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

Andrew Savonichev via llvm-dev llvm-dev at lists.llvm.org
Thu Dec 6 06:33:00 PST 2018

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.

>> 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.


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