564117 - TM: negative index check for GETELEM on 64-bit platforms is bogus?

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Nicholas Nethercote [inactive]]

Attachment: patch

denseArrayElement() has this check before getting an array element:

    /* Guard against negative index */
    if (MAX_DSLOTS_LENGTH > MAX_DSLOTS_LENGTH32 && !idx_ins->isImmI()) {
        /* Only 64-bit machines support large enough arrays for this. */
        JS_ASSERT(sizeof(jsval) == 8);
        guard(false,
              lir->ins2ImmI(LIR_lt, idx_ins, 0),
              exit);
    }

AFAICT this is totally bogus.  idx_ins is a uint32 (on both 32-bit and 64-bit platforms).  By doing a signed comparison of idx_ins against 0, all that will happen is that any index with value 2^31 (0x80000000) or higher will cause an unnecessary side-exit.

Furthermore, the !idx_ins->isImmI() test doesn't make any sense, and the maximum array size is 2^32-1 regardless of the machine word size, AIUI.

dvander agrees that it looks bogus and that it was probably written before the 64-bit TM port was working.

The attached patch removes it.  AFAICT Andreas wrote the code so I'm asking him to review.  I've been thinking about this for a while and am now royally confused, esp. how a negative index is actually handled, so input from others is welcome.

Comment 1

[Nicholas Nethercote [inactive]]

One month review ping coming up:

  PING!

Comment 2

[Andreas Gal :gal]

ooops

Comment 3

[Nicholas Nethercote [inactive]]

http://hg.mozilla.org/tracemonkey/rev/a9cdb738b187

Comment 4

[Robert Sayre]

http://hg.mozilla.org/mozilla-central/rev/a9cdb738b187