Stream: git-wasmtime

Topic: wasmtime / Issue #1088 Some RISC legalization questions?

Wasmtime GitHub notifications bot (Feb 03 2021 at 20:34):

bjorn3 commented on Issue #1088:

The new backend framework doesn't use legalizations as much as the old framework.

Wasmtime GitHub notifications bot (Feb 03 2021 at 20:41):

cfallin commented on Issue #1088:

The AArch64 backend shows how these issues can be addressed in new-style backends. Specifically, iconst is lowered to one or several instructions depending on the value, so the backend can choose whatever synthesis strategy (constant pool, synthesizing from several parts, etc.) makes the most sense. Branches can be lowered to one or several instructions as necessary -- to the specific issue above regarding value widths supported by brz/brnz, we could add widening instructions as needed in the lowering.

Wasmtime GitHub notifications bot (Feb 03 2021 at 20:41):

cfallin closed Issue #1088:

Hi,

In doing my MIPS64 port I found a couple of blockers that impede support for other RISC architectures as well.

• iconst can't load immediates larger than the instruction's slot; and
• brz brnz and possibly other instructions can only operate on native-sized operands.

How do we best implement constant materialization for RISC architectures, an arch-specific approach or a generic one with knowledge of things like li/lui/shl/or?

As for the latter, I tried widen but it didn't work. I think that's because brz and similar instructions is not "polymorphic" like with those named foo.iB. How do I specify the correct widening behavior in this case?

Last updated: Oct 23 2024 at 20:03 UTC