JIT: volatile compares should not be foldable

[AndyAyersMS]

using System;

class X
{
    volatile int a;
    
    public static int Main()
    {
        var x = new X();
        x.a = 50;
        x.a = 100;
        if (x.a == x.a) return 100;
        return -1;
    }
}

currently generates:

G_M32182_IG01:
       4883EC28             sub      rsp, 40

G_M32182_IG02:
       48B95852BB61F97F0000 mov      rcx, 0x7FF961BB5258
       E85DFA7F5F           call     CORINFO_HELP_NEWSFAST
       C7400832000000       mov      dword ptr [rax+8], 50
       C7400864000000       mov      dword ptr [rax+8], 100
       B864000000           mov      eax, 100

G_M32182_IG03:
       4883C428             add      rsp, 40
       C3                   ret

Folding the compare is unsafe in general.

See also the somewhat related #6280.

category:correctness
theme:basic-cq
skill-level:intermediate
cost:small

[mikedn]

Apparently this is due to Compiler::gtFoldExprCompare and/or GenTree::Compare. The later returns true for those 2 indirs and then the relop gets folded:
https://github.com/dotnet/coreclr/blob/0a842c03c1d60a5c57b7c60d09a657e6a66e4c95/src/jit/gentree.cpp#L12814-12820

Not clear what exactly is wrong:

• Should Compare check GTF_IND_VOLATILE?
• Should gtFoldExprCompare check GTF_ORDER_SIDEEFF?
• Something else...

[VSadov]

Is this the same issue as in #10619 ?