[llvm-dev] Potential issue with noalias @malloc and @realloc

[Flamedoge via llvm-dev]

>> For now, the only sane solution IMHO, is to say that no alias implies
>> pointer inequality, regardless of the standards. Because the above can
>> occur in any situation noalias exists but they are allowed to be pointer
>> equal (as mentioned, it's trivial to make this happen in TBAA).

>Just to be clear, you're suggesting that we no longer mark malloc's
>return value as noalias?

CMIIW, I believe the suggestion was treat p0 as unequal to p1 even under
runtime check if p0 has NoAlias relation with p1. This will at least make
ie. GVN to treat as if p0 and p1 aren't equal even if bits are, preventing
transformation of p1 to p0, thus preventing reordering. I'm not a huge fan
of this solution, and is quite hacky to GVN and pretty much to all the
transformation passes :(.
The root cause is quite clear; you cannot assume malloc'd ptrs to have
No-Alias without flow-sensitive, "lifetime" analysis.

Kevin

On Tue, Apr 11, 2017 at 6:49 PM, Sanjoy Das via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Daniel,
>
> On April 11, 2017 at 6:22:34 PM, Daniel Berlin (dberlin at dberlin.org)
> wrote:
> > Note: This is a generic problem with any situation where noalias exists
> but
> > the pointers are proven equal :)
>
> Yes.
>
> > TBAA, for example, has the same generic issue, we just drop the tbaa
> > metadata and declare it okay, even though it would have been UB at the
> > source level.
>
> Yes.  I noticed this when talking to Kostya on a thread about TBAA.
> However, I wanted to reduce the "dependencies" of this behavior so I
> did not mention TBAA here.
>
> > On Tue, Apr 11, 2017 at 3:32 PM, Sanjoy Das via llvm-dev <
> > llvm-dev at lists.llvm.org> wrote:
> >
> > > Hi all,
> > >
> > > I think I've spotted a semantic issue with marking @malloc and
> > > @realloc as noalias. Say we have the program:
> > >
> > > int f() {
> > > int* p0 = malloc(sizeof(int));
> > > free(p0);
> > >
> >
> >
> >
> > > int* p1 = malloc(sizeof(int));
> > > if (!p1) return 20;
> > > int value = 0;
> > > for (int i = 0; i < 1; i++) {
> > > *p1 = 20;
> > > value = *p1;
> > > if (false) // "false" is obscured in a way the compiler can't fathom
> > > if (p0 == p1)
> > > a();
> > > else
> > > b();
> > > }
> > > return result;
> > > }
> > >
> > > The program above is well defined, and will always return 20.
> > >
> >
> > no it won't, you never initialized result :) :) :)
>
> Yes!
>
> > > However, if we first unswitch loops:
> > >
> > > int f() {
> > > int* p0 = malloc(size of(int));
> > > free(p0);
> > > int* p1 = malloc(sizeof(int));
> > > if (!p1) return 20;
> > > int value = 0;
> > > if (p0 == p1) {
> > > for (int i = 0; i < 1; i++) {
> > > *p1 = 20;
> > > value = *p1;
> > > if (false)
> > > a();
> > > }
> > > } else {
> > > // Other copy of the loop that calls b() in dead code
> > > }
> > > return result;
> > > }
> > >
> > > and then run GVN::propgateEquality that replaces one use of p1 with p0
> > > but not the other (for some reason, say the other use was obscured
> > > behind a function call):
> > >
> > > int f() {
> > > int* p0 = malloc(size of(int));
> > > free(p0);
> > > int* p1 = malloc(sizeof(int));
> > > if (!p1) return 20;
> > > int value = 0;
> > > if (p0 == p1) {
> > > for (int i = 0; i < 1; i++) {
> > > *p0 = 20; // S0
> > > value = *p1; // L0
> > > if (false)
> > > a();
> > > }
> > > } else {
> > > // Other copy of the loop that calls b() in dead code
> > > }
> > > return result;
> > > }
> > >
> > >
> > > Now we have a problem -- since p0 NoAlias p1 (distinct @malloc()
> > > calls), we can reorder S0 and L0 (or LICM L0):
> > >
> > >
> > > int f() {
> > > int* p0 = malloc(size of(int));
> > > free(p0);
> > > int* p1 = malloc(sizeof(int));
> > > if (!p1) return 20;
> > > int value = 0;
> > > if (p0 == p1) {
> > > for (int i = 0; i < 1; i++) {
> > > value = *p1; // L0
> > > *p0 = 20; // S0
> > > if (false)
> > > a();
> > > }
> > > } else {
> > > // Other copy of the loop that calls b() in dead code
> > > }
> > > return result;
> > > }
> > >
> > > and we'll now return garbage if p0 == p1 (likely) and p1 is not null
> > > (also likely).
> > >
> >
> > The free changes p0.
> >
> > Otherwise, yes, your fundamental problem is that you are playing games
> with
> > two different language level objects whose pointers are equal, but it
> says
> > they do not alias.
> > I don't remember who i tried to explain to on irc that this can happen,
> but
> > they didn't believe me :)
> >
> > We have no way of specifying the lifetime has ended, either.
> > You can make the problem as hard or as simple as you want above, but you
> > can't solve it in general.
> > Even if you wanted to, it is in fact, trivial to come up with cases where
> > basicaa will say no alias for the load/store, but not say noalias at the
> > propagation point.
> >
> > Just put enough copies in between that it hits the various depth or
> > complexity limits and gives up and says mayalias :)
>
> As I was telling David Majnemer on IRC, I'm not annoyed by the fact
> that there is a bug, but by the fact that there is no good way of
> fixing the bug that I can think of.
>
> > It would require semantic changes to llvm ir to fix this to properly
> > express object lifetimes that is compatible with all the random babble
> > standards have written down :)
> > For now, the only sane solution IMHO, is to say that no alias implies
> > pointer inequality, regardless of the standards. Because the above can
> > occur in any situation noalias exists but they are allowed to be pointer
> > equal (as mentioned, it's trivial to make this happen in TBAA).
>
> Just to be clear, you're suggesting that we no longer mark malloc's
> return value as noalias?
>
> > I say sane because you can't actually do anything else and get it right
> all
> > the time, other than disable noalias in pretty much every case.
> >
> > FWIW outside of not propagating when tbaa sets differ, gcc does the same
> as
> > we do .
> > It definitely believes p0 and p1 do not alias in the example above, and
> > would have no trouble propagating them.
> > I can't quite get it to optimize it. but it does set the equality p0 ==
> p1,
> > and staring at the pass, it just doesn't bother to simplify the
> expression,
> > it would, in fact, otherwise propagate.
>
> I'll give writing an actual C++ program that demonstrates the problem
> this weekend, but unfortunately these things tend to be time
> consuming.
>
> -- Sanjoy
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