no need to search for safepoints when asked only for interruptibility

Author: VSadov

src/coreclr/inc/gcinfotypes.h

@@ -59,7 +59,7 @@ inline UINT32 CeilOfLog2(size_t x)
     return (UINT32)result;
 #else // _MSC_VER
     // LZCNT returns index starting from MSB, whereas BSR gives the index from LSB.
-    // 63 ^ BSR here is equivalent to 63 - BSR since the BSR result is always between 0 and 63.
+    // 63 ^ LZCNT here is equivalent to 63 - LZCNT since the LZCNT result is always between 0 and 63.
     // This saves an instruction, as subtraction from constant requires either MOV/SUB or NEG/ADD.
     return (UINT32)63 ^ (UINT32)__builtin_clzl((unsigned long)x);
 #endif // _MSC_VER

src/coreclr/vm/gcinfodecoder.cpp

@@ -350,6 +350,7 @@ GcInfoDecoder::GcInfoDecoder(
 
 #ifdef PARTIALLY_INTERRUPTIBLE_GC_SUPPORTED
     m_NumSafePoints = (UINT32) DENORMALIZE_NUM_SAFE_POINTS(m_Reader.DecodeVarLengthUnsigned(NUM_SAFE_POINTS_ENCBASE));
+    m_SafePointIndex = m_NumSafePoints;
 #endif
 
     if (slimHeader)
@@ -362,18 +363,14 @@ GcInfoDecoder::GcInfoDecoder(
     }
 
 #ifdef PARTIALLY_INTERRUPTIBLE_GC_SUPPORTED
-    if(flags & (DECODE_INTERRUPTIBILITY | DECODE_GC_LIFETIMES))
+    if(flags & (DECODE_GC_LIFETIMES))
     {
         if(m_NumSafePoints)
         {
             m_SafePointIndex = FindSafePoint(m_InstructionOffset);
         }
-        else
-        {
-            m_SafePointIndex = 0;
-        }
     }
-    else if(flags & DECODE_FOR_RANGES_CALLBACK)
+    else if(flags & (DECODE_FOR_RANGES_CALLBACK | DECODE_INTERRUPTIBILITY))
     {
         // Note that normalization as a code offset can be different than
         //  normalization as code length