Hi Dan, that sounds like an interesting idea. In fact, I considered implementing a Wasm constant pool for floating point constants, but SIMD code might benefit even more. Note that in Wasm code we do not have a root register (currently), so any access to the isolate root is a few indirections away. I am also not sure I fully understand how you plan to use external references to reference to dynamically generated constants.
An alternative to allocating on the heap might be allocating in (or close to) the Wasm code space, and use PC-relative addressing. For security reasons, we should try to make the constant pool non-executable, so if we decide to allocate *in* the Wasm code space (instead of close by) we should reserve a whole page and remove execute permissions from that page. I think a good next step would be starting a little design doc (see https://v8.dev/docs/design-review-guidelines). I would propose to initially include @Zhi An Ng <z...@chromium.org> and me, and we can add more people once we figure out a working solution. Thanks for working on this! -Clemens On Tue, Mar 16, 2021 at 11:07 PM Dan Weber <dwe...@gmail.com> wrote: > Hi everyone, > > I wanted to approach the group to understand the feasibility of creating a > root relative constant pool for WASM to support reuse of intermediate > constants generated during complex instruction sequences. > > Right now, when a complex instruction sequence like shuffle or swizzle > operates and cannot find an architectural match for the requested > operation, it generates in flight code to build shuffle masks which are > passed to pshufb on x64 and tbl on a64. Due to the current nature of the > code generator / assembler, these can be regenerated multiple times even if > the input is constant ( > https://bugs.chromium.org/p/v8/issues/detail?id=11545). > > Two options exist for resolving this. > > 1) Generating a constant pool for the code to use at compile time and > loading the data from there. > 2) Lifting sections of multi instruction code up to the graph for > optimization and reduction. > > Each is a partial solution since both can benefit from the other. > > What I'd like to enquire now is the first option -- the feasibility of > implementing an isolate root relative constant pool. > > From what I can see, this might be an easy and effective solution covering > address moves, security concerns, and alignment. > > Since isolate() has a single coherent heap that is garbage collected and > moved (https://v8.dev/blog/embedded-builtins), one can allocate objects > relative to the root. If you use it with the ExternalReference operand > mechanism we've been using, it'll automatically generate all of the address > constants relative to the root register ( > https://source.chromium.org/chromium/chromium/src/+/master:v8/src/codegen/x64/macro-assembler-x64.cc;l=124;bpv=1;bpt=1). > This should preclude any concerns about addresses moving when the heap > moves or gets reallocated. Likewise, isolate()->factory() provides > mechanisms for aligning the pointers on each allocation. As such, if an > external data structure such as a map or hash map is used to track the > constants at code generation time, then each constant can be allocated > individually on the isolate heap without respect to any other. If the heap > persists the entire duration of the executed code and is deallocated at the > end, then there are no memory management concerns. Lastly, there should be > no security concerns since the data allocated on the isolate heap will not > be executable by default. > > Is this correct? If so, what's the appropriate process for submitting and > reviewing a design proposal? > > Dan > > -- > -- > 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/CAAg-m6qF47m8fP81GoeeM4YJBaBAC8%2BY_z%3DcmanviBBeQTsD_A%40mail.gmail.com > <https://groups.google.com/d/msgid/v8-dev/CAAg-m6qF47m8fP81GoeeM4YJBaBAC8%2BY_z%3DcmanviBBeQTsD_A%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- Clemens Backes Software Engineer cleme...@google.com Google Germany GmbH Erika-Mann-Straße 33 80636 München Geschäftsführer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Diese E-Mail ist vertraulich. 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