[bigint] Set max BigInt size to 1M bits

We can bump it up later if there is need.

Taking digit size into account for the max length in digits
makes the max length in bits consistent across architectures.

Bug: v8:6791
Change-Id: Id5205d2d308bbe6a38f8ff1ee19c082bedbeb74a
Reviewed-on: https://chromium-review.googlesource.com/717176
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48514}

src/objects/bigint.h

@@ -70,8 +70,8 @@ class BigInt : public HeapObject {
   // The maximum length that the current implementation supports would be
   // kMaxInt / kDigitBits. However, we use a lower limit for now, because
   // raising it later is easier than lowering it.
-  static const int kMaxLengthBits = 20;
-  static const int kMaxLength = (1 << kMaxLengthBits) - 1;
+  // Support up to 1 million bits.
+  static const int kMaxLength = 1024 * 1024 / (kPointerSize * kBitsPerByte);
 
   class BodyDescriptor;
 
@@ -84,6 +84,8 @@ class BigInt : public HeapObject {
   static const int kDigitBits = kDigitSize * kBitsPerByte;
   static const int kHalfDigitBits = kDigitBits / 2;
   static const digit_t kHalfDigitMask = (1ull << kHalfDigitBits) - 1;
+  // kMaxLength definition assumes this:
+  STATIC_ASSERT(kDigitSize == kPointerSize);
 
   // Private helpers for public methods.
   static Handle<BigInt> Copy(Handle<BigInt> source);
@@ -164,6 +166,8 @@ class BigInt : public HeapObject {
     return static_cast<digit_t>(~x) == 0;
   }
 
+  static const int kMaxLengthBits = 20;
+  STATIC_ASSERT(kMaxLength <= ((1 << kMaxLengthBits) - 1));
   class LengthBits : public BitField<int, 0, kMaxLengthBits> {};
   class SignBits : public BitField<bool, LengthBits::kNext, 1> {};