[mono][interp] Defer compilation in bblocks with unitialized stack #108731
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Each basic block will have an emit state, not emitted, emitting or emitted. When we reach a new basic block, we will emit code into it only if the stack state is initialized (the stack state of a bblock can be initialized either from the state of the previous bblocks, if it is fallthrough, or from branching from another bblock with initialized state). If we encounter a bblock that doesn't have the state initialized we set a flag so we will retry codegen in an attempt to emit new bblocks. Once we finish emitting code, we remove all bblocks in not emitted state.
…ed ranges Following the change to only emit code in bblocks once we reach them with an initialized stack state, we have the side effect of not processing IL code in dead bblocks. This means that offset_to_bb might actually be null for some IL offsets, so we need to iterate over following il offsets until we find a mapped bblock.
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| g_assert (bb); | ||
| // If the bblock is detected as dead while traversing the IL code, the mapping for | ||
| // it is cleared. We can skip it. | ||
| if (!bb) |
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I’m concerned about relaxing the condition here. If bblock is null due to a bug, it might be incorrectly processed here. Could we explicitly annotate a bblock as dead instead?
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We do have a dead field for basic blocks, but the existing pattern is that dead bblocks are no longer linked to live bblocks, they are not reachable. In addition to being a different pattern, having them still exist in the td->offset_to_bb mapping turned out to complicate code in other places. This condition here is used only for exception clause ranges so I would say the scope is limited enough so we don't risk serious bugs.
Each basic block will have an emit state, not emitted, emitting or emitted. When we reach a new basic block, we will emit code into it only if the stack state is initialized (the stack state of a bblock can be initialized either from the state of the previous bblocks, if it is fallthrough, or from branching from another bblock with initialized state). If we encounter a bblock that doesn't have the state initialized we set a flag so we will retry codegen in an attempt to emit new bblocks.
Once we finish emitting code, we remove all bblocks in not emitted state.
Before this change, when encountering a bblock with unitialized stack, we assumed by chance that it had an empty stack, which is incorrect according to the spec. Also, in some cases we could simply crash, even if the block was indeed having an empty stack.