Andrew Savonichev via llvm-dev
2018-Nov-29 15:26 UTC
[llvm-dev] AliasAnalysis does not look though a memcpy
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?
--
Andrew
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Andrew Savonichev via llvm-dev
2018-Dec-05 15:51 UTC
[llvm-dev] AliasAnalysis does not look though a memcpy
> 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 -------------------------------------------------------------------- Joint Stock Company Intel A/O Registered legal address: Krylatsky Hills Business Park, 17 Krylatskaya Str., Bldg 4, Moscow 121614, Russian Federation This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies.
Finkel, Hal J. via llvm-dev
2018-Dec-05 17:56 UTC
[llvm-dev] AliasAnalysis does not look though a memcpy
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