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

[Nuno Lopes via llvm-commits]

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