[llvm-bugs] [Bug 31463] New: Missed optimization due to incompleteness in GVN

[via llvm-bugs]

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: unassignedbugs at nondot.org
          Reporter: dberlin at dberlin.org
                CC: llvm-bugs at lists.llvm.org
    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.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20161224/38946376/attachment.html>