Stream: git-cranelift

Topic: cranelift / Issue #631 Bitfield Extract/Create instructions

GitHub (Feb 28 2020 at 23:25):

alexcrichton transferred Issue #631:

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 06 2025 at 06:05 UTC