bitfield: add FIELD_GET_SIGNED()

Browse source

The bitfields are designed in assumption that fields contain unsigned
integer values, thus extracting the values from the field implies
zero-extending.

Some drivers need to sign-extend their fields, and currently do it like:

	dc_re += sign_extend32(FIELD_GET(0xfff000, tmp), 11);
	dc_im += sign_extend32(FIELD_GET(0xfff, tmp), 11);

It's error-prone because it relies on user to provide the correct
index of the most significant bit and proper 32 vs 64 function flavor.

Thus, introduce a FIELD_GET_SIGNED(). With the new API, the above
snippet turns into the more convenient:

	dc_re += FIELD_GET_SIGNED(0xfff000, tmp);
	dc_im += FIELD_GET_SIGNED(0xfff, tmp);

It compiles (on x86_64) into just a couple instructions: shl and sar.
When the mask includes MSB, the '<< __builtin_clzll(mask)' part becomes
a NOP, and the compiler only emits a single sar:

   long long foo(long long reg)
  {
    10:   f3 0f 1e fa             endbr64
          return FIELD_GET_SIGNED(GENMASK_ULL(63, 60), reg);
    14:   48 89 f8                mov    %rdi,%rax
    17:   48 c1 f8 3c             sar    $0x3c,%rax
  }

32-bit code generation is equally well. On arm32:

  long long foo(long long reg)
  {
         return FIELD_GET_SIGNED(0x00f00000ULL, reg);
  }

generates:

  foo(long long):
        lsls    r1, r0, #8
        asrs    r0, r1, #28
        asrs    r1, r1, #31
        bx      lr

Signed-off-by: Yury Norov <ynorov@nvidia.com>

...

This commit is contained in:

Yury Norov 2026-04-27 17:41:18 -04:00

parent 27fa82620c

commit 2c3f0541b9

1 changed files with 16 additions and 0 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

16

include/linux/bitfield.h

View file