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

Andrew Savonichev via llvm-dev
Wed, 05 Dec 2018 20:14:32 -0000

> 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.

> 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.

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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