a037582eff
Currently, a block's control instruction gets the liveness info of the last Value in the block. However, for an empty block, the control instruction gets the invalid liveness info and therefore not preemptible. One example is empty infinite loop, which has only a control instruction. The control instruction being non- preemptible makes the whole loop non-preemptible. Fix this by using a different, preemptible liveness info for empty block's control. We can choose an arbitrary preemptible liveness info, as at run time we don't really use the liveness map at that instruction. As before, if the last Value in the block is non-preemptible, so is the block control. For example, the conditional branch in the write barrier test block is still non-preemptible. Also, only update liveness info if we are actually emitting instructions. So zero-width Values' liveness info (which are always invalid) won't affect the block control's liveness info. For example, if the last Values in a block is a tuple-generating operation and a Select, the block control instruction is still preemptible. Fixes #35923. Change-Id: Ic5225f3254b07e4955f7905329b544515907642b Reviewed-on: https://go-review.googlesource.com/c/go/+/209659 Run-TryBot: Cherry Zhang <cherryyz@google.com> Reviewed-by: David Chase <drchase@google.com> |
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