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nofl space: Rework treatment of mark bits to avoid masks

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This will allow us to free up some metadata bits.
This commit is contained in:
Andy Wingo 2025-03-05 17:01:51 +01:00
parent 4d271e7492
commit 29cf0f40d3
2 changed files with 100 additions and 44 deletions
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50
src/swar.h
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@ -31,7 +31,7 @@ match_bytes_against_bits(uint64_t bytes, uint8_t mask) {
return bytes & broadcast_byte(mask);
}
static size_t
static inline size_t
scan_for_byte_with_bits(uint8_t *ptr, size_t limit, uint8_t mask) {
size_t n = 0;
size_t unaligned = ((uintptr_t) ptr) & 7;
@ -53,6 +53,52 @@ scan_for_byte_with_bits(uint8_t *ptr, size_t limit, uint8_t mask) {
return limit;
}
static inline uint64_t
match_bytes_against_tag(uint64_t bytes, uint8_t mask, uint8_t tag) {
// Precondition: tag within mask.
GC_ASSERT_EQ(tag & mask, tag);
// Precondition: high bit of mask byte is empty, so that we can add without
// overflow.
GC_ASSERT_EQ(mask & 0x7f, mask);
// Precondition: mask is low bits of byte.
GC_ASSERT(mask);
GC_ASSERT_EQ(mask & (mask + 1), 0);
uint64_t vmask = broadcast_byte(mask);
uint64_t vtest = broadcast_byte(mask + 1);
uint64_t vtag = broadcast_byte(tag);
bytes &= vmask;
uint64_t m = (bytes ^ vtag) + vmask;
return (m & vtest) ^ vtest;
}
static inline size_t
scan_for_byte_with_tag(uint8_t *ptr, size_t limit, uint8_t mask, uint8_t tag) {
// The way we handle unaligned reads by padding high bytes with zeroes assumes
// that all-zeroes is not a matching byte.
GC_ASSERT(tag);
size_t n = 0;
size_t unaligned = ((uintptr_t) ptr) & 7;
if (unaligned) {
uint64_t bytes = load_eight_aligned_bytes(ptr - unaligned) >> (unaligned * 8);
uint64_t match = match_bytes_against_tag(bytes, mask, tag);
if (match)
return count_zero_bytes(match);
n += 8 - unaligned;
}
for(; n < limit; n += 8) {
uint64_t bytes = load_eight_aligned_bytes(ptr + n);
uint64_t match = match_bytes_against_tag(bytes, mask, tag);
if (match)
return n + count_zero_bytes(match);
}
return limit;
}
static inline uint64_t
match_bytes_against_2_tags(uint64_t bytes, uint8_t mask, uint8_t tag1,
uint8_t tag2)
@ -78,7 +124,7 @@ match_bytes_against_2_tags(uint64_t bytes, uint8_t mask, uint8_t tag1,
return ((m1 & m2) & vtest) ^ vtest;
}
static size_t
static inline size_t
scan_for_byte_with_tags(uint8_t *ptr, size_t limit, uint8_t mask,
uint8_t tag1, uint8_t tag2) {
// The way we handle unaligned reads by padding high bytes with zeroes assumes