Re: amd64Translate.Operand.toAMD64Operand: strange Offset: ...

On Fri, Sep 25, 2015 at 2:38 PM, Phil Clayton <[email protected]> wrote:
> I was getting the following error when compiling with 20130715:
>
> amd64Translate.Operand.toAMD64Operand: strange Offset: base: $0x0 index:
> MEM<q>{Locals}[(localWord64)+(0x8)]
>
> Sometimes the message reported "...+(0x0)".  I have reduced this to an
> example which is attached.
>
> In the process of reducing it, I found an error in my code - an exception
> would always be raised.  Presumably that is related to this issue because I
> no longer get this error after fixing my code.

Confirmed with mlton-20130715.  Arguably, it is actually a bug in the
translation to the Machine IL, since compiling with "-type-check true"
gives:

[matthew@shadow mlton_compiler_issue-20150925-1]$ mlton
MLton 20130715 (built Wed Jul 17 03:58:25 EDT 2013 on shadowvm02)
[matthew@shadow mlton_compiler_issue-20150925-1]$ mlton -type-check
true mlton.mlb
invalid operand: XW8 (NULL, RW64(1): Word64, 1, 0): Word8
invalid statement: RW8(0): Word8  = XW8 (NULL, RW64(1): Word64, 1, 0): Word8
invalid block: L_181: {kind = Jump,
      live = (RW64(0): Word64,
      SP(24): Objptr (opt_8),
      SW64(32): Word64,
      SP(48): Objptr (opt_11),
      SP(40): Objptr (opt_10),
      SP(16): Objptr (opt_9)),
      raises = None,
      returns = Some ()}
  RW32(0): Word32  = WordU64_extdToWord32 (RW64(0): Word64)
  RW64(1): Word64  = WordS32_extdToWord64 (RW32(0): Word32)
  RW8(0): Word8  = XW8 (NULL, RW64(1): Word64, 1, 0): Word8
  XW8 (SP(24): Objptr (opt_8), RW64(0): Word64, 1, 0): Word8
   = RW8(0): Word8
  RW64(2): Word64  = Word64_add (0x1, RW64(0): Word64)
  RW64(0): Word64  = RW64(2): Word64
  Goto loop_14
Machine.typeCheck

The Machine IL program has an array offset operation on a manifestly NULL value.

But, it is already fixed in master:

[matthew@shadow mlton_compiler_issue-20150925-1]$
~/devel/mlton/builds/build.gc611cbb/bin/mlton
MLton gc611cbb (built Mon Aug  3 09:19:31 EDT 2015 on shadow)
[matthew@shadow mlton_compiler_issue-20150925-1]$
~/devel/mlton/builds/build.gc611cbb/bin/mlton mlton.mlb

I believe this was fixed by either:
  https://github.com/MLton/mlton/commit/444ca04f3630d90e316b0434e52df6c661a84923
or:
  https://github.com/MLton/mlton/commit/24b7e0bd003f7435225543b2c798da12109d995a
Probably the latter, since the commit message notes that the issue
should only arise in dead code, and you observe that your program
always raises an exception, so that the array offset operation is dead
code.

Actually, looking at you program, it also turns out that we are
missing a reasonable optimization.  MLton is able to constant
propagate the MLton.Pointer.null value through the program, to the
point that it is manifestly the base for an array offset operation.
However, that array offset is guarded by "isNull", but MLton is not
simplifying a "NULL == NULL" comparison; the RSSA IL program (both
mlton-20130715 and mlton-master) has:

      x_63: Word32 = CPointer_equal (NULL: CPointer, NULL: CPointer)

If that had been simplified to "true", then the array offset operation
would have been eliminated as dead code.

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