From: Owen Avery <[email protected]>
The function we use to calculate fnv128 had a precedence bug when
applying prime128Shift and didn't take advantage of prime128Lower
fitting into a uint32_t.
gcc/rust/ChangeLog:
* util/fnv-hash.h (FNV::write): Tweak calculation.
Signed-off-by: Owen Avery <[email protected]>
Tested on x86_64-pc-linux-gnu.
---
gcc/rust/util/fnv-hash.h | 18 ++++++------------
1 file changed, 6 insertions(+), 12 deletions(-)
diff --git a/gcc/rust/util/fnv-hash.h b/gcc/rust/util/fnv-hash.h
index 4d8a24373ee..1ef5eb617c2 100644
--- a/gcc/rust/util/fnv-hash.h
+++ b/gcc/rust/util/fnv-hash.h
@@ -47,31 +47,25 @@ public:
unsigned char c = in[i];
//
https://stackoverflow.com/questions/28868367/getting-the-high-part-of-64-bit-integer-multiplication
+ // modified since a is sub-32-bit
uint64_t a = prime128Lower;
uint64_t b = buf[1];
- uint64_t a_lo = (uint32_t) a;
- uint64_t a_hi = a >> 32;
uint64_t b_lo = (uint32_t) b;
uint64_t b_hi = b >> 32;
- uint64_t a_x_b_hi = a_hi * b_hi;
- uint64_t a_x_b_mid = a_hi * b_lo;
- uint64_t b_x_a_mid = b_hi * a_lo;
- uint64_t a_x_b_lo = a_lo * b_lo;
+ uint64_t a_x_b_mid = a * b_hi;
+ uint64_t a_x_b_lo = a * b_lo;
uint64_t carry_bit
- = ((uint64_t) (uint32_t) a_x_b_mid + (uint64_t) (uint32_t) b_x_a_mid
- + (a_x_b_lo >> 32))
- >> 32;
+ = ((uint64_t) (uint32_t) a_x_b_mid + (a_x_b_lo >> 32)) >> 32;
- uint64_t multhi
- = a_x_b_hi + (a_x_b_mid >> 32) + (b_x_a_mid >> 32) + carry_bit;
+ uint64_t multhi = (a_x_b_mid >> 32) + carry_bit;
uint64_t s0 = multhi; // high
uint64_t s1 = prime128Lower * buf[1]; // low
- s0 += buf[1] << (prime128Shift + prime128Lower * buf[0]);
+ s0 += (buf[1] << prime128Shift) + prime128Lower * buf[0];
// Update the values
buf[1] = s1;
--
2.55.0
