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If-conversion incorrectly calculates liveness of redefined registers
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Authored by kparzysz on Aug 5 2016, 7:48 AM.

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Summary

The if-convert-diamond function in the if-conversion initialized Redefs with the live-ins from the first of the two predicated blocks. This may not reflect the liveness of the defined registers correctly, since a register defined in the first block may be live across the other predicated block. The testcase illustrates this situation.

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rL LLVM

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kparzysz updated this revision to Diff 66948.Aug 5 2016, 7:48 AM
kparzysz updated this revision to Diff 66955.
kparzysz retitled this revision from to If-conversion incorrectly calculates liveness of redefined registers.
kparzysz updated this object.
kparzysz added a reviewer: MatzeB.
kparzysz set the repository for this revision to rL LLVM.
kparzysz added a subscriber: llvm-commits.

Remove a leftover debug statement.

MatzeB accepted this revision.Aug 10 2016, 6:27 PM
MatzeB edited edge metadata.

LGTM. That was a subtle issue, thanks.

Some nitpicks below:

lib/CodeGen/IfConversion.cpp
1435–1437

I don't understand the new comment (the old comment was pretty bad too though).

How about something in the line of:

// - BB2 live-in regs need implicit uses before being redefined by BB1 instructions.
// - BB1 live-out regs need implicit uses before being redefined by BB2 instructions;
//    We start with BB1 live-ins so we have the live-out regs after tracking the BB1 instructions.
test/CodeGen/Hexagon/ifcvt-impuse-livein.mir
2

Nice small and to the point test!

Would be nice if you could add one sentence about what is tested.
I would also recommend a # CHECK-LABEL: name: foo (just in case more tests get added later).

17

The %r1 livein looks unnecessary.

This revision is now accepted and ready to land.Aug 10 2016, 6:27 PM
kparzysz marked 3 inline comments as done.Aug 11 2016, 11:46 AM
This revision was automatically updated to reflect the committed changes.