interp: bail out of loops that iterate too many times#5395
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The existing loop guard (errLoopUnrolled) only fires when a loop body emits runtime instructions. Loops that are fully evaluable at compile time, such as inserting thousands of entries into a map, were not caught and could hang the compiler. Add a per-basic-block iteration counter that triggers a recoverable error (errLoopTooLong) when any block is entered more than 1000 times in a single function call. This defers the init function to runtime, which is the same behavior as other interp bailouts. Profiling showed that 83% of CPU time was spent in GC, caused by allocation pressure from the interp memory cloning on each map mutation. The iteration limit avoids this entirely by bailing out before the quadratic cost becomes significant. Performance on the reproducer from tinygo-org#2090 (map init with strconv.Itoa): entries before after 5,000 7.4s 2.1s 10,000 17.5s 2.1s 20,000 48.0s 2.8s 65,536 >180s (OOM) 3.2s
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See also #2384 |
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Yeah, I definitely read that one but of course agree that a time-based solution is a no-go. |
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Can you add a flag to make this value configurable? (With 0 == disable?). There might be some users who want the slowness in exchange for fully interpreting a loop they know to be huge. Would need to figure out how to make it play nicely with the current time-based timeout. |
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Done; please bikeshed the flag name 😄 |
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Fixes #2090
This adds a loop limiter to interp so it can still bail out even if nothing else does. Perhaps this is a bad idea (1000 is random), but it does seem to fix the problem?