[llvm-dev] Why is remalloc not marked as noalias?

[Hal Finkel via llvm-dev]

Hi, Bhatu,

I agree. The return from realloc is always a new allocation (in C's 
model, even if the same pointer is returned).

The core issue is that we don't generally add the noalias return 
attribute to malloc in a specific way, but rely on the system headers 
marking malloc with __attribute__((malloc)). The GCC docs explain why 
realloc is not marked with that attribute 
(https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Common-Function-Attributes.html#Common-Function-Attributes), 
but it has to do with a property of the GCC attribute that we don't 
capture with our noalias return attribute (that the memory contains no 
live pointers to other objects).

That having been said, the relevant property of being allocation-like 
should already be captured by LLVM's analysis:

> /// \brief Tests if a value is a call or invoke to a function that 
> returns a
> /// NoAlias pointer (including malloc/calloc/realloc/strdup-like 
> functions).
> bool llvm::isNoAliasFn(...

it recognized realloc and several other functions in this regard even if 
they're not marked with noalias. The core problem, however, is that our 
AA intrastructure is looking at the noalias attribute directly (instead 
of calling isNoAliasFn). Please feel free to submit a patch. 
Specifically, in lib/Analysis/AliasAnalysis.cpp, we have:

> bool llvm::isNoAliasCall(const Value *V) {
>   if (auto CS = ImmutableCallSite(V))
>     return CS.hasRetAttr(Attribute::NoAlias);
>   return false;
> }

We should remove the current implementation of llvm::isNoAliasCall, and 
rename llvm::isNoAliasFn to llvm::isNoAliasCall (llvm::isNoAliasFn has 
only one caller, in MemoryDependenceAnalysis, so just update that call 
site). Then update some relevant test case (e.g., 
test/Analysis/BasicAA/2008-11-23-NoaliasRet.ll). If you can't submit a 
patch, please open a bug report (and tag it with the beginner keyword).

  -Hal

On 12/26/2017 04:13 AM, Bhatu via llvm-dev wrote:
> Hello,
>
> According to my understanding, it seems that the result of realloc 
> will not technically alias the original pointer. When the realloc is 
> done in-place the reference 
> <http://en.cppreference.com/w/c/memory/realloc> says:
> "The original pointer ptr is invalidated and any access to it is 
> undefined behavior (even if reallocation was in-place)."
>
> Additionally from theC11 standard 
> <https://port70.net/%7Ensz/c/c11/n1570.html#6.2.4p2> we have:
> "The value of a pointer becomes indeterminate when the object it 
> points to (or just past) reaches the end of its lifetime"
>
> Is this enough to infer that we can safely mark realloc as noalias?
>
> -- 
> Regards
> Bhatu
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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