Add "Inline candidate has const arg that feeds string.Length." heuristic

src/coreclr/src/jit/flowgraph.cpp

@@ -4171,6 +4171,10 @@ class FgStack
     {
         Push(SLOT_ARRAYLEN);
     }
+    void PushStringLen()
+    {
+        Push(SLOT_STRINGLEN);
+    }
     void PushArgument(unsigned arg)
     {
         Push(SLOT_ARGUMENT + arg);
@@ -4193,6 +4197,10 @@ class FgStack
     {
         return value == SLOT_ARRAYLEN;
     }
+    static bool IsStringLen(unsigned value)
+    {
+        return value == SLOT_STRINGLEN;
+    }
     static bool IsArgument(unsigned value)
     {
         return value >= SLOT_ARGUMENT;
@@ -4222,7 +4230,8 @@ class FgStack
         SLOT_UNKNOWN  = 0,
         SLOT_CONSTANT = 1,
         SLOT_ARRAYLEN = 2,
-        SLOT_ARGUMENT = 3
+        SLOT_STRINGLEN = 3,
+        SLOT_ARGUMENT = 4,
     };
 
     void Push(int type)
@@ -4479,6 +4488,31 @@ void Compiler::fgFindJumpTargets(const BYTE* codeAddr, IL_OFFSET codeSize, Fixed
                         compInlineResult->Note(InlineObservation::CALLEE_LOOKS_LIKE_WRAPPER);
                     }
                 }
+
+                if (isInlining && makeInlineObservations && opcode == CEE_CALLVIRT)
+                {
+                    CORINFO_RESOLVED_TOKEN resolvedToken;
+                    impResolveToken(codeAddr, &resolvedToken, CORINFO_TOKENKIND_Method);
+
+                    CORINFO_CALL_INFO corinfo;
+                    info.compCompHnd->getCallInfo(&resolvedToken, nullptr, info.compMethodHnd, CORINFO_CALLINFO_CALLVIRT, &corinfo);
+
+                    // ugly hack to check if this CEE_CALLVIRT is actually `string.get_Length`
+                    if (corinfo.hMethod != nullptr &&
+                        corinfo.hMethod != (CORINFO_METHOD_HANDLE)0xcccccccccccccccc) //isInvalidHandle
+                    {
+                        const char* className;
+                        const char* methodName = eeGetMethodName(corinfo.hMethod, &className);
+                        if (!strcmp("System.String", className) && !strcmp("get_Length", methodName))
+                        {
+                            if (makeInlineObservations)
+                            {
+                                pushedStack.PushStringLen();
+                            }
+                        }
+                    }
+                }
+
             }
             break;
 
@@ -5150,6 +5184,15 @@ void Compiler::fgObserveInlineConstants(OPCODE opcode, const FgStack& stack, boo
         compInlineResult->Note(InlineObservation::CALLEE_ARG_FEEDS_RANGE_CHECK);
     }
 
+    // Const string feeds String.Length
+    if ((FgStack::IsConstant(slot0) && FgStack::IsStringLen(slot1)) ||
+        (FgStack::IsConstant(slot1) && FgStack::IsStringLen(slot0)))
+    {
+        compInlineResult->Note(InlineObservation::CALLSITE_CONSTANT_ARG_FEEDS_STRLEN);
+        // TODO: check `oper` and `slot3` for const to apply the observation only const tests, e.g.:
+        // `cnsStr.Length > 0`  (CEE_CEQ, CEE_CGT, etc..)
+    }
+
     // Check for an incoming arg that's a constant
     if (isInlining)
     {

src/coreclr/src/jit/inline.def

@@ -165,6 +165,7 @@ INLINE_OBSERVATION(RARE_GC_STRUCT,            bool,   "rarely called, has gc str
 // ------ Call Site Information -------
 
 INLINE_OBSERVATION(CONSTANT_ARG_FEEDS_TEST,   bool,   "constant argument feeds test",  INFORMATION, CALLSITE)
+INLINE_OBSERVATION(CONSTANT_ARG_FEEDS_STRLEN, bool,   "constant argument feeds StrLen",INFORMATION, CALLSITE)
 INLINE_OBSERVATION(DEPTH,                     int,    "depth",                         INFORMATION, CALLSITE)
 INLINE_OBSERVATION(FREQUENCY,                 int,    "rough call site frequency",     INFORMATION, CALLSITE)
 INLINE_OBSERVATION(IN_LOOP,                   bool,   "call site is in a loop",        INFORMATION, CALLSITE)

src/coreclr/src/jit/inlinepolicy.cpp

@@ -315,6 +315,13 @@ void DefaultPolicy::NoteBool(InlineObservation obs, bool value)
                 m_ConstantArgFeedsConstantTest++;
                 break;
 
+            case InlineObservation::CALLSITE_CONSTANT_ARG_FEEDS_STRLEN:
+                // We shouldn't see this for a prejit root since
+                // we don't know anything about callers.
+                assert(!m_IsPrejitRoot);
+                m_ConstantArgFeedsStrLen++;
+                break;
+
             case InlineObservation::CALLEE_BEGIN_OPCODE_SCAN:
             {
                 // Set up the state machine, if this inline is
@@ -658,6 +665,13 @@ double DefaultPolicy::DetermineMultiplier()
         multiplier += 3.0;
         JITDUMP("\nInline candidate has const arg that feeds a conditional.  Multiplier increased to %g.", multiplier);
     }
+
+    if (m_ConstantArgFeedsStrLen > 0)
+    {
+        multiplier += 7.0; 
+        JITDUMP("\nInline candidate has const arg that feeds string.Length.  Multiplier increased to %g.", multiplier);
+    }
+
     // For prejit roots we do not see the call sites. To be suitably optimistic
     // assume that call sites may pass constants.
     else if (m_IsPrejitRoot && ((m_ArgFeedsConstantTest > 0) || (m_ArgFeedsTest > 0)))
@@ -1729,6 +1743,7 @@ void DiscretionaryPolicy::EstimateCodeSize()
          -0.238 * m_IsInstanceCtor +
          -5.357 * m_IsFromPromotableValueClass +
          -7.901 * (m_ConstantArgFeedsConstantTest > 0 ? 1 : 0)  +
+         -7.901 * (m_ConstantArgFeedsStrLen > 0 ? 1 : 0) +
           0.065 * m_CalleeNativeSizeEstimate;
     // clang-format on
 
@@ -1930,6 +1945,7 @@ void DiscretionaryPolicy::DumpData(FILE* file) const
     fprintf(file, ",%u", m_MethodIsMostlyLoadStore ? 1 : 0);
     fprintf(file, ",%u", m_ArgFeedsRangeCheck);
     fprintf(file, ",%u", m_ConstantArgFeedsConstantTest);
+    fprintf(file, ",%u", m_ConstantArgFeedsStrLen);
     fprintf(file, ",%d", m_CalleeNativeSizeEstimate);
     fprintf(file, ",%d", m_CallsiteNativeSizeEstimate);
     fprintf(file, ",%d", m_ModelCodeSizeEstimate);

src/coreclr/src/jit/inlinepolicy.h

@@ -95,6 +95,7 @@ class DefaultPolicy : public LegalPolicy
         , m_ArgFeedsConstantTest(0)
         , m_ArgFeedsRangeCheck(0)
         , m_ConstantArgFeedsConstantTest(0)
+        , m_ConstantArgFeedsStrLen(0)
         , m_CalleeNativeSizeEstimate(0)
         , m_CallsiteNativeSizeEstimate(0)
         , m_IsForceInline(false)
@@ -160,6 +161,7 @@ class DefaultPolicy : public LegalPolicy
     unsigned                m_ArgFeedsConstantTest;
     unsigned                m_ArgFeedsRangeCheck;
     unsigned                m_ConstantArgFeedsConstantTest;
+    unsigned                m_ConstantArgFeedsStrLen;
     int                     m_CalleeNativeSizeEstimate;
     int                     m_CallsiteNativeSizeEstimate;
     bool                    m_IsForceInline : 1;