Hi Sam, Just so that everybody is on the same page: these stack checks are really interrupt checks rather than stack-overflow checks or something like that.
And yea, performing them on every loop iteration is a bit of a waste of time, but we kinda need them in every loop where we can't prove that the loop won't run forever (or even: where we can't prove that the loop won't run for a long time). There is already a mechanism in place in Turboshaft to try to estimate the number of iterations of specific loops, and then remove stack checks based on this, cf LoopStackCheckElisionReducer, whose analysis is done by LoopUnrollingAnalyzer in the LoopUnrollingPhase. However, as I write this, I'm noticing that the Wasm pipeline doesn't use the LoopStackCheckElisionReducer outside of loop unrolling (and loop unrolling only runs if there is at least one small(ish) inner loop). So, it would make sense to put a LoopStackCheckElisionReducer in the next phase after LoopUnrollingPhase: WasmGCOptimizePhase... except that this phase only runs for WasmGC. Maybe we can tweak a little bit the LoopStackCheckElisionReducer to put it in both WasmGCOptimizePhase and WasmLoweringPhase (the next phase, which runs unconditionally), but it would be a nop in WasmLoweringPhase if it has already ran in LoopStackCheckElisionReducer. Additional limitation of LoopUnrollingAnalyzer+LoopStackCheckElisionReducer: the LoopUnrollingAnalyzer only detects the number of iterations of very specific loops (something like "for (i = <constant>; i <comparison operation> <constant>; i += <constant>)"). But it's perfectly reasonable to improve the pattern matching there to support more loops. Another way to improve these interrupt checks that we've talked about before would be to remove all loop stack checks, and patch the code in-place when we have an interrupt to trigger. This way, we wouldn't have to keep repeating those stack checks that most of the time don't do anything. Probably not super trivial to implement, but doable I would think. If you want to go down that route, I suggest writing a design doc and gathering approvals from V8 folks, since some might have concerns (regarding security for instance). Cheers, Darius On Thu, 12 Jun 2025 at 17:02, 'Emanuel Ziegler' via v8-dev < v8-dev@googlegroups.com> wrote: > Hi Sam, > > In principle, loop unrolling should already reduce the number of stack > checks, but it could be that it's insufficient or that for whatever reason > this optimization does not get applied here. Did you take a look at the > generated code? > > Cheers, > Emanuel > > On Thu, Jun 12, 2025 at 4:44 PM Sam Parker-Haynes <sam.par...@arm.com> > wrote: > >> Hi! >> >> While running some AI code, the loop header WasmStackCheck was appearing >> quite heavily in the profile. Disabling the checks results in ~1.5% >> speedup. So, is it necessary to execute these for every iteration? Or could >> we wrap inner loops, devoid of a stack check, in a new loop with one so >> that these checks are executed for a fraction of what they are now? >> >> Cheers, >> Sam >> >> >> >> -- >> -- >> v8-dev mailing list >> v8-dev@googlegroups.com >> http://groups.google.com/group/v8-dev >> --- >> You received this message because you are subscribed to the Google Groups >> "v8-dev" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to v8-dev+unsubscr...@googlegroups.com. >> To view this discussion visit >> https://groups.google.com/d/msgid/v8-dev/b4a77d25-21f9-4a0d-a467-8cbe48275bfbn%40googlegroups.com >> <https://groups.google.com/d/msgid/v8-dev/b4a77d25-21f9-4a0d-a467-8cbe48275bfbn%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- > -- > v8-dev mailing list > v8-dev@googlegroups.com > http://groups.google.com/group/v8-dev > --- > You received this message because you are subscribed to the Google Groups > "v8-dev" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to v8-dev+unsubscr...@googlegroups.com. > To view this discussion visit > https://groups.google.com/d/msgid/v8-dev/CAPAU7RyGTVCKFUgJoTiDH546up4d8qJ4fQCr8aTKFBVtxUt%2BoQ%40mail.gmail.com > <https://groups.google.com/d/msgid/v8-dev/CAPAU7RyGTVCKFUgJoTiDH546up4d8qJ4fQCr8aTKFBVtxUt%2BoQ%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > Darius Mercadier Software Engineer dmercad...@google.com Google Germany GmbH Erika-Mann-Straße 33 80636 München Geschäftsführer: Paul Manicle, Liana Sebastian Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Diese E-Mail ist vertraulich. 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