[llvm-dev] AliasAnalysis does not look though a memcpy

[Andrew Savonichev via llvm-dev]

Finkel, Hal J. writes:
> On 12/6/18 8:33 AM, Andrew Savonichev wrote:
>> Finkel, Hal J. writes:
>>> On 12/5/18 2:14 PM, Andrew Savonichev wrote:
>>>>> On 12/5/18 9:51 AM, Andrew Savonichev via llvm-dev wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> I'm trying to get AA results for two pointers, but it seems that AA
>>>>>>> cannot look though a memcpy. For example:
>>>>>>>
>>>>>>>     define dso_local spir_func void @fun() {
>>>>>>>     entry:
>>>>>>>       ; Store an address of `var'
>>>>>>>       %var = alloca i32, align 4
>>>>>>>       store i32 42, i32* %var, align 4
>>>>>>>       %var.addr = alloca i32*, align 8
>>>>>>>       store i32* %var, i32** %var.addr, align 8
>>>>>>>
>>>>>>>       ; Memcpy `var.addr' to `var.addr.tmp'
>>>>>>>       %var.addr.tmp = alloca i32*, align 8
>>>>>>>       %0 = bitcast i32** %var.addr.tmp to i8*
>>>>>>>       %1 = bitcast i32** %var.addr to i8*
>>>>>>>       call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 8 %0, i8* align 8 %1, i64 8, i1 false)
>>>>>>>
>>>>>>>       ; Load a copy of `var'
>>>>>>>       %var.tmp = load i32*, i32** %var.addr.tmp
>>>>>>>       %should.be.42 = load i32, i32* %var.tmp
>>>>>>>       ret void
>>>>>>>     }
>>>>>>>
>>>>>>>     ; Function Attrs: argmemonly nounwind
>>>>>>>     declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i1) #1
>>>>>>>     attributes #1 = { argmemonly nounwind }
>>>>>>>
>>>>>>> I run it with opt, and get the following:
>>>>>>>
>>>>>>>     $ opt -basicaa -print-alias-sets memcpy.ll -disable-output
>>>>>>>     Alias sets for function 'fun':
>>>>>>>     Alias Set Tracker: 3 alias sets for 6 pointer values.
>>>>>>>       AliasSet[0x5b5df0, 2] may alias, Mod/Ref   Pointers: (i32* %var, LocationSize::precise(4)), (i32* %var.tmp, LocationSize::precise(4))
>>>>>>>       AliasSet[0x5b5e90, 2] must alias, Mod/Ref   Pointers: (i32** %var.addr, LocationSize::precise(8)), (i8* %1, LocationSize::precise(8))
>>>>>>>       AliasSet[0x5b7390, 2] must alias, Mod/Ref   Pointers: (i8* %0, LocationSize::precise(8)), (i32** %var.addr.tmp, LocationSize::precise(8))
>>>>>>>
>>>>>>> So AA says that %var and %var.tmp "may alias", but I'd expect to get a
>>>>>>> "must alias". What can be done to determine that %var and %var.tmp are
>>>>>>> actually the same pointer in this case?
>>>>>> Can anyone suggest how to get the expected results from AA?
>>>>>>
>>>>>> InstCombiner and other LLVM passes can probably remove a memcpy and make
>>>>>> IR a lot easier for AA, but I'd like to get the analysis working for
>>>>>> not-optimized IR as well.
>>>>> Hi, Andrew,
>>>>>
>>>>> To be clear, you'd like BasicAA to look back, not just trough the
>>>>> memcpy, but also through the loads and stores of the addresses?
>>>>>
>>>> Yes, exactly. I was mostly concerned about memcpy, but you're right,
>>>> loads/stores of addresses seems to have the same issue.
>>> We've not generally seen a need to put this kind of logic into BasicAA,
>>> and even if we were motivated to do so, we'd need to be very careful
>>> about compile-time impact.
>>>
>> Understood. I assume that the best way to ensure that is to add this
>> logic into a new AA.
>>
>>>>> BasicAA currently doesn't do any of that under the presumption that,
>>>>> once SROA, InstCombine, etc. do promotion and simplification, AA will
>>>>> be able to understand the rest.
>>>> That is what I thought. Thanks for the confirmation.
>>>>
>>>>> Can you say more about the use case?
>>>> OpenCL C has a notion of Generic Address Space (GAS), allowing you to
>>>> cast a pointer from any (named) address space to a GAS pointer. Then you
>>>> can use this GAS pointer instead of a named AS pointer. Compiler is
>>>> responsible to infer the original address space of a GAS pointer when it
>>>> is actually used (for load/store), otherwise this is a compilation
>>>> error.
>>> That seems scary :-) -- Can this inference not be done syntactically in
>>> Clang?
>>>
>> From a frontend perspective, a GAS pointer is just a pointer:
>>
>>   void foo(int *p, int *q) { *p = 42; *q = 43; };
>>
>> Until Clang reaches a call site, it has no idea about real address
>> spaces of `p' and `q'. When we do reach a call site, `foo()' can already
>> be CodeGen'ed, so we can't really change anything.
>
> Is this supposed to work like template instantiation? Are you guaranteed
> to only get one (unique) set of address spaces for the function
> arguments?
>

Yes, just like in C++ template, if `foo' is called with different sets
of address spaces, a compiler have to create different function
instantiations for each set.

>
> We can change the order that functions are emitted in Clang if necessary.
>

I haven't thought this is actually configurable. I'd really appreciate
if you can give me a pointer on how to do this.

>>>> So I'm trying to follow a pointer from a point of origin
>>>> (i.e. addrspacecast private ptr -> generic ptr) and look through a
>>>> sequence of bitcasts, GEPs, function calls and pointer copies through
>>>> memory. AliasAnalysis and MemDepAnalysis give me all the required
>>>> information to do intraprocedural analysis, except for the memcpy case
>>>> that I raised in this thread.
>>>>
>>>> Since a failure to infer an address space means a compilation error, I'm
>>>> trying to get this analysis working for debug builds, where we do not
>>>> run InstCombine and other aggressive optimizations.
>>>>

-- 
Andrew

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