[llvm-bugs] [Bug 26940] New: [Scheduler] Recent improvements lead to compile time bloat

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26940

            Bug ID: 26940
           Summary: [Scheduler] Recent improvements lead to compile time
                    bloat
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: bruno.cardoso at gmail.com
                CC: llvm-bugs at lists.llvm.org, paulsson at linux.vnet.ibm.com
    Classification: Unclassified

Commit r259673 (“[ScheduleDAGInstrs::buildSchedGraph()] Handling of memory
dependecies rewritten”) introduced a huge slowdown in an internal benchmark:

=== Before r259673

$ clang -arch arm64 -Os -c t.mi
real    1389.12s
user    1314.71s => 22min
sys     14.92s

$ clang -arch x86_64 -Os -c t.mi
real    76.90s
user    75.46s => 1min15s
sys     0.92s

=== After r259673

real    10045.62s
user    9131.03s => 3h20min
sys     95.60s

real    2217.71s
user    2174.12s => 36min23s
sys     33.07s

ARM64 is a lot worse than x86, but compile time for both are pretty bad! Note
that before r259673 arm64 time was already bad (22min) but got even worse
after. Gonna try to reduce the testcase / provide a testcase with the same
reproducible slowdown factor.

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