JIT: boost inlining for methods that may return small arrays (#114806)

Look for inlinees that may be allocating and returning small fixed sized arrays.
When inlined, these array allocations may end up non-escaping and be stack allocated.

This analysis is approximate; we can't tell for sure in the IL scan what the array
size is, and we can't easily tell if the allocated array is actually returned.

Contributes to #113236

Author: AndyAyersMS

src/coreclr/jit/fgbasic.cpp

@@ -915,12 +915,14 @@ void Compiler::fgFindJumpTargets(const BYTE* codeAddr, IL_OFFSET codeSize, Fixed
     var_types   varType      = DUMMY_INIT(TYP_UNDEF); // TYP_ type
     bool        typeIsNormed = false;
     FgStack     pushedStack;
-    const bool  isForceInline = (info.compFlags & CORINFO_FLG_FORCEINLINE) != 0;
-    const bool  isInlining    = compIsForInlining();
-    unsigned    retBlocks     = 0;
-    int         prefixFlags   = 0;
-    bool        preciseScan   = makeInlineObservations && compInlineResult->GetPolicy()->RequiresPreciseScan();
-    const bool  resolveTokens = preciseScan;
+    const bool  isForceInline       = (info.compFlags & CORINFO_FLG_FORCEINLINE) != 0;
+    const bool  isInlining          = compIsForInlining();
+    unsigned    retBlocks           = 0;
+    int         prefixFlags         = 0;
+    bool        preciseScan         = makeInlineObservations && compInlineResult->GetPolicy()->RequiresPreciseScan();
+    const bool  resolveTokens       = preciseScan;
+    bool        isReturnsArrayKnown = false;
+    bool        returnsArray        = false;
 
     // Track offsets where IL instructions begin in DEBUG builds. Used to
     // validate debug info generated by the JIT.
@@ -2441,6 +2443,36 @@ void Compiler::fgFindJumpTargets(const BYTE* codeAddr, IL_OFFSET codeSize, Fixed
                 retBlocks++;
                 break;
 
+            case CEE_NEWARR:
+
+                if (makeInlineObservations)
+                {
+                    if (!isReturnsArrayKnown)
+                    {
+                        if (info.compRetType == TYP_REF)
+                        {
+                            CORINFO_CLASS_HANDLE retClass = info.compMethodInfo->args.retTypeClass;
+                            if (retClass != NO_CLASS_HANDLE)
+                            {
+                                uint32_t retClassAttribs = info.compCompHnd->getClassAttribs(retClass);
+                                returnsArray             = (retClassAttribs & CORINFO_FLG_ARRAY) != 0;
+                            }
+                        }
+                        isReturnsArrayKnown = true;
+                    }
+
+                    if (returnsArray && pushedStack.IsStackAtLeastOneDeep())
+                    {
+                        FgStack::FgSlot slot0 = pushedStack.GetSlot0();
+
+                        if (FgStack::IsConstantOrConstArg(slot0, impInlineInfo))
+                        {
+                            compInlineResult->Note(InlineObservation::CALLEE_MAY_RETURN_SMALL_ARRAY);
+                        }
+                    }
+                }
+                break;
+
             default:
                 break;
         }

src/coreclr/jit/inline.def

@@ -101,6 +101,7 @@ INLINE_OBSERVATION(IS_SIZE_DECREASING_INLINE, bool,   "size decreasing inline",
 INLINE_OBSERVATION(LOG_REPLAY_ACCEPT,         bool,   "accepted by log replay",               INFORMATION, CALLEE)
 INLINE_OBSERVATION(LOOKS_LIKE_WRAPPER,        bool,   "thin wrapper around a call",           INFORMATION, CALLEE)
 INLINE_OBSERVATION(MAXSTACK,                  int,    "maxstack",                             INFORMATION, CALLEE)
+INLINE_OBSERVATION(MAY_RETURN_SMALL_ARRAY,    bool,   "may return a small new array",         INFORMATION, CALLEE)
 INLINE_OBSERVATION(OPCODE,                    int,    "next opcode in IL stream",             INFORMATION, CALLEE)
 INLINE_OBSERVATION(OPCODE_NORMED,             int,    "next opcode in IL stream",             INFORMATION, CALLEE)
 INLINE_OBSERVATION(NUMBER_OF_ARGUMENTS,       int,    "number of arguments",                  INFORMATION, CALLEE)

src/coreclr/jit/inlinepolicy.cpp

@@ -1341,6 +1341,10 @@ void ExtendedDefaultPolicy::NoteBool(InlineObservation obs, bool value)
             m_ArgUnboxExact++;
             break;
 
+        case InlineObservation::CALLEE_MAY_RETURN_SMALL_ARRAY:
+            m_MayReturnSmallArray = true;
+            break;
+
         default:
             DefaultPolicy::NoteBool(obs, value);
             break;
@@ -1776,6 +1780,12 @@ double ExtendedDefaultPolicy::DetermineMultiplier()
         }
     }
 
+    if (m_MayReturnSmallArray)
+    {
+        multiplier += 4.0;
+        JITDUMP("\nInline candidate may return small known-size array.  Multiplier increased to %g.", multiplier);
+    }
+
     if (m_HasProfileWeights)
     {
         // There are cases when Profile Data can be misleading or polluted:
@@ -1889,6 +1899,7 @@ void ExtendedDefaultPolicy::OnDumpXml(FILE* file, unsigned indent) const
     XATTR_B(m_IsCallsiteInNoReturnRegion)
     XATTR_B(m_HasProfileWeights)
     XATTR_B(m_InsideThrowBlock)
+    XATTR_B(m_MayReturnSmallArray)
 }
 #endif
 

src/coreclr/jit/inlinepolicy.h

@@ -226,6 +226,7 @@ class ExtendedDefaultPolicy : public DefaultPolicy
         , m_NonGenericCallsGeneric(false)
         , m_IsCallsiteInNoReturnRegion(false)
         , m_HasProfileWeights(false)
+        , m_MayReturnSmallArray(false)
     {
         // Empty
     }
@@ -281,6 +282,7 @@ class ExtendedDefaultPolicy : public DefaultPolicy
     bool     m_NonGenericCallsGeneric     : 1;
     bool     m_IsCallsiteInNoReturnRegion : 1;
     bool     m_HasProfileWeights          : 1;
+    bool     m_MayReturnSmallArray        : 1;
 };
 
 // DiscretionaryPolicy is a variant of the default policy.  It