https://llvm.org/bugs/show_bug.cgi?id=31463
Bug ID: 31463
Summary: Missed optimization due to incompleteness in GVN
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Code that looks like this:
%.0 = phi i32 [ 10, %4 ], [ 5, %2 ]
%11 = add nsw i32 %.0, 5
generates a constant (phi i32 [15, %4], [10, %2])), but neither GVN nor NewGVN
will detect this.
This is due to one of the standard forms of GVN incompleteness:
the missing equivalence of A op Phi (B, C) and Phi(A op B, A op C)
(IE add 5, phi [10], [5] == phi [add 5, 10], [add 5, 5])
It's exponential time/space to do the transforms back and forth in every case
(because the arguments can be N levels deep)
However, for phis of all constants, and operations on them, we may want to do
it.
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