https://llvm.org/bugs/show_bug.cgi?id=28142
Bug ID: 28142
Summary: RegScavenger (as used by BranchFolding) pessimises
liveness
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: ahmed.bouga...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
This came up in http://reviews.llvm.org/D21085; on the original testcase:
Looks like BranchFolding, when merging tails, indiscriminately marks every
register defined but not killed (it might really be live, but not necessarily)
before the tail as live-in the new tail block. It queries the
RegisterScavenger, which walks the original block forward, using kill flags;
maybe it should walk the tail block backward instead?
That patch adds forward liveness computation to X86FixupBWInsts (using flags,
via LivePhysRegs) as a stopgap.
Instead, seems like we should:
- teach RegScavenger to compute liveness backwards (or replace it with
something else)
- fix the various users; in this case, BranchFolding
- remove the X86FixupBWInsts workaround, and probably more
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