JIT: enable removal of try/catch if the try can't throw. (#110273)

If no tree in the try region of a try/catch can throw, then
we can remove the try and delete the catch.

If no tree in the try region of a try/finally can throw, we can
remove the try and inline the finally. This slightly generalizes
the empty-try/finally opt we have been doing for a long time.
(We should do something similar for try/fault, but don't, yet).

Since these optimization passes are cheap, and opportunities
for them arise after other optimizations and unblock subsequent
optimizations, run them early, middle, and late.

Resolves #107191.

I expect we'll see more of these cases in the future, say if
we unblock cloning of loops with EH.

Author: AndyAyersMS

src/coreclr/jit/compiler.cpp

@@ -4637,10 +4637,14 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     DoPhase(this, PHASE_SWIFT_ERROR_RET, &Compiler::fgAddSwiftErrorReturns);
 #endif // SWIFT_SUPPORT
 
-    // Remove empty try regions
+    // Remove empty try regions (try/finally)
     //
     DoPhase(this, PHASE_EMPTY_TRY, &Compiler::fgRemoveEmptyTry);
 
+    // Remove empty try regions (try/catch)
+    //
+    DoPhase(this, PHASE_EMPTY_TRY_CATCH, &Compiler::fgRemoveEmptyTryCatch);
+
     // Remove empty finally regions
     //
     DoPhase(this, PHASE_EMPTY_FINALLY, &Compiler::fgRemoveEmptyFinally);
@@ -4810,6 +4814,18 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
         //
         DoPhase(this, PHASE_UNROLL_LOOPS, &Compiler::optUnrollLoops);
 
+        // Try again to remove empty try finally/fault clauses
+        //
+        DoPhase(this, PHASE_EMPTY_FINALLY_2, &Compiler::fgRemoveEmptyFinally);
+
+        // Remove empty try regions (try/finally)
+        //
+        DoPhase(this, PHASE_EMPTY_TRY_2, &Compiler::fgRemoveEmptyTry);
+
+        // Remove empty try regions (try/catch)
+        //
+        DoPhase(this, PHASE_EMPTY_TRY_CATCH_2, &Compiler::fgRemoveEmptyTryCatch);
+
         // Compute dominators and exceptional entry blocks
         //
         DoPhase(this, PHASE_COMPUTE_DOMINATORS, &Compiler::fgComputeDominators);
@@ -5039,7 +5055,16 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
 #endif
 
     // Try again to remove empty try finally/fault clauses
-    DoPhase(this, PHASE_EMPTY_FINALLY_2, &Compiler::fgRemoveEmptyFinally);
+    //
+    DoPhase(this, PHASE_EMPTY_FINALLY_3, &Compiler::fgRemoveEmptyFinally);
+
+    // Remove empty try regions (try/finally)
+    //
+    DoPhase(this, PHASE_EMPTY_TRY_3, &Compiler::fgRemoveEmptyTry);
+
+    // Remove empty try regions (try/catch)
+    //
+    DoPhase(this, PHASE_EMPTY_TRY_CATCH_3, &Compiler::fgRemoveEmptyTryCatch);
 
     if (UsesFunclets())
     {

src/coreclr/jit/compiler.h

@@ -5270,7 +5270,6 @@ class Compiler
 
     bool fgModified = false;             // True if the flow graph has been modified recently
     bool fgPredsComputed = false;        // Have we computed the bbPreds list
-    bool fgOptimizedFinally = false;     // Did we optimize any try-finallys?
 
     bool fgHasSwitch = false; // any BBJ_SWITCH jumps?
 
@@ -5352,12 +5351,16 @@ class Compiler
 
     PhaseStatus fgRemoveEmptyTry();
 
+    PhaseStatus fgRemoveEmptyTryCatch();
+
     PhaseStatus fgRemoveEmptyFinally();
 
     PhaseStatus fgMergeFinallyChains();
 
     PhaseStatus fgCloneFinally();
 
+    void fgUpdateACDsBeforeEHTableEntryRemoval(unsigned XTnum);
+
     void fgCleanupContinuation(BasicBlock* continuation);
 
     PhaseStatus fgTailMergeThrows();

src/coreclr/jit/compphases.h

@@ -37,6 +37,7 @@ CompPhaseNameMacro(PHASE_MORPH_ADD_INTERNAL,         "Morph - Add internal block
 CompPhaseNameMacro(PHASE_SWIFT_ERROR_RET,            "Add Swift error returns",        false, -1, true)
 CompPhaseNameMacro(PHASE_ALLOCATE_OBJECTS,           "Allocate Objects",               false, -1, false)
 CompPhaseNameMacro(PHASE_EMPTY_TRY,                  "Remove empty try",               false, -1, false)
+CompPhaseNameMacro(PHASE_EMPTY_TRY_CATCH,            "Remove empty try/catch",         false, -1, false)
 CompPhaseNameMacro(PHASE_EMPTY_FINALLY,              "Remove empty finally",           false, -1, false)
 CompPhaseNameMacro(PHASE_MERGE_FINALLY_CHAINS,       "Merge callfinally chains",       false, -1, false)
 CompPhaseNameMacro(PHASE_CLONE_FINALLY,              "Clone finally",                  false, -1, false)
@@ -74,6 +75,9 @@ CompPhaseNameMacro(PHASE_FIND_LOOPS,                 "Find loops",
 CompPhaseNameMacro(PHASE_CLONE_LOOPS,                "Clone loops",                    false, -1, false)
 CompPhaseNameMacro(PHASE_UNROLL_LOOPS,               "Unroll loops",                   false, -1, false)
 CompPhaseNameMacro(PHASE_MORPH_MDARR,                "Morph array ops",                false, -1, false)
+CompPhaseNameMacro(PHASE_EMPTY_FINALLY_2,            "Remove empty finally 2",         false, -1, false)
+CompPhaseNameMacro(PHASE_EMPTY_TRY_2,                "Remove empty try 2",             false, -1, false)
+CompPhaseNameMacro(PHASE_EMPTY_TRY_CATCH_2,          "Remove empty try-catch 2",       false, -1, false)
 CompPhaseNameMacro(PHASE_HOIST_LOOP_CODE,            "Hoist loop code",                false, -1, false)
 CompPhaseNameMacro(PHASE_MARK_LOCAL_VARS,            "Mark local vars",                false, -1, false)
 CompPhaseNameMacro(PHASE_OPTIMIZE_BOOLS,             "Optimize bools",                 false, -1, false)
@@ -96,7 +100,9 @@ CompPhaseNameMacro(PHASE_OPTIMIZE_BRANCHES,          "Redundant branch opts",
 CompPhaseNameMacro(PHASE_ASSERTION_PROP_MAIN,        "Assertion prop",                 false, -1, false)
 CompPhaseNameMacro(PHASE_IF_CONVERSION,              "If conversion",                  false, -1, false)
 CompPhaseNameMacro(PHASE_VN_BASED_DEAD_STORE_REMOVAL,"VN-based dead store removal",    false, -1, false)
-CompPhaseNameMacro(PHASE_EMPTY_FINALLY_2,            "Remove empty finally 2",         false, -1, false)
+CompPhaseNameMacro(PHASE_EMPTY_FINALLY_3,            "Remove empty finally 3",         false, -1, false)
+CompPhaseNameMacro(PHASE_EMPTY_TRY_3,                "Remove empty try 3",             false, -1, false)
+CompPhaseNameMacro(PHASE_EMPTY_TRY_CATCH_3,          "Remove empty try-catch 3",       false, -1, false)
 CompPhaseNameMacro(PHASE_OPT_UPDATE_FLOW_GRAPH,      "Update flow graph opt pass",     false, -1, false)
 CompPhaseNameMacro(PHASE_STRESS_SPLIT_TREE,          "Stress gtSplitTree",             false, -1, false)
 CompPhaseNameMacro(PHASE_EXPAND_RTLOOKUPS,           "Expand runtime lookups",         false, -1, true)