[llvm-dev] MemorySSA question

Siddharth Bhat via llvm-dev llvm-dev at lists.llvm.org
Tue Dec 19 09:31:49 PST 2017


> MemoryUses can reach back past the nearest def, so that doesn't affect
uses

Ahh, I see. I did not know this. Thanks for the clarification.

I now believe I had passed insufficient AA information when I was trying to
use memorySSA.

Can I add a detailed explanation that explains this to the memorySSA docs,
or the "analysis passes" list?

Thanks,
Siddharth

On Tue 19 Dec, 2017, 21:26 Daniel Berlin, <dberlin at dberlin.org> wrote:

> 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!
>>
>> _______________________________________________
>> 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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