[llvm-dev] MemorySSA question
Daniel Berlin via llvm-dev
llvm-dev at lists.llvm.org
Tue Dec 19 09:26:24 PST 2017
On Tue, Dec 19, 2017 at 9:10 AM, Siddharth Bhat via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I could be entirely wrong, but from my understanding of memorySSA, each
> def defines an "abstract heap state" which has the coarsest possible
> definition - any write will be modelled as a "new heap state".
>
This is true for def-def relationships, but doesn't;'t matter here.
>
>
So in that sense, from what I understand, it does not actually model the
> heap in a fine grained way.
>
> Any write to any part of the heap will create a new memorydef node.
>
> Yes, but MemoryUses can reach back past the nearest def, so that doesn't
affect uses.
The limitation here is deliberately done to keep it only requiring a single
phi.
All data from building this for years in GCC (which also moved from
"precise" to "imprecise" for the same reason) told us that the massive
amount of def-use chains you end up with from trying to model def-def
relationships precisely was not worth it by far.
(It degrades into putting N^2 variables into SSA, and attaching N variables
to each def/use).
With respect to that model, memorySSA is right. The value of A could depend
> on the abstract heap state of the definition of array "e".
>
> I'm on my phone, so this may not make much sense, but I hope this helps,
> Siddharth.
>
> On Tue 19 Dec, 2017, 15:13 Venugopal Raghavan via llvm-dev, <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> I am new to MemorySSA and wanted to understand its capabilities. Hence I
>> wrote the following program (test.c):
>>
>> int N;
>>
>> void test(int *restrict a, int *restrict b, int *restrict c, int
>> *restrict d, int *restrict e) {
>> int i;
>> for (i = 0; i < N; i = i + 5) {
>> a[i] = b[i] + c[i];
>> }
>>
>> for (i = 0; i < N - 5; i = i + 5) {
>> e[i] = a[i] * d[i];
>> }
>> }
>>
>> I compiled this program using the following commands:
>>
>> clang -c -o test_clang_out.ll -emit-llvm -O3 test.c
>> opt -o test_opt_out.ll -O3 -passes='print<memoryssa>' -disable-output
>> test_clang_out.ll > out 2>&1
>>
>> The relevant parts of the file "out" are shown below:
>> .
>> .
>> .
>>
>> for.body: ; preds = %
>> for.body.lr.ph, %for.body
>> ; 3 = MemoryPhi({for.body.lr.ph,liveOnEntry},{for.body,1})
>> %indvars.iv35 = phi i64 [ 0, %for.body.lr.ph ], [ %indvars.iv.next36,
>> %for.body ]
>> %arrayidx = getelementptr inbounds i32, i32* %b, i64 %indvars.iv35
>> ; MemoryUse(3)
>> %2 = load i32, i32* %arrayidx, align 4, !tbaa !2
>> %arrayidx2 = getelementptr inbounds i32, i32* %c, i64 %indvars.iv35
>> ; MemoryUse(3)
>> %3 = load i32, i32* %arrayidx2, align 4, !tbaa !2
>> %add = add nsw i32 %3, %2
>> %arrayidx4 = getelementptr inbounds i32, i32* %a, i64 %indvars.iv35
>> *; 1 = MemoryDef(3)*
>> store i32 %add, i32* %arrayidx4, align 4, !tbaa !2
>> %indvars.iv.next36 = add nuw nsw i64 %indvars.iv35, 5
>> %cmp = icmp slt i64 %indvars.iv.next36, %1
>> br i1 %cmp, label %for.body, label %for.end
>>
>> for.end: ; preds = %for.body
>> %cmp729 = icmp sgt i32 %0, 5
>> br i1 %cmp729, label %for.body8.lr.ph, label %for.end17
>>
>> for.body8.lr.ph: ; preds = %for.end
>> %sub = add nsw i32 %0, -5
>> %4 = sext i32 %sub to i64
>> br label %for.body8
>>
>> for.body8: ; preds = %
>> for.body8.lr.ph, %for.body8
>> *; 4 = MemoryPhi({for.body8.lr.ph
>> <http://for.body8.lr.ph>,1},{for.body8,2})*
>> %indvars.iv = phi i64 [ 0, %for.body8.lr.ph ], [ %indvars.iv.next,
>> %for.body8 ]
>> %arrayidx10 = getelementptr inbounds i32, i32* %a, i64 %indvars.iv
>> *; MemoryUse(4)*
>> %5 = load i32, i32* %arrayidx10, align 4, !tbaa !2
>> %arrayidx12 = getelementptr inbounds i32, i32* %d, i64 %indvars.iv
>> ; MemoryUse(4)
>> %6 = load i32, i32* %arrayidx12, align 4, !tbaa !2
>> %mul = mul nsw i32 %6, %5
>> %arrayidx14 = getelementptr inbounds i32, i32* %e, i64 %indvars.iv
>> *; 2 = MemoryDef(4)*
>> store i32 %mul, i32* %arrayidx14, align 4, !tbaa !2
>> %indvars.iv.next = add nuw nsw i64 %indvars.iv, 5
>> %cmp7 = icmp slt i64 %indvars.iv.next, %4
>> br i1 %cmp7, label %for.body8, label %for.end17
>>
>>
>>
>> I have highlighted the interesting lines in bold.
>>
>> I was interested in the use of array "a" in the second loop and and
>> wanted to check if memorySSA would show the reaching definitions for these
>> uses to emanate from the definitions in 1 = MemoryDef(3) and, indeed,
>> the MemoryUse(4) corresponding to the use of "a" shows the reaching
>> definition to be from the MemoryPhi node 4, which, in turn has one of its
>> reaching definitions as 1 = MemoryDef(3). But this MemoryPHi node also has
>> another reaching definition which is 2 = MemoryDef(4) which corresponds to
>> the definition of array e in the second loop.
>>
>> This seems to make the MemorySSA form imprecise because it seems to
>> indicate that the use of "a" in the second loop could be having a reaching
>> definition from the definition of "a" in the first loop or the definition
>> of "e" in the second loop (through the MemoryPhi). I would have expected
>> only the first reaching definition to be inferred.
>>
>> Am I mis-interpreting the information here or mis-understanding the
>> capabilities of MemorySSA? If not, can someone explain why the information
>> is imprecise? Maybe the underlying alias analysis is unable to disambiguate
>> the different arrays? But I would have thought that this would not be a
>> difficult case for alias analysis.
>>
>> Thanks.
>>
>> Regards,
>> Venugopal Raghavan.
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
> --
> Sending this from my phone, please excuse any typos!
>
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