNum)
                 structPromotionInfo.fieldCnt, varDsc->lvFieldAccessed);
         shouldPromote = false;
     }
-    else if (varDsc->lvIsMultiRegRet && structPromotionInfo.anySignificantPadding)
-    {
-        JITDUMP("Not promoting multi-reg returned struct local V%02u with significant padding.\n", lclNum);
-        shouldPromote = false;
-    }
 #if defined(TARGET_LOONGARCH64) || defined(TARGET_RISCV64)
     else if ((structPromotionInfo.fieldCnt == 2) && (varTypeIsFloating(structPromotionInfo.fields[0].fldType) ||
                                                      varTypeIsFloating(structPromotionInfo.fields[1].fldType)))
@@ -2739,13 +2740,8 @@ bool Compiler::StructPromotionHelper::ShouldPromoteStructVar(unsigned lclNum)
         // multiple registers?
         if (compiler->lvaIsMultiregStruct(varDsc, compiler->info.compIsVarArgs))
         {
-            if (structPromotionInfo.anySignificantPadding)
-            {
-                JITDUMP("Not promoting multi-reg struct local V%02u with significant padding.\n", lclNum);
-                shouldPromote = false;
-            }
-            else if ((structPromotionInfo.fieldCnt != 2) &&
-                     !((structPromotionInfo.fieldCnt == 1) && varTypeIsSIMD(structPromotionInfo.fields[0].fldType)))
+            if ((structPromotionInfo.fieldCnt != 2) &&
+                !((structPromotionInfo.fieldCnt == 1) && varTypeIsSIMD(structPromotionInfo.fields[0].fldType)))
             {
                 JITDUMP("Not promoting multireg struct local V%02u, because lvIsParam is true, #fields != 2 and it's "
                         "not a single SIMD.\n",
@@ -2834,11 +2830,10 @@ void Compiler::StructPromotionHelper::PromoteStructVar(unsigned lclNum)
     assert(varDsc->GetLayout()->GetClassHandle() == structPromotionInfo.typeHnd);
     assert(structPromotionInfo.canPromote);
 
-    varDsc->lvFieldCnt              = structPromotionInfo.fieldCnt;
-    varDsc->lvFieldLclStart         = compiler->lvaCount;
-    varDsc->lvPromoted              = true;
-    varDsc->lvContainsHoles         = structPromotionInfo.containsHoles;
-    varDsc->lvAnySignificantPadding = structPromotionInfo.anySignificantPadding;
+    varDsc->lvFieldCnt      = structPromotionInfo.fieldCnt;
+    varDsc->lvFieldLclStart = compiler->lvaCount;
+    varDsc->lvPromoted      = true;
+    varDsc->lvContainsHoles = structPromotionInfo.containsHoles;
 
 #ifdef DEBUG
     // Don't stress this in LCL_FLD stress.

src/coreclr/jit/liveness.cpp

@@ -1024,7 +1024,7 @@ bool Compiler::fgComputeLifeUntrackedLocal(VARSET_TP&           life,
     {
         // Do not consider this store dead if the parent local variable is an address exposed local or
         // if the struct has any significant padding we must retain the value of.
-        return !varDsc.IsAddressExposed() && !varDsc.lvAnySignificantPadding;
+        return !varDsc.IsAddressExposed();
     }
 
     return false;

src/coreclr/jit/morph.cpp

@@ -14728,12 +14728,11 @@ PhaseStatus Compiler::fgRetypeImplicitByRefArgs()
                 }
 
                 // Copy the struct promotion annotations to the new temp.
-                LclVarDsc* newVarDsc               = lvaGetDesc(newLclNum);
-                newVarDsc->lvPromoted              = true;
-                newVarDsc->lvFieldLclStart         = varDsc->lvFieldLclStart;
-                newVarDsc->lvFieldCnt              = varDsc->lvFieldCnt;
-                newVarDsc->lvContainsHoles         = varDsc->lvContainsHoles;
-                newVarDsc->lvAnySignificantPadding = varDsc->lvAnySignificantPadding;
+                LclVarDsc* newVarDsc       = lvaGetDesc(newLclNum);
+                newVarDsc->lvPromoted      = true;
+                newVarDsc->lvFieldLclStart = varDsc->lvFieldLclStart;
+                newVarDsc->lvFieldCnt      = varDsc->lvFieldCnt;
+                newVarDsc->lvContainsHoles = varDsc->lvContainsHoles;
 #ifdef DEBUG
                 newVarDsc->lvKeepType = true;
 #endif // DEBUG

src/coreclr/jit/morphblock.cpp

@@ -359,12 +359,6 @@ void MorphInitBlockHelper::TryInitFieldByField()
         return;
     }
 
-    if (destLclVar->lvAnySignificantPadding)
-    {
-        JITDUMP(" dest has significant padding.\n");
-        return;
-    }
-
     if (m_dstLclOffset != 0)
     {
         JITDUMP(" dest not at a zero offset.\n");
@@ -775,22 +769,6 @@ void MorphCopyBlockHelper::MorphStructCases()
         requiresCopyBlock = true;
     }
 
-    // Can we use field by field copy for the dest?
-    if (m_dstDoFldStore && m_dstVarDsc->lvAnySignificantPadding)
-    {
-        JITDUMP(" dest has significant padding");
-        // C++ style CopyBlock with holes
-        requiresCopyBlock = true;
-    }
-
-    // Can we use field by field copy for the src?
-    if (m_srcDoFldStore && m_srcVarDsc->lvAnySignificantPadding)
-    {
-        JITDUMP(" src has significant padding");
-        // C++ style CopyBlock with holes
-        requiresCopyBlock = true;
-    }
-
 #if defined(TARGET_ARM)
     if (m_store->OperIsIndir() && m_store->AsIndir()->IsUnaligned())
     {