Fix fgValueNumberArrIndexVal for wide reads

[EgorBo]

Fixes#57914

Minimal repro:

privatestaticboolTest1(){byte[]array=newbyte[2];array[0]=1;array[1]=2;bytea1=Unsafe.ReadUnaligned<byte>(refarray[0]);shorta2=Unsafe.ReadUnaligned<short>(refarray[0]);array[1]=42;shorta3=Unsafe.ReadUnaligned<short>(refarray[0]);returna1==a2&&a1==a3;// returns True currently}

a1, a2 and a3 all have different values here with the same liberal VN resulting Test1 to return true.

a1:

\--* IND byte <l:$42, c:$401>
\--* ADD byref $300
+--* LCL_VAR ref V04 tmp1 u:2 $240
\--* CNS_INT long 16 Fseq[#FirstElem] $181

a2:

\--* IND short <l:$500, c:$5c1>
\--* LCL_VAR byref V07 tmp4 u:2 (last use) $300

($500 here is $42 with NullRefExc)

a3:

\--* IND byte <l:$42, c:$402>
\--* ADD byref $300
+--* LCL_VAR ref V04 tmp1 u:2 (last use) $240
\--* CNS_INT long 16 Fseq[#FirstElem] $181

The fix is quite conservative - it treats such array element loads as "non-proper" and assign a unique VN.
SuperPMI diffs are empty.

PS: Doesn't reproduce on .NET 5.0

/cc @dotnet/jit-contrib

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Issue Details

---

Minimal repro:

privatestaticboolTest1(){byte[]array=newbyte[2];array[0]=1;array[1]=2;bytea1=Unsafe.ReadUnaligned<byte>(refarray[0]);shorta2=Unsafe.ReadUnaligned<short>(refarray[0]);array[1]=42;shorta3=Unsafe.ReadUnaligned<short>(refarray[0]);returna1==a2&&a1==a3;}

a1, a2 and a3 all have different values here with the same liberal VN resulting Test1 to return true.

The fix is quite conservative - it treats such array element loads as "non-proper" and assign a unique VN.
Ideas for a less conservative fix are welcomed 🙂

PS: Doesn't reproduce on .NET 5.0

/cc @dotnet/jit-contrib

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[SingleAccretion]

Heh, so I always wondered if it is possible to make VN use CASTs where it ought to use BITCASTs. Turns out it is very possible.

privatestaticfloatBitcast(){int[]array=newint[2];array[0]=1;returnUnsafe.ReadUnaligned<float>(refUnsafe.As<int,byte>(refarray[0]));}

 wholeElem $41 is MapSelect(hAtArrTypeAtArr($340), ind=$142).
VNForCastOper(float) is $42VNForCast($41, $42) returns $440 {FltCns[1.000000]}
selectedElem is $440 after applying selectors.

But that is a separate issue (filed as #58312).

[sandreenko]

LGTM

[briansull]

Looks Good,

[EgorBo]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1185769084