On 14/12/2016 11:16, Mike Williams wrote:
On 14/12/2016 08:44, Dominique Pellé wrote:
Yegappan Lakshmanan wrote:
eval.c:4432:10: runtime error: negation of -9223372036854775808 cannot
be represented in type 'varnumber_T' (aka 'long long'); cast to an
unsigned type to negate this value to itself
I see it too.
It's coming from this line in Test_num64() in test_viml.vim:
call assert_equal(-9223372036854775808, 0 / 0)
Running :echo -9223372036854775808
also gives the ubsan error.
viml parser is considering the number 9223372036854775808
(which is not a valid long long) and then negating it, instead
of considering it as number -9223372036854775808
(which is a valid long long). Not sure how it can be fix.
Even though it's undefined behavior, it probably works
on all platforms in practice. The test would probably
fail if it did not work. But it would be good to avoid undefined
behavior.
The usual idiom for writing an integer constant for the largest negative
value 2s complement integer is to negate the largest positive value and
subtract 1. So use (-9223372036854775807 - 1)?
Apologies. That just solves the issue with the test and not the
underlying problem. Caffeine took too long to kick in.
The cause is a combination of things. eval7() parses integers as
unsigned values but uses the signed value returned from vim_str2nr()
before applying any unary minus. In turn vim_str2nr() casts the
unsigned value of the integer to a signed value, so the value it returns
depends on whether the top bit is set from the unsigned calculation.
(Casting is lying to the compiler who will have its revenge one day -
looks like today is that day ;))
VIM doesn't signal (AFAIK) out of range integer values. At the moment,
with the wrapping unsigned integer math in vim_str2nr(), the result for
large integers is unpredictable (for those not concerned with integer
representation) Applying a saturating calculation would at least return
results as close as possible to the original integer value maintaining sign.
So the simplest change would be to have vim_str2nr() saturate and return
9223372036854775807 as the largest positive varnumber_T supported.
eval7() would then apply the unary minus and return -9223372036854775807
You will still need to use -9223372036854775807 - 1 in the original test
to generate the largest negative integer value possible in VIM.
TTFN
Mike
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