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

[Finkel, Hal J. via llvm-dev]

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? 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. Can you say more about the use case?

Thanks,

Hal

>
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory