2774ab189b
The InlineSpiller tries to shorten the live-ranges used when storing a value that is defined by a sibling register by performing the following transformation: ``` a = copy b store a ``` => ``` store b ``` That is, it eliminates the copy and store the original value at the copy location. As far as `b`'s live-range is concerned, this transformation is neutral as long as the store is inserted in place of the copy being removed. That doesn't hold when the copy is part of the basic block prologue. In that case, the earliest insertion point for the store is after the prologue: ``` a = copy b # part of bb prologue ... # more prologue instrs store a <-- earliest insertion point ``` Therefore when this optimization kicks in and the copy is in the bb's prologue, b's live-range needs to be extended to that insertion point. This is exactly what this fix does. Note: This issue is fairly easy to understand from an IR stand point and one would expect a small reproducer. Indeed, we would need only something that looks like: ``` a = copy b # prologue compatible copy ... # some more prologue instrs ... # high pressure point = use a ``` The problem is the InlineSpiller optimization (implemented in InlineSpiller::hoistSpillInsideBB) triggers only if `a` and `b` are sibling registers, meaning they are the product of a live-range splitting. Controlling live-range splitting is hard and I couldn't come up with a smaller test case. The problem also disappears if we run greedy alone because the live-ranges are repacked (well the slot indices to be exact) compared to what the scheduler produces. That changes the allocation order and the bug is not hit anymore. The alternative would be to run the pipeline from the coalescer/scheduler but that's not a whole lot more robust than the LLVM IR path, plus the reproducer would be ~20k line long! The other option (maybe something we should do in the long term) would be to serialize/deserialize the VirtRegMap object so that we can set the Virt2SplitMap map in a state that register a and b as siblings.
69 KiB
69 KiB