Stream: git-wasmtime

Topic: wasmtime / issue #1067 Bitfield Extract/Create instructions

Wasmtime GitHub notifications bot (May 04 2022 at 20:42):

cfallin labeled issue #1067:

This would add two new instructions to cranelift, bextr and bmak. They provide a easy to optimize, and easy to generate mechanism for bitfield manipulation.

bextr

a = bextr target, size, offset

bextr will extract a _n_ bit large field at the provided offset, and return it in a, shifted so that the LSB of the field is also the LSB of a.
a's type is inferred from target

Visual example

offset = 4
size = 4
typeof target = i8

xxxx **** 
\__/----V
0000 xxxx

bmak

a = bmak target, size, offset

bmak will fill a _n_ bit large field at the provided offset with 1s, the rest with 0. The generated field will then be applied to target using a bitwise AND, and the result will be returned in a.
a's type is inferred from target
bmak is designed so that you can simply use a bitwise or to combine the fields.

Visual Example

offset = 2
size = 4
typeof target = i8

xx0101xx
  \__/
  /  \
00010100

Wasmtime GitHub notifications bot (May 04 2022 at 20:42):

cfallin labeled issue #1067:

This would add two new instructions to cranelift, bextr and bmak. They provide a easy to optimize, and easy to generate mechanism for bitfield manipulation.

bextr

a = bextr target, size, offset

bextr will extract a _n_ bit large field at the provided offset, and return it in a, shifted so that the LSB of the field is also the LSB of a.
a's type is inferred from target

Visual example

offset = 4
size = 4
typeof target = i8

xxxx **** 
\__/----V
0000 xxxx

bmak

a = bmak target, size, offset

bmak will fill a _n_ bit large field at the provided offset with 1s, the rest with 0. The generated field will then be applied to target using a bitwise AND, and the result will be returned in a.
a's type is inferred from target
bmak is designed so that you can simply use a bitwise or to combine the fields.

Visual Example

offset = 2
size = 4
typeof target = i8

xx0101xx
  \__/
  /  \
00010100

Last updated: Dec 23 2024 at 12:05 UTC