Undefined Behavior in lib/msun/src/e_pow.c (was Re: New math library from ARM)
pfg at FreeBSD.org
Tue Jan 1 15:01:50 UTC 2019
On 1/1/19 1:20 AM, Pedro Giffuni wrote:
> On 1/1/19 12:52 AM, Steve Kargl wrote:
>> On Tue, Jan 01, 2019 at 12:14:38AM -0500, Pedro Giffuni wrote:
>>>> I'll defer to Bruce on this. My only comments are
>>>> 1) declarations belong at the top of the file where all
>>>> declarations occur
>>> Modern standards let you declare variables within blocks. For style we
>>> do prefer to start the function with them but in this case we can't.
>> I'am aware of what modern standards allow. I'm also
>> aware of the code style of fdlibm, which I think
>> should be maintain.
> Well, the alternative is the musl-libc patch, which declares the
> variables upon use (twice).
> I think my patch is a little bit more acceptable.
Actually, perhaps a cast is just enough ...
See below ...
>>>> 2) j is already declared as int32_t
>>> And it has to be a signed integer: we later check for negative values.
>> Yeah, I know. That's why I pointed out the collision.
>>>> 3) uint32_t should be written as u_int32_t.
>>> No, uint32_t is the standard type, u_int32_t is a BSDism. Also, the
>>> consistently uses uint32_t.
>> Ah, no. e_pow.c uses u_int32_t on line 107.
> Duh! I completely misread, sorry about that I probably was still
> looking at musl-libc instead.
> Yes, we should use u_int32_t.
>> This would be easy
>> to see if all declarations are grouped together. In additional,
>> don't you need to include stdint.h to get uint32_t?
> Well, my patch did compile, but I will match the existing style if
> the change is accepted/desired.
>> % grep uint32_t /usr/src/lib/msun/src/e_pow.c
>> % grep u_int32_t /usr/src/lib/msun/src/e_pow.c
>> u_int32_t lx,ly;
>> n = ((u_int32_t)hx>>31)-1;
>> Code churn to placate lint for an event that cannot occur
>> seems dubious to me.
> UBsan should be able to detect it. I think Undefined Behavior is
> considered a portability bug.
> There is no urgency but it is still a bug.
In the new patch: the first j comparison has to be made against an
unsigned value anyways and the second comparison will never be against
the magical 31 so there is no need to cast that one.
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