This (in both cases) is a trivially unsafe transformation though (if applied without any range analysis), since you can have:
X = MaxFloat64 C1 = 2 C2 = 4 (X*C1)/C2 = (MaxFloat64 * 2) / 4 = Infinity / 4 = Infinity X*(C1/C2) = MaxFloat64 * (2 / 4) = MaxFloat64 * 0.5 = whatever half of MaxFloat64 is On Wed, 13 Sept 2023, 10:57 'Marja Hölttä' via v8-dev, < v8-dev@googlegroups.com> wrote: > > > ```(X * C1) / C2 —> X / (C2 / C1)``` > > You probably meant "X * (C2 / C1)" instead of "X / (C2 / C1)". > > Nah they probably meant just that. (X * 2) / 10 is X / (10 / 2) aka X / 5, > not X * (10 / 2) aka X * 5. > > It would be the same as X * (C1 / C2) though, that's another possible > simplification (if things don't overflow and so on). > > > > On Wed, Sep 13, 2023 at 9:43 AM 'dmerc...@google.com' via v8-dev < > v8-dev@googlegroups.com> wrote: > >> Hi, >> >> In general, LLVM and ahead-of-time compilers have all the time in the >> world to optimize a function, while Turbofan tries to save every >> millisecond it can (it's not quite true: LLVM also tries to compile >> somewhat quickly, but it's orders of magnitude slower that Turbofan). As a >> result, Turbofan does fewer optimizations than LLVM, in particular when >> they have non-linear cost. That being said, Turbofan is not set in stone, >> and we are always happy to add new optimizations, provided that their cost >> can be justified by the improvements in generated code. >> >> > ```(A&B)|(A&C) —> A&(B|C)``` >> Optimizations such as this one are fairly cheap and straight-forward to >> implement. I'm guessing that the reason for their absence is that we didn't >> think about them or didn't see a specific case where they would improve the >> generated code. Feel free to submit patch to add them to >> Turboshaft's MachineOptimizationReducer ( >> https://source.chromium.org/chromium/chromium/src/+/main:v8/src/compiler/turboshaft/machine-optimization-reducer.h >> ). >> >> > ```(X * C1) / C2 —> X / (C2 / C1)``` >> You probably meant "X * (C2 / C1)" instead of "X / (C2 / C1)". Such >> mistakes are a good argument against implementing such optimizations unless >> we see a clear use-case: if they are wrong but almost never used, then they >> might cause random crashes (or security issues) that would be very hard to >> debug. >> Additionally, this simplification could be invalid depending on >> multiplication overflow, integer division and floating point >> approximations, which once again could easily introduce subtle bugs. >> >> > ```A+B --> A|B``` provided that A and B have no overlapping bits set. >> This one as well, I'm not sure we'd really want. First, it's probably >> rare that we know for a fact that 2 values don't have overlapping bits. And >> second, on most architectures that we support, an addition and a bitwise or >> have the same latency and throughput. >> >> Best, >> Darius >> >> >> On Wednesday, September 13, 2023 at 6:51:18 AM UTC+2 doit_man wrote: >> >>> Hi v8 developers, >>> >>> I’m curious about the specific optimizations that TurboFan currently not >>> support. >>> >>> Other ahead-of-time (AOT) compilers perform a wide range of >>> optimizations, while TurboFan comparatively has fewer. >>> >>> >>> For instance, the LLVM compiler employs optimization rules such as >>> ```A+B --> A|B``` provided that A and B have no overlapping bits set. >>> >>> Similarly, ```(A&B)|(A&C) —> A&(B|C)``` and ```(X * C1) / C2 —> X / (C2 >>> / C1)``` are examples of optimizations present in LLVM but absent in >>> TurboFan. >>> >>> I think these kinds of rules could enhance TurboFan’s optimization >>> capabilities. >>> >>> I’m interested in understanding whether this absence is due to >>> TurboFan’s current state of implementation, inherent limitations of a >>> just-in-time (JIT) compiler, or if there are other factors at play. >>> >>> >>> Thanks. >>> >> -- >> -- >> 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 on the web visit >> https://groups.google.com/d/msgid/v8-dev/9fcd075d-14b8-41f5-a793-7ddee3f4e322n%40googlegroups.com >> <https://groups.google.com/d/msgid/v8-dev/9fcd075d-14b8-41f5-a793-7ddee3f4e322n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > > > -- > > 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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