[llvm-dev] MemorySSA question
Siddharth Bhat via llvm-dev
llvm-dev at lists.llvm.org
Tue Dec 19 09:25:54 PST 2017
@daniel: so, memorySSA is as precise as AA allows it to be?
Thanks
Siddharth
On Tue 19 Dec, 2017, 21:17 Daniel Berlin via llvm-dev, <
llvm-dev at lists.llvm.org> wrote:
> On Tue, Dec 19, 2017 at 3:13 AM, 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.
>>
>> Yes, that is correct.
>
>
>> 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.
>>
>
> If AA could prove it, MemorySSA would have a MemoryUse(1) instead.
>
>>
>> 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.
>>
>
> Here, the underlying AA is unable to distinguish, AFAICT, after some
> transformation.
>
> clang -c -o test_clang_out.ll -emit-llvm -O3 test.c -S
> bin/opt -print-memoryssa -debug test_clang_out.ll
>
> (note lack of other passes):
>
> ; <label>:15: ; preds = %15, %8
> ; 3 = MemoryPhi({%8,liveOnEntry},{%15,1})
> %16 = phi i64 [ 0, %8 ], [ %23, %15 ]
> %17 = getelementptr inbounds i32, i32* %1, i64 %16
> ; MemoryUse(liveOnEntry)
> %18 = load i32, i32* %17, align 4, !tbaa !2
> %19 = getelementptr inbounds i32, i32* %2, i64 %16
> ; MemoryUse(liveOnEntry)
> %20 = load i32, i32* %19, align 4, !tbaa !2
> %21 = add nsw i32 %20, %18
> %22 = getelementptr inbounds i32, i32* %0, i64 %16
> ; 1 = MemoryDef(3)
> store i32 %21, i32* %22, align 4, !tbaa !2
> %23 = add nuw nsw i64 %16, 5
> %24 = icmp slt i64 %23, %9
> br i1 %24, label %15, label %10
>
> ; <label>:25: ; preds = %25, %12
> ; 4 = MemoryPhi({%12,1},{%25,2})
> %26 = phi i64 [ 0, %12 ], [ %33, %25 ]
> %27 = getelementptr inbounds i32, i32* %0, i64 %26
> ; MemoryUse(1)
> %28 = load i32, i32* %27, align 4, !tbaa !2
> %29 = getelementptr inbounds i32, i32* %3, i64 %26
> ; MemoryUse(liveOnEntry)
> %30 = load i32, i32* %29, align 4, !tbaa !2
> %31 = mul nsw i32 %30, %28
> %32 = getelementptr inbounds i32, i32* %4, i64 %26
> ; 2 = MemoryDef(4)
> store i32 %31, i32* %32, align 4, !tbaa !2
> %33 = add nuw nsw i64 %26, 5
> %34 = icmp slt i64 %33, %14
> br i1 %34, label %25, label %35
>
>
> Note memoryuse reaching back through memoryphi.
>
>
> MemorySSA is deliberately imprecise for def-def relationships, but this is
> not the cause of your issue. Yours is caused by AA not being able to
> distinguish anymore.
>
>
> _______________________________________________
> 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!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20171219/6cf71285/attachment.html>
More information about the llvm-dev
mailing list