[LLVMbugs] [Bug 17419] New: AsmMatcherEmitter's ClassInfo comparator not transitive

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Mon Sep 30 16:02:20 PDT 2013


            Bug ID: 17419
           Summary: AsmMatcherEmitter's ClassInfo comparator not
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: willdtz at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Similar to the recent fix posted for the sorting predicate used in
DAGISelEmitter[1], the sorting predicate used in AsmMatcherEmitter for
ClassInfo objects is also non-transitive.

This results in unspecified sorting behavior and in particular
breaks tests relying on a particular match ordering
(possibly producing inferior assembly or other problems).

The comparison operator is in AsmMatcherEmitter.cpp:265.

Examples of lack of transitivity are:

class A subclass B, B.Kind < A.Kind.
when a class 'C' has a kind between B and A, this breaks:

A < B (A subclass B)
B < C (unrelated classes, B.Kind < C.Kind)
C < A (unrelated classes, C.Kind < A.Kind)

And a similar failure with ValueName causing the failure
instead of the Kind comparison.

I've seen both of these failures happen when emitting the AsmMatcher for X86,
I haven't tested other platforms.  These were detected while debugging building
LLVM w/an alternative C++ implementation, with the following sanity check added
to std::sort():

for (_RandomAccessIterator cur = first; cur != last; ++cur)
  for (_RandomAccessIterator before = first; before != cur; ++before)
    assert((comp(*before, *cur) || !comp(*cur, *before)) && 
           "Bad sort predicate");

Perhaps a preprocessing pass over the ClassInfo's that builds a numbering would
work?  A postorder traversal of the class heirarchy should provide a reasonable
such numbering, I believe, and can use Kind/ValueName as we presently do to
break ties amongst children for output that's as deterministic as what we have

Regardless if someone could take a look that'd be great.  Thanks! :)


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