cranelift: Round inline stack probes down, not up #8397
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When we have `enable_probestack` turned on and set `probestack_strategy` to "inline", we have to compute how many pages of the stack we'll probe. The current implementation rounds our stack frame size up to the nearest multiple of the page size, then probes each page once. However, if our stack frame is not a multiple of the page size, that means there's a partial page at the end. It's not necessary to probe that partial page, just like it's unnecessary to probe at all if the frame is smaller than one page. Either way, any signal handler needs to be prepared for stack accesses on that last page to fault at any time during the function's execution.
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| let probe_count = frame_size / guard_size; | ||
| if probe_count == 0 { | ||
| // No probe necessary | ||
| } else if probe_count <= PROBE_MAX_UNROLL { |
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Mind leaving a comment about why this doesn't need aligning up here? Kind of unfortunate it would need to be copied to all the other inline probestack code paths. Maybe that is a sign that we should factor this little bit of logic out to a generic helper?
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When we have
enable_probestackturned on and setprobestack_strategyto "inline", we have to compute how many pages of the stack we'll probe.The current implementation rounds our stack frame size up to the nearest multiple of the page size, then probes each page once.
However, if our stack frame is not a multiple of the page size, that means there's a partial page at the end. It's not necessary to probe that partial page, just like it's unnecessary to probe at all if the frame is smaller than one page. Either way, any signal handler needs to be prepared for stack accesses on that last page to fault at any time during the function's execution.