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Restore the augmented strlen commentary

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... lost in revert
This commit is contained in:
Mateusz Guzik 2021-02-07 19:53:34 +00:00
parent 81e074d57d
commit 3acea07c18
2 changed files with 8 additions and 22 deletions
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15
lib/libc/string/strlen.c
View file

@ -35,10 +35,6 @@ __FBSDID("$FreeBSD$");
/*
* Portable strlen() for 32-bit and 64-bit systems.
*
* Rationale: it is generally much more efficient to do word length
* operations and avoid branches on modern computer systems, as
* compared to byte-length operations with a lot of branches.
*
* The expression:
*
* ((x - 0x01....01) & ~x & 0x80....80)
@ -46,15 +42,12 @@ __FBSDID("$FreeBSD$");
* would evaluate to a non-zero value iff any of the bytes in the
* original word is zero.
*
* On multi-issue processors, we can divide the above expression into:
* a) (x - 0x01....01)
* b) (~x & 0x80....80)
* c) a & b
*
* Where, a) and b) can be partially computed in parallel.
*
* The algorithm above is found on "Hacker's Delight" by
* Henry S. Warren, Jr.
*
* Note: this leaves performance on the table and each architecture
* would be best served with a tailor made routine instead, even if
* using the same trick.
*/
/* Magic numbers for the algorithm */

15
sys/libkern/strlen.c
View file

@ -34,10 +34,6 @@ __FBSDID("$FreeBSD$");
/*
* Portable strlen() for 32-bit and 64-bit systems.
*
* Rationale: it is generally much more efficient to do word length
* operations and avoid branches on modern computer systems, as
* compared to byte-length operations with a lot of branches.
*
* The expression:
*
* ((x - 0x01....01) & ~x & 0x80....80)
@ -45,15 +41,12 @@ __FBSDID("$FreeBSD$");
* would evaluate to a non-zero value iff any of the bytes in the
* original word is zero.
*
* On multi-issue processors, we can divide the above expression into:
* a) (x - 0x01....01)
* b) (~x & 0x80....80)
* c) a & b
*
* Where, a) and b) can be partially computed in parallel.
*
* The algorithm above is found on "Hacker's Delight" by
* Henry S. Warren, Jr.
*
* Note: this leaves performance on the table and each architecture
* would be best served with a tailor made routine instead, even if
* using the same trick.
*/
/* Magic numbers for the algorithm */