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: unassignedb...@nondot.org
Reporter: dber...@dberlin.org
CC: llvm-bugs@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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