Make high only to arm64

Author: kunalspathak

src/coreclr/jit/lsrabuild.cpp

@@ -2825,20 +2825,26 @@ void LinearScan::buildIntervals()
         availableRegCount = REG_INT_COUNT;
     }
 
+#ifdef HAS_MORE_THAN_64_REGISTERS
     static_assert(sizeof(regMaskTP) == 2 * sizeof(regMaskSmall));
+#else
+    static_assert(sizeof(regMaskTP) == sizeof(regMaskSmall));
+#endif
 
     if (availableRegCount < (sizeof(regMaskSmall) * 8))
     {
         // Mask out the bits that are between (8 * regMaskSmall) ~ availableRegCount
         actualRegistersMask = regMaskTP((1ULL << availableRegCount) - 1);
     }
+#ifdef HAS_MORE_THAN_64_REGISTERS
     else if (availableRegCount < (sizeof(regMaskTP) * 8))
     {
         // Mask out the bits that are between (8 * regMaskTP) ~ availableRegCount
         // Subtract one extra for stack.
         unsigned topRegCount = availableRegCount - sizeof(regMaskSmall) * 8 - 1;
         actualRegistersMask  = regMaskTP(~RBM_NONE, (1ULL << topRegCount) - 1);
     }
+#endif
     else
     {
         actualRegistersMask = regMaskTP(~RBM_NONE, ~RBM_NONE);

src/coreclr/jit/target.h

@@ -229,20 +229,31 @@ typedef uint64_t regMaskSmall;
 #define REG_MASK_ALL_FMT "%016llX"
 #endif
 
+#ifdef TARGET_ARM64
+#define HAS_MORE_THAN_64_REGISTERS 1
+#define MORE_THAN_64_REGISTERS(x)  x
+#else
+#define MORE_THAN_64_REGISTERS(x)
+#endif // TARGET_ARM64
+
 typedef regMaskSmall    SingleTypeRegSet;
 inline SingleTypeRegSet genSingleTypeRegMask(regNumber reg);
 
 struct regMaskTP
 {
 private:
     regMaskSmall low;
+#ifdef HAS_MORE_THAN_64_REGISTERS
     regMaskSmall high;
+#endif
 
 public:
 
     regMaskTP(regMaskSmall lowMask, regMaskSmall highMask)
         : low(lowMask)
+#ifdef HAS_MORE_THAN_64_REGISTERS
         , high(highMask)
+#endif
     {
     }