JIT: introduce durable EH region ID (#113497)

Give each EH clause a permanent ID unique to that clause. Use this
to revise how we model GT_END_LFIN's nesting depth -- the node now
refers to a region ID, and we use that plus a lookup table built
during `FinalizeEH` to figure out the nesting depth.

Contributes to #108900.

Author: AndyAyersMS

docs/design/coreclr/botr/clr-abi.md

@@ -452,7 +452,7 @@ The code this finally returns to looks like this:
 
 In this case, it zeros out the ShadowSP slot that it previously set to 0xFC, then jumps to the address that is the actual target of the leave from the finally.
 
-The JIT does this "end finally restore" by creating a GT_END_LFIN tree node, with the appropriate stack level as an operand, that generates this code.
+The JIT does this "end finally restore" by creating a GT_END_LFIN tree node, with the appropriate EH region ID as an operand, that generates this code.
 
 In the case of an exceptional 'finally' invocation, the VM sets up the 'return address' to whatever address it wants the JIT to return to.
 
@@ -476,7 +476,7 @@ The VM walks the ShadowSP slots in the function `GetHandlerFrameInfo()`, and set
 
 An aside on the JIT implementation for x86.
 
-The JIT creates BBJ_CALLFINALLY/BBJ_ALWAYS pairs for calling the 'finally' clause. The BBJ_CALLFINALLY block will have a series of CORINFO_JIT_ENDCATCH calls appended at the end, if we need to "leave" a series of nested catches before calling the finally handler (due to a single 'leave' opcode attempting to leave multiple levels of different types of handlers). Then, a GT_END_LFIN statement with the finally clause handler nesting level as an argument is added to the step block where the finally returns to. This is used to generate code to zero out the appropriate level of the ShadowSP slot array after the finally has been executed. The BBJ_CALLFINALLY block itself generates the code to insert the 0xFC value into the ShadowSP slot array. If the 'finally' is invoked by the VM, in exceptional cases, then the VM itself updates the ShadowSP slot array before invoking the 'finally'.
+The JIT creates BBJ_CALLFINALLY/BBJ_ALWAYS pairs for calling the 'finally' clause. The BBJ_CALLFINALLY block will have a series of CORINFO_JIT_ENDCATCH calls appended at the end, if we need to "leave" a series of nested catches before calling the finally handler (due to a single 'leave' opcode attempting to leave multiple levels of different types of handlers). Then, a GT_END_LFIN statement with EH region ID as an argument is added to the step block where the finally returns to. This is used to generate code to zero out the appropriate level of the ShadowSP slot array after the finally has been executed and the final EH nesting depth is known. The BBJ_CALLFINALLY block itself generates the code to insert the 0xFC value into the ShadowSP slot array. If the 'finally' is invoked by the VM, in exceptional cases, then the VM itself updates the ShadowSP slot array before invoking the 'finally'.
 
 At the end of a finally or filter, a GT_RETFILT is inserted. For a finally, this is a TYP_VOID which is just a placeholder. For a filter, it takes an argument which evaluates to the return value from the filter. On legacy JIT, this tree triggers the generation of both the return value load (for filters) and the "funclet" exit sequence, which is either a "pop eax; jmp eax" for a finally, or a "ret" for a filter. When processing the BBJ_EHFINALLYRET or BBJ_EHFILTERRET block itself (at the end of code generation for the block), nothing is generated. In RyuJIT, the GT_RETFILT only loads up the return value (for filters) and does nothing for finally, and the block type processing after all the tree processing triggers the exit sequence to be generated. There is no real difference between these, except to centralize all "exit sequence" generation in the same place.
 

src/coreclr/jit/codegenxarch.cpp

@@ -2200,13 +2200,22 @@ void CodeGen::genCodeForTreeNode(GenTree* treeNode)
 
 #if defined(FEATURE_EH_WINDOWS_X86)
         case GT_END_LFIN:
+        {
+            // Find the eh table entry via the eh ID
+            //
+            unsigned const ehID = (unsigned)treeNode->AsVal()->gtVal1;
+            assert(ehID < compiler->compEHID);
+            assert(compiler->m_EHIDtoEHblkDsc != nullptr);
+
+            EHblkDsc* HBtab = nullptr;
+            bool      found = compiler->m_EHIDtoEHblkDsc->Lookup(ehID, &HBtab);
+            assert(found);
+            assert(HBtab != nullptr);
 
             // Have to clear the ShadowSP of the nesting level which encloses the finally. Generates:
             //     mov dword ptr [ebp-0xC], 0  // for some slot of the ShadowSP local var
-
-            size_t finallyNesting;
-            finallyNesting = treeNode->AsVal()->gtVal1;
-            noway_assert(treeNode->AsVal()->gtVal1 < compiler->compHndBBtabCount);
+            //
+            const size_t finallyNesting = HBtab->ebdHandlerNestingLevel;
             noway_assert(finallyNesting < compiler->compHndBBtabCount);
 
             // The last slot is reserved for ICodeManager::FixContext(ppEndRegion)
@@ -2220,6 +2229,7 @@ void CodeGen::genCodeForTreeNode(GenTree* treeNode)
             GetEmitter()->emitIns_S_I(INS_mov, EA_PTRSIZE, compiler->lvaShadowSPslotsVar, (unsigned)curNestingSlotOffs,
                                       0);
             break;
+        }
 #endif // FEATURE_EH_WINDOWS_X86
 
         case GT_PINVOKE_PROLOG:

src/coreclr/jit/compiler.cpp

@@ -5207,6 +5207,21 @@ void Compiler::FinalizeEH()
         lvaSetVarAddrExposed(lvaShadowSPslotsVar DEBUGARG(AddressExposedReason::EXTERNALLY_VISIBLE_IMPLICITLY));
     }
 
+    // Build up a mapping from EH IDs to EHblkDsc*
+    //
+    assert(m_EHIDtoEHblkDsc == nullptr);
+
+    if (compHndBBtabCount > 0)
+    {
+        m_EHIDtoEHblkDsc = new (getAllocator()) EHIDtoEHblkDscMap(getAllocator());
+
+        for (unsigned XTnum = 0; XTnum < compHndBBtabCount; XTnum++)
+        {
+            EHblkDsc* const HBtab = &compHndBBtab[XTnum];
+            m_EHIDtoEHblkDsc->Set(HBtab->ebdID, HBtab);
+        }
+    }
+
 #endif // FEATURE_EH_WINDOWS_X86
 
     // We should not make any more alterations to the EH table structure.

src/coreclr/jit/compiler.h

@@ -2722,8 +2722,12 @@ class Compiler
     // etc.
     unsigned ehMaxHndNestingCount = 0;
 
+    typedef JitHashTable<unsigned, JitSmallPrimitiveKeyFuncs<unsigned>, EHblkDsc*> EHIDtoEHblkDscMap;
+    EHIDtoEHblkDscMap* m_EHIDtoEHblkDsc = nullptr;
+
 #endif // FEATURE_EH_WINDOWS_X86
 
+    EHblkDsc* ehFindEHblkDscById(unsigned short ehID);
     bool ehTableFinalized = false;
     void FinalizeEH();
 
@@ -10977,9 +10981,10 @@ class Compiler
     size_t compInfoBlkSize;
     BYTE*  compInfoBlkAddr;
 
-    EHblkDsc* compHndBBtab           = nullptr; // array of EH data
-    unsigned  compHndBBtabCount      = 0;       // element count of used elements in EH data array
-    unsigned  compHndBBtabAllocCount = 0;       // element count of allocated elements in EH data array
+    EHblkDsc*      compHndBBtab           = nullptr; // array of EH data
+    unsigned       compHndBBtabCount      = 0;       // element count of used elements in EH data array
+    unsigned       compHndBBtabAllocCount = 0;       // element count of allocated elements in EH data array
+    unsigned short compEHID               = 0;       // unique ID for EH data array entries
 
 #if defined(FEATURE_EH_WINDOWS_X86)
 

src/coreclr/jit/fgbasic.cpp

@@ -3646,6 +3646,7 @@ void Compiler::fgFindBasicBlocks()
             BADCODE3("end of hnd block beyond end of method for try", " at offset %04X", tryBegOff);
         }
 
+        HBtab->ebdID              = impInlineRoot()->compEHID++;
         HBtab->ebdTryBegOffset    = tryBegOff;
         HBtab->ebdTryEndOffset    = tryEndOff;
         HBtab->ebdFilterBegOffset = filterBegOff;

src/coreclr/jit/fgehopt.cpp

@@ -690,7 +690,9 @@ PhaseStatus Compiler::fgRemoveEmptyTry()
         // Handler index of any nested blocks will update when we
         // remove the EH table entry.  Change handler exits to jump to
         // the continuation.  Clear catch type on handler entry.
-        // Decrement nesting level of enclosed GT_END_LFINs.
+        //
+        // GT_END_LFIN no longer need updates here, now their gtVal1 fields refer to EH IDs.
+        //
         for (BasicBlock* const block : Blocks(firstHandlerBlock, lastHandlerBlock))
         {
             if (block == firstHandlerBlock)
@@ -725,25 +727,6 @@ PhaseStatus Compiler::fgRemoveEmptyTry()
                     }
                 }
             }
-
-#if defined(FEATURE_EH_WINDOWS_X86)
-            if (!UsesFunclets())
-            {
-                // If we're in a non-funclet model, decrement the nesting
-                // level of any GT_END_LFIN we find in the handler region,
-                // since we're removing the enclosing handler.
-                for (Statement* const stmt : block->Statements())
-                {
-                    GenTree* expr = stmt->GetRootNode();
-                    if (expr->gtOper == GT_END_LFIN)
-                    {
-                        const size_t nestLevel = expr->AsVal()->gtVal1;
-                        assert(nestLevel > 0);
-                        expr->AsVal()->gtVal1 = nestLevel - 1;
-                    }
-                }
-            }
-#endif // FEATURE_EH_WINDOWS_X86
         }
 
         // (6) Update any impacted ACDs.
@@ -2870,6 +2853,8 @@ BasicBlock* Compiler::fgCloneTryRegion(BasicBlock* tryEntry, CloneTryInfo& info,
         compHndBBtab[XTnum]    = compHndBBtab[originalXTnum];
         EHblkDsc* const ebd    = &compHndBBtab[XTnum];
 
+        ebd->ebdID = impInlineRoot()->compEHID++;
+
         // Note the outermost region enclosing indices stay the same, because the original
         // clause entries got adjusted when we inserted the new clauses.
         //
@@ -3028,6 +3013,22 @@ BasicBlock* Compiler::fgCloneTryRegion(BasicBlock* tryEntry, CloneTryInfo& info,
                 newBlock->bbRefs++;
             }
         }
+
+#if defined(FEATURE_EH_WINDOWS_X86)
+        // Update the EH ID for any cloned GT_END_LFIN.
+        //
+        for (Statement* const stmt : newBlock->Statements())
+        {
+            GenTree* const rootNode = stmt->GetRootNode();
+            if (rootNode->OperIs(GT_END_LFIN))
+            {
+                GenTreeVal* const endNode = rootNode->AsVal();
+                EHblkDsc* const   oldEbd  = ehFindEHblkDscById((unsigned short)endNode->gtVal1);
+                EHblkDsc* const   newEbd  = oldEbd + indexShift;
+                endNode->gtVal1           = newEbd->ebdID;
+            }
+        }
+#endif
     }
     JITDUMP("Done fixing region indices\n");
 

src/coreclr/jit/flowgraph.cpp

@@ -1463,6 +1463,7 @@ void Compiler::fgAddSyncMethodEnterExit()
 
         // Initialize the new entry
 
+        newEntry->ebdID          = impInlineRoot()->compEHID++;
         newEntry->ebdHandlerType = EH_HANDLER_FAULT;
 
         newEntry->ebdTryBeg  = tryBegBB;

src/coreclr/jit/gentree.cpp

@@ -12333,7 +12333,7 @@ void Compiler::gtDispLeaf(GenTree* tree, IndentStack* indentStack)
 
 #if defined(FEATURE_EH_WINDOWS_X86)
         case GT_END_LFIN:
-            printf(" endNstLvl=%d", tree->AsVal()->gtVal1);
+            printf(" ehID=%d", tree->AsVal()->gtVal1);
             break;
 #endif // FEATURE_EH_WINDOWS_X86
 

src/coreclr/jit/importer.cpp

@@ -4747,12 +4747,15 @@ void Compiler::impImportLeaveEHRegions(BasicBlock* block)
             }
 #endif
 
-            unsigned finallyNesting = compHndBBtab[XTnum].ebdHandlerNestingLevel;
-            assert(finallyNesting <= compHndBBtabCount);
+            // We now record the EH region ID on GT_END_LFIN instead of the finally nesting depth,
+            // as the later can change as we optimize the code.
+            //
+            unsigned const ehID = compHndBBtab[XTnum].ebdID;
+            assert(ehID <= impInlineRoot()->compEHID);
 
-            GenTree* endLFin = new (this, GT_END_LFIN) GenTreeVal(GT_END_LFIN, TYP_VOID, finallyNesting);
-            endLFinStmt      = gtNewStmt(endLFin);
-            endCatches       = NULL;
+            GenTree* const endLFin = new (this, GT_END_LFIN) GenTreeVal(GT_END_LFIN, TYP_VOID, ehID);
+            endLFinStmt            = gtNewStmt(endLFin);
+            endCatches             = NULL;
 
             encFinallies++;
         }

src/coreclr/jit/jiteh.cpp

@@ -241,7 +241,7 @@ bool EHblkDsc::ebdIsSameTry(BasicBlock* ebdTryBeg, BasicBlock* ebdTryLast)
 
 void EHblkDsc::DispEntry(unsigned XTnum)
 {
-    printf(" %2u  ::", XTnum);
+    printf(" %2u     %2u  ::", ebdID, XTnum);
 
 #if defined(FEATURE_EH_WINDOWS_X86)
     if (ebdHandlerNestingLevel == 0)
@@ -1258,6 +1258,30 @@ EHblkDsc* Compiler::ehInitTryBlockRange(BasicBlock* blk, BasicBlock** tryBeg, Ba
     return tryTab;
 }
 
+//------------------------------------------------------------------------
+// ehFindEHblkDscById: find an eh table entry by its ID
+//
+// Argument:
+//     ID to use in search
+//
+// Returns:
+//     Pointer to the entry, or nullptr
+//
+EHblkDsc* Compiler::ehFindEHblkDscById(unsigned short id)
+{
+    EHblkDsc* result = nullptr;
+    for (EHblkDsc* const xtab : EHClauses(this))
+    {
+        if (xtab->ebdID == id)
+        {
+            result = xtab;
+            break;
+        }
+    }
+
+    return result;
+}
+
 /*****************************************************************************
  *  This method updates the value of ebdTryBeg
  */
@@ -3230,13 +3254,21 @@ void Compiler::fgVerifyHandlerTab()
     // block (case 3)?
     bool multipleLastBlockNormalizationDone = false; // Currently disabled
 
+    BitVecTraits traits(impInlineRoot()->compEHID, this);
+    BitVec       ids(BitVecOps::MakeEmpty(&traits));
+
     assert(compHndBBtabCount <= compHndBBtabAllocCount);
 
     unsigned  XTnum;
     EHblkDsc* HBtab;
 
     for (XTnum = 0, HBtab = compHndBBtab; XTnum < compHndBBtabCount; XTnum++, HBtab++)
     {
+        // EH IDs should be unique and in range
+        //
+        assert(HBtab->ebdID < impInlineRoot()->compEHID);
+        assert(BitVecOps::TryAddElemD(&traits, ids, HBtab->ebdID));
+
         assert(HBtab->ebdTryBeg != nullptr);
         assert(HBtab->ebdTryLast != nullptr);
         assert(HBtab->ebdHndBeg != nullptr);
@@ -3763,7 +3795,7 @@ void Compiler::fgDispHandlerTab()
         return;
     }
 
-    printf("\nindex  ");
+    printf("\n  id,  index  ");
 #if defined(FEATURE_EH_WINDOWS_X86)
     if (!UsesFunclets())
     {