Hayrapetyan, Anahit via llvm-dev
2018-Aug-09 13:00 UTC
[llvm-dev] llvm MemorySSA def-use chains
Hi,
I have a question about how llvm MemorySSA works, as seems I misunderstand
something.
Consider following code snippet and corresponding IR with MemorySSA annotations
(got with opt -print-memoryssa)
void foo(int* b) {
int a = 0;
int d = 12;
if (b) {
a = 42;
d = 32;
}
int c = a;
int e = d;
}
; Function Attrs: noinline nounwind optnone uwtable
define void @foo(i32* %b) #0 {
entry:
%b.addr = alloca i32*, align 8
%a = alloca i32, align 4
%d = alloca i32, align 4
%c = alloca i32, align 4
%e = alloca i32, align 4
; 1 = MemoryDef(liveOnEntry)
store i32* %b, i32** %b.addr, align 8
; 2 = MemoryDef(1)
store i32 0, i32* %a, align 4
; 3 = MemoryDef(2)
store i32 12, i32* %d, align 4
; MemoryUse(1)
%0 = load i32*, i32** %b.addr, align 8
%tobool = icmp ne i32* %0, null
br i1 %tobool, label %if.then, label %if.end
if.then: ; preds = %entry
; 4 = MemoryDef(3)
store i32 42, i32* %a, align 4
; 5 = MemoryDef(4)
store i32 32, i32* %d, align 4
br label %if.end
if.end: ; preds = %if.then, %entry
; 9 = MemoryPhi({entry,3},{if.then,5})
; MemoryUse(9)
%1 = load i32, i32* %a, align 4
; 6 = MemoryDef(9)
store i32 %1, i32* %c, align 4
; MemoryUse(9)
%2 = load i32, i32* %d, align 4
; 7 = MemoryDef(6)
store i32 %2, i32* %e, align 4
ret void
}
Tracking back def-use chain for `%1 = load i32, i32* %a, align 4` first I get `9
= MemoryPhi({entry,3},{if.then,5})` and then as definitions MemoryDef(2) which
corresponds to `store i32 12, i32* %d, align 4` and MemoryDef(4) corresponding
to `store i32 32, i32* %d, align 4`. However neither of these stores define
`a`, they both define `d`. What I was expecting to get was MemoryDef(3) and
MemoryDef(1), which are real definitions for `a`.
Could you please explain what's the reason MemorySSA gives incorrect
definitions, and is there any way to get the correct ones.
Thanks.
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Alexandros Lamprineas via llvm-dev
2018-Aug-09 13:37 UTC
[llvm-dev] llvm MemorySSA def-use chains
Hi,
My understanding is that MemorySSA doesn't know whether memory locations can
alias or not.
Alexandros
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Hayrapetyan, Anahit via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Thursday, August 9, 2018 2:00:14 PM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] llvm MemorySSA def-use chains
Hi,
I have a question about how llvm MemorySSA works, as seems I misunderstand
something.
Consider following code snippet and corresponding IR with MemorySSA annotations
(got with opt -print-memoryssa)
void foo(int* b) {
int a = 0;
int d = 12;
if (b) {
a = 42;
d = 32;
}
int c = a;
int e = d;
}
; Function Attrs: noinline nounwind optnone uwtable
define void @foo(i32* %b) #0 {
entry:
%b.addr = alloca i32*, align 8
%a = alloca i32, align 4
%d = alloca i32, align 4
%c = alloca i32, align 4
%e = alloca i32, align 4
; 1 = MemoryDef(liveOnEntry)
store i32* %b, i32** %b.addr, align 8
; 2 = MemoryDef(1)
store i32 0, i32* %a, align 4
; 3 = MemoryDef(2)
store i32 12, i32* %d, align 4
; MemoryUse(1)
%0 = load i32*, i32** %b.addr, align 8
%tobool = icmp ne i32* %0, null
br i1 %tobool, label %if.then, label %if.end
if.then: ; preds = %entry
; 4 = MemoryDef(3)
store i32 42, i32* %a, align 4
; 5 = MemoryDef(4)
store i32 32, i32* %d, align 4
br label %if.end
if.end: ; preds = %if.then, %entry
; 9 = MemoryPhi({entry,3},{if.then,5})
; MemoryUse(9)
%1 = load i32, i32* %a, align 4
; 6 = MemoryDef(9)
store i32 %1, i32* %c, align 4
; MemoryUse(9)
%2 = load i32, i32* %d, align 4
; 7 = MemoryDef(6)
store i32 %2, i32* %e, align 4
ret void
}
Tracking back def-use chain for `%1 = load i32, i32* %a, align 4` first I get `9
= MemoryPhi({entry,3},{if.then,5})` and then as definitions MemoryDef(2) which
corresponds to `store i32 12, i32* %d, align 4` and MemoryDef(4) corresponding
to `store i32 32, i32* %d, align 4`. However neither of these stores define
`a`, they both define `d`. What I was expecting to get was MemoryDef(3) and
MemoryDef(1), which are real definitions for `a`.
Could you please explain what's the reason MemorySSA gives incorrect
definitions, and is there any way to get the correct ones.
Thanks.
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Hi, try adding some alias analysis to the pipeline: see examples in llvm/test/Analysis/MemorySSA/*.ll for example assume.ll has ; RUN: opt -aa-pipeline=basic-aa -passes='print<memoryssa>,verify<memoryssa>' without basic-aa the memory ssa will safely assume that the stores to %a and %d alias which may be proven disjoint by one of the alias analyses. On Thu, Aug 9, 2018 at 8:37 AM Alexandros Lamprineas via llvm-dev <llvm-dev at lists.llvm.org> wrote:> > Hi, > > > My understanding is that MemorySSA doesn't know whether memory locations can alias or not. > > > Alexandros > > ________________________________ > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Hayrapetyan, Anahit via llvm-dev <llvm-dev at lists.llvm.org> > Sent: Thursday, August 9, 2018 2:00:14 PM > To: llvm-dev at lists.llvm.org > Subject: [llvm-dev] llvm MemorySSA def-use chains > > > Hi, > > > I have a question about how llvm MemorySSA works, as seems I misunderstand something. > > Consider following code snippet and corresponding IR with MemorySSA annotations (got with opt -print-memoryssa) > > > void foo(int* b) { > > int a = 0; > > int d = 12; > > if (b) { > > a = 42; > > d = 32; > > } > > int c = a; > > int e = d; > > } > > > ; Function Attrs: noinline nounwind optnone uwtable > > define void @foo(i32* %b) #0 { > > entry: > > %b.addr = alloca i32*, align 8 > > %a = alloca i32, align 4 > > %d = alloca i32, align 4 > > %c = alloca i32, align 4 > > %e = alloca i32, align 4 > > ; 1 = MemoryDef(liveOnEntry) > > store i32* %b, i32** %b.addr, align 8 > > ; 2 = MemoryDef(1) > > store i32 0, i32* %a, align 4 > > ; 3 = MemoryDef(2) > > store i32 12, i32* %d, align 4 > > ; MemoryUse(1) > > %0 = load i32*, i32** %b.addr, align 8 > > %tobool = icmp ne i32* %0, null > > br i1 %tobool, label %if.then, label %if.end > > > if.then: ; preds = %entry > > ; 4 = MemoryDef(3) > > store i32 42, i32* %a, align 4 > > ; 5 = MemoryDef(4) > > store i32 32, i32* %d, align 4 > > br label %if.end > > > if.end: ; preds = %if.then, %entry > > ; 9 = MemoryPhi({entry,3},{if.then,5}) > > ; MemoryUse(9) > > %1 = load i32, i32* %a, align 4 > > ; 6 = MemoryDef(9) > > store i32 %1, i32* %c, align 4 > > ; MemoryUse(9) > > %2 = load i32, i32* %d, align 4 > > ; 7 = MemoryDef(6) > > store i32 %2, i32* %e, align 4 > > ret void > > } > > > Tracking back def-use chain for `%1 = load i32, i32* %a, align 4` first I get `9 = MemoryPhi({entry,3},{if.then,5})` and then as definitions MemoryDef(2) which corresponds to `store i32 12, i32* %d, align 4` and MemoryDef(4) corresponding to `store i32 32, i32* %d, align 4`. However neither of these stores define `a`, they both define `d`. What I was expecting to get was MemoryDef(3) and MemoryDef(1), which are real definitions for `a`. > > Could you please explain what's the reason MemorySSA gives incorrect definitions, and is there any way to get the correct ones. > > > Thanks. > > > > IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev