public inbox for gcc-cvs@sourceware.org
help / color / mirror / Atom feed
* [gcc r14-8994] [libiberty] remove TBAA violation in iterative_hash, improve code-gen
@ 2024-02-15 8:14 Richard Biener
0 siblings, 0 replies; only message in thread
From: Richard Biener @ 2024-02-15 8:14 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:52ac4c6be8664e9dab5b90e7c64df03985791893
commit r14-8994-g52ac4c6be8664e9dab5b90e7c64df03985791893
Author: Richard Biener <rguenther@suse.de>
Date: Wed Feb 14 14:00:23 2024 +0100
[libiberty] remove TBAA violation in iterative_hash, improve code-gen
The following removes the TBAA violation present in iterative_hash.
As we eventually LTO that it's important to fix. This also improves
code generation for the >= 12 bytes loop by using | to compose the
4 byte words as at least GCC 7 and up can recognize that pattern
and perform a 4 byte load while the variant with a + is not
recognized (not on trunk either), I think we have an enhancement bug
for this somewhere.
Given we reliably merge and the bogus "optimized" path might be
only relevant for archs that cannot do misaligned loads efficiently
I've chosen to keep a specialization for aligned accesses.
libiberty/
* hashtab.c (iterative_hash): Remove TBAA violating handling
of aligned little-endian case in favor of just keeping the
aligned case special-cased. Use | for composing a larger word.
Diff:
---
libiberty/hashtab.c | 23 ++++++++++-------------
1 file changed, 10 insertions(+), 13 deletions(-)
diff --git a/libiberty/hashtab.c b/libiberty/hashtab.c
index 48f28078114e..e3a07256a300 100644
--- a/libiberty/hashtab.c
+++ b/libiberty/hashtab.c
@@ -940,26 +940,23 @@ iterative_hash (const void *k_in /* the key */,
c = initval; /* the previous hash value */
/*---------------------------------------- handle most of the key */
-#ifndef WORDS_BIGENDIAN
- /* On a little-endian machine, if the data is 4-byte aligned we can hash
- by word for better speed. This gives nondeterministic results on
- big-endian machines. */
- if (sizeof (hashval_t) == 4 && (((size_t)k)&3) == 0)
- while (len >= 12) /* aligned */
+ /* Provide specialization for the aligned case for targets that cannot
+ efficiently perform misaligned loads of a merged access. */
+ if ((((size_t)k)&3) == 0)
+ while (len >= 12)
{
- a += *(hashval_t *)(k+0);
- b += *(hashval_t *)(k+4);
- c += *(hashval_t *)(k+8);
+ a += (k[0] | ((hashval_t)k[1]<<8) | ((hashval_t)k[2]<<16) | ((hashval_t)k[3]<<24));
+ b += (k[4] | ((hashval_t)k[5]<<8) | ((hashval_t)k[6]<<16) | ((hashval_t)k[7]<<24));
+ c += (k[8] | ((hashval_t)k[9]<<8) | ((hashval_t)k[10]<<16)| ((hashval_t)k[11]<<24));
mix(a,b,c);
k += 12; len -= 12;
}
else /* unaligned */
-#endif
while (len >= 12)
{
- a += (k[0] +((hashval_t)k[1]<<8) +((hashval_t)k[2]<<16) +((hashval_t)k[3]<<24));
- b += (k[4] +((hashval_t)k[5]<<8) +((hashval_t)k[6]<<16) +((hashval_t)k[7]<<24));
- c += (k[8] +((hashval_t)k[9]<<8) +((hashval_t)k[10]<<16)+((hashval_t)k[11]<<24));
+ a += (k[0] | ((hashval_t)k[1]<<8) | ((hashval_t)k[2]<<16) | ((hashval_t)k[3]<<24));
+ b += (k[4] | ((hashval_t)k[5]<<8) | ((hashval_t)k[6]<<16) | ((hashval_t)k[7]<<24));
+ c += (k[8] | ((hashval_t)k[9]<<8) | ((hashval_t)k[10]<<16)| ((hashval_t)k[11]<<24));
mix(a,b,c);
k += 12; len -= 12;
}
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2024-02-15 8:14 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2024-02-15 8:14 [gcc r14-8994] [libiberty] remove TBAA violation in iterative_hash, improve code-gen Richard Biener
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).