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

Andrew Savonichev via llvm-dev
Wed, 05 Dec 2018 15:51:50 -0000

> 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.

--
Andrew

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