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Adopted shift-table hack for lfsr_tag_mask

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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:

                          .-- LFSR_TAG_MASK0
                         .|-- LFSR_TAG_MASK2
                        .||-- LFSR_TAG_MASK8
                       .|||-- LFSR_TAG_MASK12
                       vvvv
  0x0fff & (-1U << ((0xc820 >> (4*((tag >> 12) & 0x3))) & 0xf))
  '--.-'      ^                   '--------.--------'
  key mask  gcc complains w/o this     mask bits

Saves a bit of code at the cost of some stack. I guess because GCC is
trying to avoid multiple constant pool lookups? This may just be
compiler noise:

           code          stack          ctx
  before: 35692           2432          640
  after:  35688 (-0.0%)   2440 (+0.3%)  640 (+0.0%)
This commit is contained in:
Christopher Haster
2025-04-20 17:13:14 -05:00
parent 670b8e6732
commit 7cd4c1f12f
1 changed files with 1 additions and 5 deletions
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lfs.c
+1 -5
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@@ -1255,12 +1255,8 @@ static inline bool lfsr_tag_ismask12(lfsr_tag_t tag) {
return ((tag >> 12) & 0x3) == 3;
}
static const uint16_t lfsr_tag_masktable[4] = {
0x0fff, 0x0ffc, 0x0f00, 0x0000
};
static inline lfsr_tag_t lfsr_tag_mask(lfsr_tag_t tag) {
return lfsr_tag_masktable[(tag >> 12) & 0x3];
return 0x0fff & (-1U << ((0xc820 >> (4*((tag >> 12) & 0x3))) & 0xf));
}
// alt operations