Add signed and unsigned integer types?

[abrown]

Feature

Add unsigned integer types to the Cranelift type system.

Benefit

Halve the number of cranelift instructions of the form uload*/sload*, uadd_sat/sadd_sat, etc.

Implementation

The real question would be whether the Type struct (cranelift-codegen/src/ir/types.rs) has enough index space to fit all of the new unsigned variants; if it does, the work could proceed along the lines of:

• Alter cranelift-codegen/meta/src/shared/types.rs to include SignedInt and UnsignedInt enums
• Add the new type to the CDSL, e.g. LaneType in cranelift-codegen/meta/src/cdsl/types.rs
• Generate the new types in cranelift-codegen/meta/src/gen_types.rs
• Add necessary helper functions in cranelift-codegen/src/ir/types.rs
• Modify the CLIF parser to differentiate between types u* and i*
• Finally, replace the extra instructions with simpler ones that use, e.g., the u32 vs i32 controlling type to determine the encoding

Alternatives

This is more an RFC than a real proposal so the alternative is to leave things as they are.

[sunfishcode]

One downside of this is that it requires new instructions for signed<->unsigned casts. These would need to exist in the IR, but they'd be effectively no-op instructions, so we'd likely end up teaching pattern-matching code, GVN, and other things to look past them, and to teach register allocation to coalesce them like copies.

Also, since wasm doesn't have signed/unsigned types, wasm translation would be a little awkward, because at the point where we see eg. an i32.load, we wouldn't know whether to give it a signed or unsigned type until we see a user of the loaded value that cares about the signedness. So we'd either have to lookahead, or we'd have to guess and insert signed<->unsigned casts to fix things up when we guess wrong.

[abrown]

since wasm doesn't have signed/unsigned types, wasm translation would be a little awkward

Yup. Closing this; it would be nice to have to reduce the number of Cranelift instructions but I see what you mean by the added awkwardness.