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bitfield: add FIELD_GET_SIGNED()

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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
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16
include/linux/bitfield.h
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@@ -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