[LLVMbugs] [Bug 18747] New: Add an option to allow an exhaustive search during last chance recoloring

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18747

            Bug ID: 18747
           Summary: Add an option to allow an exhaustive search during
                    last chance recoloring
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Register Allocator
          Assignee: unassignedbugs at nondot.org
          Reporter: qcolombet at apple.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Since r200883, the greedy register allocator features a last chance recoloring
mechanism.
This mechanism allows to find a valid assignment of the variables (if any) when
the regular heuristic fails.
It uses a backtracking algorithm which is currently cutoff with some heuristics
to avoid uncontrolled compile time.

There is two things to improve here:
1. When the assignment fails because of the early cutoffs, this should be
reported instead of the current obscur “ran out of registers”.
2. We should provide an option (e.g., -fexhaustive-register-search) to allow to
explore all the possibilities when the allocation failed because of a cutoffs
(which we would know thanks to 1.).

For testing the reporting in 1., one can use the test-case of r200883 and
artificially cut the search space with:
- lcr-max-depth=<uint>
- lcr-max-interf=<uint>

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