JIT: removed unneeded throw helper blocks

Various optimizations can happen between the time the throw helper blocks
are first requested (morph) and finally used (codegen). Prune away ones
that aren't needed during the stack level setter.

Fixes dotnet#93948.

src/coreclr/jit/compiler.h

@@ -6295,6 +6295,7 @@ class Compiler
         BasicBlock*     acdDstBlk; // block  to  which we jump
         unsigned        acdData;
         SpecialCodeKind acdKind; // what kind of a special block is this?
+        bool            acdUsed; // do we need to keep this helper block?
 #if !FEATURE_FIXED_OUT_ARGS
         bool     acdStkLvlInit; // has acdStkLvl value been already set?
         unsigned acdStkLvl;     // stack level in stack slots.

src/coreclr/jit/flowgraph.cpp

@@ -3568,6 +3568,10 @@ void Compiler::fgAddCodeRef(BasicBlock* srcBlk, SpecialCodeKind kind)
     add->acdStkLvlInit = false;
 #endif // !FEATURE_FIXED_OUT_ARGS
 
+    // This gets set true in the stack level setter
+    // if there's still a need for this helper
+    add->acdUsed = false;
+
     fgAddCodeList = add;
 
     // Defer creating of the blocks until later.

src/coreclr/jit/stacklevelsetter.cpp

@@ -14,9 +14,9 @@ StackLevelSetter::StackLevelSetter(Compiler* compiler)
     , maxStackLevel(0)
     , memAllocator(compiler->getAllocator(CMK_CallArgs))
     , putArgNumSlots(memAllocator)
+    , throwHelperBlocksUsed(comp->fgUseThrowHelperBlocks() && comp->compUsesThrowHelper)
 #if !FEATURE_FIXED_OUT_ARGS
     , framePointerRequired(compiler->codeGen->isFramePointerRequired())
-    , throwHelperBlocksUsed(comp->fgUseThrowHelperBlocks() && comp->compUsesThrowHelper)
 #endif // !FEATURE_FIXED_OUT_ARGS
 {
     // The constructor reads this value to skip iterations that could set it if it is already set.
@@ -59,8 +59,25 @@ PhaseStatus StackLevelSetter::DoPhase()
     comp->fgSetPtrArgCntMax(maxStackLevel);
     CheckArgCnt();
 
+    // Check if there are any unused throw helper blocks, and if so, remove them.
+    bool madeChanges = false;
+
+    for (Compiler::AddCodeDsc* add = comp->fgGetAdditionalCodeDescriptors(); add != nullptr; add = add->acdNext)
+    {
+        if (add->acdUsed)
+        {
+            continue;
+        }
+
+        BasicBlock* const block = add->acdDstBlk;
+        JITDUMP("Throw help block " FMT_BB " is unused\n", block->bbNum);
+        block->bbFlags &= ~BBF_DONT_REMOVE;
+        comp->fgRemoveBlock(block, /* unreachable */ true);
+        madeChanges = true;
+    }
+
     // Might want an "other" category for things like this...
-    return PhaseStatus::MODIFIED_NOTHING;
+    return madeChanges ? PhaseStatus::MODIFIED_EVERYTHING : PhaseStatus::MODIFIED_NOTHING;
 }
 
 //------------------------------------------------------------------------
@@ -89,16 +106,10 @@ void StackLevelSetter::ProcessBlock(BasicBlock* block)
             SubStackLevel(numSlots);
         }
 
-#if !FEATURE_FIXED_OUT_ARGS
-        // Set throw blocks incoming stack depth for x86.
-        if (throwHelperBlocksUsed && !framePointerRequired)
+        if (throwHelperBlocksUsed && node->OperMayThrow(comp))
         {
-            if (node->OperMayThrow(comp))
-            {
-                SetThrowHelperBlocks(node, block);
-            }
+            SetThrowHelperBlocks(node, block);
         }
-#endif // !FEATURE_FIXED_OUT_ARGS
 
         if (node->IsCall())
         {
@@ -112,7 +123,6 @@ void StackLevelSetter::ProcessBlock(BasicBlock* block)
     assert(currentStackLevel == 0);
 }
 
-#if !FEATURE_FIXED_OUT_ARGS
 //------------------------------------------------------------------------
 // SetThrowHelperBlocks: Set throw helper blocks incoming stack levels targeted
 //                       from the node.
@@ -171,6 +181,11 @@ void StackLevelSetter::SetThrowHelperBlock(SpecialCodeKind kind, BasicBlock* blo
 {
     Compiler::AddCodeDsc* add = comp->fgFindExcptnTarget(kind, comp->bbThrowIndex(block));
     assert(add != nullptr);
+    // We expect we'll actually need this helper.
+    add->acdUsed = true;
+
+#if !FEATURE_FIXED_OUT_ARGS
+
     if (add->acdStkLvlInit)
     {
         // If different range checks happen at different stack levels,
@@ -217,9 +232,8 @@ void StackLevelSetter::SetThrowHelperBlock(SpecialCodeKind kind, BasicBlock* blo
 #endif // Debug
         add->acdStkLvl = currentStackLevel;
     }
-}
-
 #endif // !FEATURE_FIXED_OUT_ARGS
+}
 
 //------------------------------------------------------------------------
 // PopArgumentsFromCall: Calculate the number of stack arguments that are used by the call.

src/coreclr/jit/stacklevelsetter.h

@@ -16,10 +16,8 @@ class StackLevelSetter final : public Phase
 private:
     void ProcessBlock(BasicBlock* block);
 
-#if !FEATURE_FIXED_OUT_ARGS
     void SetThrowHelperBlocks(GenTree* node, BasicBlock* block);
     void SetThrowHelperBlock(SpecialCodeKind kind, BasicBlock* block);
-#endif // !FEATURE_FIXED_OUT_ARGS
 
     unsigned PopArgumentsFromCall(GenTreeCall* call);
     void AddStackLevel(unsigned value);
@@ -35,10 +33,10 @@ class StackLevelSetter final : public Phase
     CompAllocator memAllocator;
 
     typedef JitHashTable<GenTreePutArgStk*, JitPtrKeyFuncs<GenTreePutArgStk>, unsigned> PutArgNumSlotsMap;
-    PutArgNumSlotsMap putArgNumSlots; // The hash table keeps stack slot sizes for active GT_PUTARG_STK nodes.
+    PutArgNumSlotsMap putArgNumSlots;        // The hash table keeps stack slot sizes for active GT_PUTARG_STK nodes.
+    bool              throwHelperBlocksUsed; // Were any throw helper blocks requested for this method.
 
 #if !FEATURE_FIXED_OUT_ARGS
-    bool framePointerRequired;  // Is frame pointer required based on the analysis made by this phase.
-    bool throwHelperBlocksUsed; // Were any throw helper blocks created for this method.
-#endif                          // !FEATURE_FIXED_OUT_ARGS
+    bool framePointerRequired; // Is frame pointer required based on the analysis made by this phase.
+#endif                         // !FEATURE_FIXED_OUT_ARGS
 };