[llvm-dev] AliasAnalysis does not look though a memcpy (original) (raw)

Andrew Savonichev via llvm-dev llvm-dev at lists.llvm.org
Wed Dec 5 07:51:50 PST 2018

Hi,

I'm trying to get AA results for two pointers, but it seems that AA cannot look though a memcpy. For example: define dsolocal spirfunc 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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