[llvm-bugs] [Bug 31868] New: NewGVN needs support for multiple fake expressions for instructions

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31868

            Bug ID: 31868
           Summary: NewGVN needs support for multiple fake expressions for
                    instructions
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: dberlin at dberlin.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Some canonicalizations can only be represented as multiple expressions.

For example,

F(phi(a,b), phi(c, d)) == phi(F(a, c), F(b, d))

In the attached example, the multiplication    %21 = mul nsw i64 %18, %19

is completely redundant
we do not detect it because one of the multiplications is in one of the above
forms, and the other is in the other.

If we canonicalize  both to the same (instcombine does this), we would discover
it's completely redundant).

WE can either move that code out of instcombine, and run it first, or we could
support multiple fake expressions that get inserted at elimination time.


We end up needing the latter support for PRE anyway...

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