Github/littlefs
1
Fork 0
Code Actions Activity

Added shift-table hack comment to lfs3_tag_mask

Browse Source 
Was reading through some old commits (7cd4c1f), and realized this should
really be committed into the code.
This commit is contained in:
Christopher Haster
2025-12-04 00:33:05 -06:00
parent 7ccdee255b
commit ee50dc5307
1 changed files with 13 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lfs3.c
+13
View File
@@ -1107,7 +1107,20 @@ static inline bool lfs3_tag_ismask12(lfs3_tag_t tag) {
} }
static inline lfs3_tag_t lfs3_tag_mask(lfs3_tag_t tag) { static inline lfs3_tag_t lfs3_tag_mask(lfs3_tag_t tag) {
// this is based off the parity impl in Sean Eron Anderson's Bit
// Twiddling Hacks, who attributes the idea to Mathew Hendry
//
// basically the idea is to encode a small lookup table in an
// integer, and extract using a shift + mask
//
// .-- LFS3_tag_MASK0
// .|-- LFS3_tag_MASK2
// .||-- LFS3_tag_MASK8
// .|||-- LFS3_tag_MASK12
// vvvv
return 0x0fff & (-1U << ((0xc820 >> (4*((tag >> 12) & 0x3))) & 0xf)); return 0x0fff & (-1U << ((0xc820 >> (4*((tag >> 12) & 0x3))) & 0xf));
// '--.-' ^ '--------.--------'
// key mask gcc complains w/o this mask bits
} }
// alt operations // alt operations