JIT: boost inlining when callee unboxes an arg

Especially so if the caller is passing an exact type. This may lead
to the JIT being able to stack allocate the box and promote the underlying
payload.

Fixes dotnet#104479

src/coreclr/jit/fgbasic.cpp

@@ -1106,6 +1106,24 @@ void Compiler::fgFindJumpTargets(const BYTE* codeAddr, IL_OFFSET codeSize, Fixed
                 break;
             }
 
+            case CEE_UNBOX:
+            case CEE_UNBOX_ANY:
+            {
+                if (makeInlineObservations)
+                {
+                    FgStack::FgSlot slot = pushedStack.Top();
+                    if (FgStack::IsExactArgument(slot, impInlineInfo))
+                    {
+                        compInlineResult->Note(InlineObservation::CALLSITE_UNBOX_EXACT_ARG);
+                    }
+                    else if (FgStack::IsArgument(slot))
+                    {
+                        compInlineResult->Note(InlineObservation::CALLEE_UNBOX_ARG);
+                    }
+                }
+                break;
+            }
+
             case CEE_CASTCLASS:
             case CEE_ISINST:
             {

src/coreclr/jit/inline.def

@@ -106,6 +106,7 @@ INLINE_OBSERVATION(NUMBER_OF_ARGUMENTS,       int,    "number of arguments",
 INLINE_OBSERVATION(NUMBER_OF_BASIC_BLOCKS,    int,    "number of basic blocks",               INFORMATION, CALLEE)
 INLINE_OBSERVATION(NUMBER_OF_LOCALS,          int,    "number of locals",                     INFORMATION, CALLEE)
 INLINE_OBSERVATION(RANDOM_ACCEPT,             bool,   "random accept",                        INFORMATION, CALLEE)
+INLINE_OBSERVATION(UNBOX_ARG,                 int,    "callee unboxes arg",                   INFORMATION, CALLEE)
 INLINE_OBSERVATION(UNSUPPORTED_OPCODE,        bool,   "unsupported opcode",                   INFORMATION, CALLEE)
 
 // ------ Caller Correctness -------
@@ -194,6 +195,7 @@ INLINE_OBSERVATION(LOG_REPLAY_ACCEPT,         bool,   "accepted by log replay",
 INLINE_OBSERVATION(PROFILE_FREQUENCY,         double, "frequency from profile data",          INFORMATION, CALLSITE)
 INLINE_OBSERVATION(RANDOM_ACCEPT,             bool,   "random accept",                        INFORMATION, CALLSITE)
 INLINE_OBSERVATION(WEIGHT,                    int,    "frequency from block weight",          INFORMATION, CALLSITE)
+INLINE_OBSERVATION(UNBOX_EXACT_ARG,           int,    "unbox of arg with exact class",        INFORMATION, CALLSITE)
 
 // ------ Final Sentinel -------
 

src/coreclr/jit/inlinepolicy.cpp

@@ -1362,6 +1362,14 @@ void ExtendedDefaultPolicy::NoteBool(InlineObservation obs, bool value)
             m_IsCallsiteInNoReturnRegion = value;
             break;
 
+        case InlineObservation::CALLEE_UNBOX_ARG:
+            m_ArgUnbox++;
+            break;
+
+        case InlineObservation::CALLSITE_UNBOX_EXACT_ARG:
+            m_ArgUnboxExact++;
+            break;
+
         default:
             DefaultPolicy::NoteBool(obs, value);
             break;
@@ -1716,6 +1724,30 @@ double ExtendedDefaultPolicy::DetermineMultiplier()
         JITDUMP("\nPrejit root candidate has arg that feeds a conditional.  Multiplier increased to %g.", multiplier);
     }
 
+    if (m_ArgUnboxExact > 0)
+    {
+        // Callee has unbox(arg), caller supplies exact type (a box)
+        // We can likely optimize
+        multiplier += 4.0;
+        JITDUMP("\nInline candidate has %d exact arg unboxes.  Multiplier increased to %g.", m_ArgUnboxExact,
+                multiplier);
+    }
+
+    if (m_ArgUnbox > 0)
+    {
+        // Callee has unbox(arg), caller arg not known type
+        if (m_IsPrejitRoot)
+        {
+            // Assume these might be met with exact type args
+            multiplier += 4.0;
+        }
+        else
+        {
+            multiplier += 1.0;
+        }
+        JITDUMP("\nInline candidate has %d arg unboxes.  Multiplier increased to %g.", m_ArgUnboxExact, multiplier);
+    }
+
     switch (m_CallsiteFrequency)
     {
         case InlineCallsiteFrequency::RARE:

src/coreclr/jit/inlinepolicy.h

@@ -219,6 +219,8 @@ class ExtendedDefaultPolicy : public DefaultPolicy
         , m_UnrollableMemop(0)
         , m_Switch(0)
         , m_DivByCns(0)
+        , m_ArgUnbox(0)
+        , m_ArgUnboxExact(0)
         , m_ReturnsStructByValue(false)
         , m_IsFromValueClass(false)
         , m_NonGenericCallsGeneric(false)
@@ -272,6 +274,8 @@ class ExtendedDefaultPolicy : public DefaultPolicy
     unsigned m_UnrollableMemop;
     unsigned m_Switch;
     unsigned m_DivByCns;
+    unsigned m_ArgUnbox;
+    unsigned m_ArgUnboxExact;
     bool     m_ReturnsStructByValue       : 1;
     bool     m_IsFromValueClass           : 1;
     bool     m_NonGenericCallsGeneric     : 1;