Commit 4fc1eb4f authored by Linus Torvalds's avatar Linus Torvalds Committed by Ben Hutchings

Make hash_64() use a 64-bit multiply when appropriate

commit 23d0db76 upstream.

The hash_64() function historically does the multiply by the
GOLDEN_RATIO_PRIME_64 number with explicit shifts and adds, because
unlike the 32-bit case, gcc seems unable to turn the constant multiply
into the more appropriate shift and adds when required.

However, that means that we generate those shifts and adds even when the
architecture has a fast multiplier, and could just do it better in
hardware.

Use the now-cleaned-up CONFIG_ARCH_HAS_FAST_MULTIPLIER (together with
"is it a 64-bit architecture") to decide whether to use an integer
multiply or the explicit sequence of shift/add instructions.
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
[bwh: This has no immediate effect in 3.16 because nothing defines
 CONFIG_ARCH_HAS_FAST_MULTIPLIER. However the following fix removes
 that condition.]
parent 17cabcb4
......@@ -37,6 +37,9 @@ static __always_inline u64 hash_64(u64 val, unsigned int bits)
{
u64 hash = val;
#if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
hash = hash * GOLDEN_RATIO_PRIME_64;
#else
/* Sigh, gcc can't optimise this alone like it does for 32 bits. */
u64 n = hash;
n <<= 18;
......@@ -51,6 +54,7 @@ static __always_inline u64 hash_64(u64 val, unsigned int bits)
hash += n;
n <<= 2;
hash += n;
#endif
/* High bits are more random, so use them. */
return hash >> (64 - bits);
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment