Coverity has found undefined behavior in dtoa.c:d2b(). lo0bits() can return 32 which z >>= 32, where z is an uint32. I've talked to doku at PyCon. He suggested to update dtoa.c to a more recent version. Our copy is based on a version from 2001. There are more modern versions available, e.g. https://searchcode.com/codesearch/view/52748288/ from 2006.

CID 1202735 (#1 of 1): Bad bit shift operation (BAD_SHIFT)
large_shift: In expression z >>= k, right shifting by more than 31 bits has undefined behavior. The shift amount, k, is 32.

I'm pretty sure that our code was based on something rather more recent than 2001: it was the most recent version available at the time (around 2008?), and it incorporates subsequent fixes from David Gay.

Please don't replace our dtoa.c with a current version: ours has diverged from the original, and includes fixes that aren't available upstream.

Looking more closely, the report doesn't make sense to me:  `k` is the return value from a call to `lo0bits`.  From the source of `lo0bits`, I don't see any way that `k` can be 32: it's always going to be in the range [0, 31].  Christian: do you have any more information from Coverity?  This looks like a false positive to me.

Ah, sorry; I see it.  Fix on the way.

Okay, so after looking more closely, this *still* looks like a false positive:  `lo0bits` *can* return 32, but only for an input of zero.  In the code in question, we're doing `k = lo0bits(&y)`, so the only way we can get a `k` of `32` is if `y = 0`.  But the whole thing is inside an "if" block that looks like `if ((y = word1(d))) { ... }` (yep, completely with the extra parentheses and the misleading equality-test-lookalike assignment), so that `if` block won't be executed if `y` is zero.

I edited the code to print out debugging information if `k` is ever 32 at that point, and saw no output.  So I don't think that line ever gets executed with `k = 32`.

> saw no output

Bah; missed a bit.  I saw no output when running the Python test suite, that is.  That's not definitive, of course.

You could be right. I didn't track all paths manually. All this bit shifting is making my head dizzy... :)

Anyways I have sent you an invite for Coverity, so you can check the result yourself. The Python test suite passes with assert(k < 32); inside the problematic block, too.