[LLVMbugs] [Bug 18996] New: Changing regalloc order breaks "lencod" on native arm linux builds.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18996

            Bug ID: 18996
           Summary: Changing regalloc order breaks "lencod" on native arm
                    linux builds.
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: atrick at apple.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The LNT run of lencod broke here:
http://lab.llvm.org:8011/builders/clang-native-arm-lnt/builds/5356

After this commit:

Author: atrick
Date: Wed Feb 26 16:07:26 2014
New Revision: 202304

URL: http://llvm.org/viewvc/llvm-project?rev=202304&view=rev
Log:
Add a limit to the heuristic that register allocates instructions in local
order.

This handles pathological cases in which we see 2x increase in spill
code for large blocks (~50k instructions). I don't have a unit test
for this behavior.

Fixes rdar://16072279.

Modified:
   llvm/trunk/lib/CodeGen/RegAllocGreedy.cpp

There is a downstream codegen bug that is only exposed by a certain register
allocation order, and only currently reproducible on native arm linux builds.

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