Björn Pettersson f321456e1a [ScheduleDAG] Dirty height/depth in addPred/removePred even for latency zero (#102915) ...

A long time ago (back in 2009) there was a commit 52d4d8244b
that changed the scheduler to not dirty height/depth when adding or
removing SUnit predecessors when the latency on the edge was zero. That
commit message is claiming that the depth or height isn't affected when
the latency is zero.

As a matter of fact, the depth/height can change even with a zero
latency on the edge. If for example adding a new SUnit A, with zero
latency, but as a predecessor to a SUnit B, then both height of A and
depth of B should be marked as dirty. If for example B has a greater
height than A, then the height of A needs to be adjusted even if the
latency is zero.

I think this has been wrong for many years. Downstream we have had
commit 52d4d8244b reverted since back in 2016. There is no
motivating lit test for 52d4d8244b (only an incomplete C level
reproducer in https://github.com/llvm/llvm-project/issues/3613).

After commit 13d04fa560 there finally appeared an upstream
lit test that shows that we get better code if marking height/depth as
dirty (llvm/test/CodeGen/AArch64/abds.ll).

2024-08-20 13:38:18 +02:00

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