[TableGen] Allow 2^63-1 and 2^63-2 as int literals.

[simon_tatham]

Summary

These two values correspond to the 'Empty' and 'Tombstone' special
keys defined by DenseMapInfo<int64_t>, which means that neither one
can be used as a key in DenseMap<int64_t, anything>. Hence, if you try
to use either of those values as an int literal, IntInit::get() fails
an assertion when it tries to insert them into its static cache of
int-literal objects.

A simple workaround is to detect those two special values in advance
and cache them separately from the DenseMap.

[efriedma]

Instead of adding special cases for the tombstone, can we use some generic map that doesn't require user-defined tombstones, like std::map?

[simon_tatham]

I'm happy to do that if you prefer – I left the DenseMap in place on the Chesterton's Fence principle. (I didn't know why it was originally chosen over std::map, so hesitated to unilaterally reverse the decision.)

[simon_tatham]

Simpler version that just replaces DenseMap with std::map.

[efriedma]

TableGen is not very performance sensitive; it's fine to just do the simple thing here.

DenseMap is generally faster than std::map both because it's a hashtable and because it allocates the keys and values inline... but clearly it's not a good idea to use it when it produces wrong results. Longer-term, we should probably come up with some alternative that has performance like a DenseMap, but doesn't misbehave for cases like this.

llvm/lib/TableGen/Record.cpp

 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/SMLoc.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Record.h"
 #include <cassert>
 #include <cstdint>
 #include <memory>
+#include <map>
 #include <string>
 #include <utility>
 #include <vector>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "tblgen-records"
 
Init *BitsInit::resolveReferences(Resolver &R) const {
 
   if (Changed)
     return BitsInit::get(NewBits);
 
   return const_cast<BitsInit *>(this);
 }
 
 IntInit *IntInit::get(int64_t V) {
-  static DenseMap<int64_t, IntInit*> ThePool;
+  static std::map<int64_t, IntInit*> ThePool;
 
   IntInit *&I = ThePool[V];
   if (!I) I = new(Allocator) IntInit(V);
   return I;
 }
 
 std::string IntInit::getAsString() const {
   return itostr(Value);

llvm/test/TableGen/IntSpecialValues.td

+// RUN: llvm-tblgen %s | FileCheck %s
+
+def TestRecord {
+  // CHECK: int X = 9223372036854775807;
+  int X = 0x7FFFFFFFFFFFFFFF;
+  // CHECK: int Y = 9223372036854775806;
+  int Y = 0x7FFFFFFFFFFFFFFE;
+}