[PATCH v2 1/9] bitfield: add FIELD_GET_SIGNED()

Yury Norov
2026-04-27T21:41:52Z

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>
---
include/linux/bitfield.h | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 54aeeef1f0ec..cd44013281c7 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -178,6 +178,22 @@
__FIELD_GET(_mask, _reg, "FIELD_GET: "); \
})

+/**
+ * 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
--
2.51.0