[llvm] r321309 - [ModRefInfo] Add must alias info to ModRefInfo.

[Nuno Lopes via llvm-commits]

You're right :)
Interpreting Must as NoModRef, now reading the code makes sense.  Having 
NoModRef as the bottom of the lattice is sufficient, of course. I don't know 
what the heck I was thinking of..

I must confess I still don't understand exactly the purpose or meaning of 
Must and the explanation in the enum ModRefInfo isn't very helpful.

BTW, right now we can already end up with Must in all those functions that 
do intersection in at least 2 cases: 1) a buggy AA result, leading to 2 
different results from different AAs, and 2) the program has UB which is 
determinized to different results by different AAs.

Thanks,
Nuno

-----Original Message----- 
From: Alina Sbirlea
Sent: Monday, December 25, 2017 11:05 PM
To: Nuno Lopes
Cc: llvm-commits
Subject: Re: [llvm] r321309 - [ModRefInfo] Add must alias info to 
ModRefInfo.


On Sun, Dec 24, 2017 at 4:55 AM, Nuno Lopes <nunoplopes at sapo.pt> wrote:
Ah, this was my misunderstanding; the picture of the lattice I sent before 
was wrong..

Ok, now assuming I understand the meaning of each value, I'll claim the 
lattice now has 3 bottom values: NoModRef, MustRef, MustMod. These 3 are the 
most precise values and intersection of these with any other value cannot 
yield a more precision value (well, assuming there's no UB that could lead 
to some ⊥ value, which we don't have).


Hmm, I haven't thought about this a lot, but I was intending to have only 
NoModRef as the bottom of the lattice.
>From MustRef and MustMod you could in theory go to Must, which will 
"default" to NoModRef (meaning, I'm going in the code and re-setting that 
Must bit to get NoModRef if we reach Must).
The reason for not having MustRef or MustMod as bottoms, is that, say you 
have a call with only one argument, A, so you know there's Must alias with 
A. So you have MustModRef. Then say you find the call cannot write, you'll 
get MustRef. Then say you find the call does not read either, you're left 
with Must. (As I mentioned above this has to be reset to NoModRef).

In this patch already committed I removed most (all?) of the processing for 
calls, and I have a follow-up patch that addresses precisely the above.
In this follow-up it's pretty obvious you can end up with Must, and you have 
to rest that to NoModRef as the single lattice bottom.

I send the follow-up patch out after the holidays.

Let me know what you think in the mean time :)


This function doesn't seem right to me:
inline bool isNoModRef(const ModRefInfo MRI) {
return (MRI & MustModRef) == Must;
}

There's only one value representing NoModRef, so shouldn't this function 
just do a simple comparison "MRI == NoModRef"?


This function is then used here (and in other similar places):

ModRefInfo AAResults::getArgModRefInfo(ImmutableCallSite CS, unsigned 
ArgIdx) {
ModRefInfo Result = ModRefInfo::ModRef;

for (const auto &AA : AAs) {
   Result = intersectModRef(Result, AA->getArgModRefInfo(CS, ArgIdx));

   // Early-exit the moment we reach the bottom of the lattice.
   if (isNoModRef(Result))
     return Result;
}


NoModRef is only one of the bottom lattice values. I think that's not the 
correct function to call here. I think you need that check if the value is a 
power of 2 or 0 (just in case of UB):
inline bool isBottomModRef(const ModRefInfo MRI) {
return MRI & (MRI-1) == 0;
}


Have a nice Christmas! :)


Thanks, you too! :)

Best,
Alina

Nuno


-----Original Message----- From: Alina Sbirlea
Sent: Friday, December 22, 2017 9:19 PM
To: Nuno Lopes
Cc: llvm-commits
Subject: Re: [llvm] r321309 - [ModRefInfo] Add must alias info to 
ModRefInfo.



Hi Nuno,

The definition for setting Must is whether must alias with the location was 
found and there is no alias with other locations.
Yes, it's possible a write does not occur for cmpxchg, and that's fine. 
ModRef means it may read and it may write, but it doesn't necessarily mean 
it will. MustModRef means it may read and it may write, and if so, there is 
a must alias with that location and with no other locations. It doesn't mean 
it must read and write to that location.


The opposite scenario: if, say, you knew for sure statement S writes to A, 
and may write to B, then Must will not be set, because of B. So you get Mod.
Whereas if S *may* write to A and accesses nothing else, then you get 
MustMod.


Does this make sense?

Please let me know if I'm missing something obvious here or whether I should 
update the documentation to make this clearer.

Thanks,
Alina



On Fri, Dec 22, 2017 at 8:10 AM, Nuno Lopes <nunoplopes at sapo.pt> wrote:
Hi Alina,

Some comments below; I think some operations cannot be marked as MustModRef:


@@ -440,9 +479,17 @@ ModRefInfo AAResults::getModRefInfo(cons
  if (isStrongerThanMonotonic(CX->getSuccessOrdering()))
    return ModRefInfo::ModRef;

-  // If the cmpxchg address does not alias the location, it does not access 
it.
-  if (Loc.Ptr && !alias(MemoryLocation::get(CX), Loc))
-    return ModRefInfo::NoModRef;
+  if (Loc.Ptr) {
+    AliasResult AR = alias(MemoryLocation::get(CX), Loc);
+    // If the cmpxchg address does not alias the location, it does not 
access
+    // it.
+    if (AR == NoAlias)
+      return ModRefInfo::NoModRef;
+
+    // If the cmpxchg address aliases the pointer as must alias, set Must.
+    if (AR == MustAlias)
+      return ModRefInfo::MustModRef;
+  }

According to the manual (http://llvm.org/docs/LangRef.html#id205), cmpxchg 
may or may not write to the location, depending on whether the comparison 
succeeds. For weak operations, the write may not occur even if the 
comparison succeeds.


@@ -453,9 +500,17 @@ ModRefInfo AAResults::getModRefInfo(cons
  if (isStrongerThanMonotonic(RMW->getOrdering()))
    return ModRefInfo::ModRef;

-  // If the atomicrmw address does not alias the location, it does not 
access it.
-  if (Loc.Ptr && !alias(MemoryLocation::get(RMW), Loc))
-    return ModRefInfo::NoModRef;
+  if (Loc.Ptr) {
+    AliasResult AR = alias(MemoryLocation::get(RMW), Loc);
+    // If the atomicrmw address does not alias the location, it does not 
access
+    // it.
+    if (AR == NoAlias)
+      return ModRefInfo::NoModRef;
+
+    // If the atomicrmw address aliases the pointer as must alias, set 
Must.
+    if (AR == MustAlias)
+      return ModRefInfo::MustModRef;
+  }


According to the manual (http://llvm.org/docs/LangRef.html#id210) not all 
'atomicrmw' operations read the pointer. The instruction always writes, but 
may not read (depending on the underlying operation).

Nuno