Adding EVEX encoding pathways for emitOutputRRR(). (#75934)

Adding flag to turn on EVEX encoding.

src/coreclr/jit/compiler.cpp

@@ -2290,6 +2290,11 @@ void Compiler::compSetProcessor()
 #ifdef TARGET_XARCH
     if (!compIsForInlining())
     {
+        if (canUseEvexEncoding())
+        {
+            codeGen->GetEmitter()->SetUseEvexEncoding(true);
+            // TODO-XArch-AVX512: Revisit other flags to be set once avx512 instructions are added.
+        }
         if (canUseVexEncoding())
         {
             codeGen->GetEmitter()->SetUseVEXEncoding(true);

src/coreclr/jit/compiler.h

@@ -8937,6 +8937,39 @@ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 #endif
     }
 
+    //------------------------------------------------------------------------
+    // canUseEvexEncoding - Answer the question: Is Evex encoding supported on this target.
+    //
+    // Returns:
+    //    TRUE if Evex encoding is supported, FALSE if not.
+    bool canUseEvexEncoding() const
+    {
+#ifdef TARGET_XARCH
+        return compOpportunisticallyDependsOn(InstructionSet_AVX512F);
+#else
+        return false;
+#endif
+    }
+
+    //------------------------------------------------------------------------
+    // DoJitStressEvexEncoding- Answer the question: Do we force EVEX encoding.
+    //
+    // Returns:
+    //    TRUE if user requests EVEX encoding and it's safe, FALSE if not.
+    bool DoJitStressEvexEncoding() const
+    {
+#ifdef TARGET_XARCH
+        // Using JitStressEVEXEncoding flag will force instructions which would
+        // otherwise use VEX encoding but can be EVEX encoded to use EVEX encoding
+        // This requires AVX512VL support.
+        if (JitConfig.JitStressEVEXEncoding() && compOpportunisticallyDependsOn(InstructionSet_AVX512F_VL))
+        {
+            return true;
+        }
+#endif
+        return false;
+    }
+
     /*
     XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
     XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

src/coreclr/jit/emit.h

@@ -445,6 +445,7 @@ class emitter
 
 #ifdef TARGET_XARCH
         SetUseVEXEncoding(false);
+        SetUseEvexEncoding(false);
 #endif // TARGET_XARCH
 
         emitDataSecCur = nullptr;