HashSetFn's calculation of maxSize can cause Overflow during functor instantiation

[Skyb0rg007]

Version

110.99.3 (Latest)

Operating System

• Any
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OS Version

No response

Processor

• Any
• Arm (using Rosetta)
• PowerPC
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• x86 (32-bit)
• x86-64 (64-bit)
• Other

System Component

SML/NJ Library

Severity

Major

Description

The HashSetFn functor (hash-set-fn.sml) calculates maxSize as the largest power of 2 that is still less than Array.maxLen.
However, the calculation does not properly handle systems in which Int.maxInt = SOME Array.maxLen, causing an exception to be raised during functor instantiation.

Note: This bug does not occur on the SML/NJ compiler, since Array.maxLen is much smaller than Int.maxInt.
The issue is reproducible for MLton (latest version: 20230523-gd082c4a36), since there SOME Array.maxLen = Int.maxInt.

Transcript

(* test.sml *)
structure X = HashSetFn(
   struct
      type hash_key = int
      val hashVal = Word.fromInt
      val sameKey: int * int -> bool = op =
   end)
val () = print "Okay!\n"


(* test.mlb *)
$(SML_LIB)/basis/basis.mlb
$(SML_LIB)/smlnj-lib/Util/smlnj-lib.mlb
test.sml


$ mlton -output a.out test.mlb
$ ./a.out
unhandled exception: Overflow


$ # The `-default-type intinf` flag causes Int.int to have infinite precision, but keeps Array.maxLen the same
$ mlton -output a.out -default-type intinf test.mlb
$ ./a.out
Okay!

Expected Behavior

Instantiating the HashSetFn should never raise Overflow.

Steps to Reproduce

Just run the code from the transcript.

Additional Information

This can be fixed by modifying the code at line 37 in hash-set-fn.sml:

(* old *)   
val maxSize = let
     fun f i = let
             val i' = i+i
             in
               if i' < Array.maxLen then f i' else i
             end
     in
       f 0x10000
     end

(* new*)   
val maxSize = let
     fun f i = let
             val i' = i+i
             in
               if i' < Array.maxLen then f i' else i
             end
             handle Overflow => i
     in
       f 0x10000
     end

Email address

ssoss AT uchicago DOT edu

[MatthewFluet]

MLton patches the SML/NJ Library that it ships to avoid a similar problem in structure HashTableRep (see https://github.com/MLton/mlton/blob/b1f1f0f0916d28c0d183fba85549d5bf96f1fa41/lib/smlnj-lib/smlnj-lib.patch#L4498-L4510).

[Skyb0rg007]

I'll resubmit this report there. However I do still believe this should be addressed in the SML/NJ repo, if for nothing but future-proofing against a possible Array.maxLen increase or Int.maxInt decrease.

[JohnReppy]

I've pushed a fix that computes floor(log2(Array.maxSize)) to use as the maxSize limit for hash tables.

[MatthewFluet]

This computation of floor(log2(Array.maxSize)) is not robust when structure Word = Word32 and structure Int = IntInf; in general, if the default word size is smaller than the default integer size.

$ cat z.sml 
val maxSize = let
      fun lp (0w0, k) = Word.toIntX(Word.<<(0w1, k-0w1))
        | lp (w, k) = lp (Word.>>(w, 0w1), k+0w1)
      in
        lp (Word.fromInt Array.maxLen, 0w0)
      end

val _ = print (concat ["maxSize: ", Int.toString maxSize, "\n"])
$ mlton -default-type intinf z.sml 
$ ./z 
maxSize: ~2147483648

[JohnReppy]

Rewrote the code to just use the Int module, so it should be portable now.

[MatthewFluet]

The max-hash-table-size.sml file was not added in the commit.

[JohnReppy]

I missed it in the initial commit, but I added it about 4 hours ago.