diff options
| author | Yury Norov <ynorov@nvidia.com> | 2026-04-27 17:41:18 -0400 |
|---|---|---|
| committer | Yury Norov <ynorov@nvidia.com> | 2026-05-20 11:18:10 -0400 |
| commit | 2c3f0541b99c60ac079228627ea325317e7d8b2a (patch) | |
| tree | 6d42e918d416a16763a816f039969da86bf88075 /include/linux | |
| parent | 27fa82620cbaa89a7fc11ac3057701d598813e87 (diff) | |
bitfield: add FIELD_GET_SIGNED()
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>
Diffstat (limited to 'include/linux')
| -rw-r--r-- | include/linux/bitfield.h | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h index 54aeeef1f0ec..989c214ec8a5 100644 --- a/include/linux/bitfield.h +++ b/include/linux/bitfield.h @@ -179,6 +179,22 @@ }) /** + * FIELD_GET_SIGNED() - extract a signed bitfield element + * @mask: shifted mask defining the field's length and position + * @reg: value of entire bitfield + * + * Returns the sign-extended field specified by @_mask from the + * bitfield passed in as @reg by masking and shifting it down. + */ +#define FIELD_GET_SIGNED(mask, reg) \ + ({ \ + __BF_FIELD_CHECK(mask, reg, 0U, "FIELD_GET_SIGNED: "); \ + ((__signed_scalar_typeof(mask)) \ + (((long long)(reg) << __builtin_clzll(mask)) >> \ + (__builtin_clzll(mask) + __builtin_ctzll(mask)))); \ + }) + +/** * FIELD_MODIFY() - modify a bitfield element * @_mask: shifted mask defining the field's length and position * @_reg_p: pointer to the memory that should be updated |