[llvm-bugs] [Bug 36794] New: [MISched] indeterministic effects of enabling bidirectional scheduling pre-RA

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36794

            Bug ID: 36794
           Summary: [MISched] indeterministic effects of enabling
                    bidirectional scheduling pre-RA
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: paulsson at linux.vnet.ibm.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 20090
  --> https://bugs.llvm.org/attachment.cgi?id=20090&action=edit
reduced testcase (1 spill more bidirectional)

I found a test case where enabling bidirectional scheduling increased the
number of spilled live ranges. I reduced this to a small test case (attached),
and found to my surprise that even when bidirectional produces the same final
schedule for a region, it may give different slot indexes if it was picked from
Top, like:

BOTTOM-UP                               BIDIRECTIONAL
2080B     CallBRASL @strchr, $r2d, $r   2080B     CallBRASL @strchr, $r2d, $r
2112B     %81:gr64bit = COPY $r2d     | 2088B     %81:gr64bit = COPY $r2d
2120B     ADJCALLSTACKUP 0, 0         | 2096B     ADJCALLSTACKUP 0, 0
2160B     %82:gr64bit = IMPLICIT_DEF    2160B     %82:gr64bit = IMPLICIT_DEF

I suspect that this change in slot indexes causes the register allocator to
produce a different result based on interval weighting etc.

Is this something that should be fixed? When experimenting with pre-RA
scheduling it is of course desirable to reduce side effects like this, I think.


bin/llc -mtriple=s390x-linux-gnu -mcpu=z13 -o out.s ./tc1.ll --stats |& grep
"spilled live ranges"
  4 regalloc              - Number of spilled live ranges

bin/llc -mtriple=s390x-linux-gnu -mcpu=z13 -o out.s ./tc1.ll --stats
-misched-topdown=false -misched-bottomup=false |& grep "spilled live ranges"
  5 regalloc              - Number of spilled live ranges

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