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* [glibc/release/2.33/master] x86: Cleanup bounds checking in large memcpy case
@ 2022-07-19 5:54 Sunil Pandey
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From: Sunil Pandey @ 2022-07-19 5:54 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4e7f3b34a494b18e00f3b49d7794149c4bf03afc
commit 4e7f3b34a494b18e00f3b49d7794149c4bf03afc
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Wed Jun 15 10:41:28 2022 -0700
x86: Cleanup bounds checking in large memcpy case
1. Fix incorrect lower-bound threshold in L(large_memcpy_2x).
Previously was using `__x86_rep_movsb_threshold` and should
have been using `__x86_shared_non_temporal_threshold`.
2. Avoid reloading __x86_shared_non_temporal_threshold before
the L(large_memcpy_4x) bounds check.
3. Document the second bounds check for L(large_memcpy_4x)
more clearly.
(cherry picked from commit 89a25c6f64746732b87eaf433af0964b564d4a92)
Diff:
---
.../x86_64/multiarch/memmove-vec-unaligned-erms.S | 29 ++++++++++++++++------
1 file changed, 21 insertions(+), 8 deletions(-)
diff --git a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
index 7b27cbdda5..618d46d8ce 100644
--- a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
+++ b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
@@ -118,7 +118,13 @@
# define LARGE_LOAD_SIZE (VEC_SIZE * 4)
#endif
-/* Amount to shift rdx by to compare for memcpy_large_4x. */
+/* Amount to shift __x86_shared_non_temporal_threshold by for
+ bound for memcpy_large_4x. This is essentially use to to
+ indicate that the copy is far beyond the scope of L3
+ (assuming no user config x86_non_temporal_threshold) and to
+ use a more aggressively unrolled loop. NB: before
+ increasing the value also update initialization of
+ x86_non_temporal_threshold. */
#ifndef LOG_4X_MEMCPY_THRESH
# define LOG_4X_MEMCPY_THRESH 4
#endif
@@ -724,9 +730,14 @@ L(skip_short_movsb_check):
.p2align 4,, 10
#if (defined USE_MULTIARCH || VEC_SIZE == 16) && IS_IN (libc)
L(large_memcpy_2x_check):
- cmp __x86_rep_movsb_threshold(%rip), %RDX_LP
- jb L(more_8x_vec_check)
+ /* Entry from L(large_memcpy_2x) has a redundant load of
+ __x86_shared_non_temporal_threshold(%rip). L(large_memcpy_2x)
+ is only use for the non-erms memmove which is generally less
+ common. */
L(large_memcpy_2x):
+ mov __x86_shared_non_temporal_threshold(%rip), %R11_LP
+ cmp %R11_LP, %RDX_LP
+ jb L(more_8x_vec_check)
/* To reach this point it is impossible for dst > src and
overlap. Remaining to check is src > dst and overlap. rcx
already contains dst - src. Negate rcx to get src - dst. If
@@ -774,18 +785,21 @@ L(large_memcpy_2x):
/* ecx contains -(dst - src). not ecx will return dst - src - 1
which works for testing aliasing. */
notl %ecx
+ movq %rdx, %r10
testl $(PAGE_SIZE - VEC_SIZE * 8), %ecx
jz L(large_memcpy_4x)
- movq %rdx, %r10
- shrq $LOG_4X_MEMCPY_THRESH, %r10
- cmp __x86_shared_non_temporal_threshold(%rip), %r10
+ /* r11 has __x86_shared_non_temporal_threshold. Shift it left
+ by LOG_4X_MEMCPY_THRESH to get L(large_memcpy_4x) threshold.
+ */
+ shlq $LOG_4X_MEMCPY_THRESH, %r11
+ cmp %r11, %rdx
jae L(large_memcpy_4x)
/* edx will store remainder size for copying tail. */
andl $(PAGE_SIZE * 2 - 1), %edx
/* r10 stores outer loop counter. */
- shrq $((LOG_PAGE_SIZE + 1) - LOG_4X_MEMCPY_THRESH), %r10
+ shrq $(LOG_PAGE_SIZE + 1), %r10
/* Copy 4x VEC at a time from 2 pages. */
.p2align 4
L(loop_large_memcpy_2x_outer):
@@ -850,7 +864,6 @@ L(large_memcpy_2x_end):
.p2align 4
L(large_memcpy_4x):
- movq %rdx, %r10
/* edx will store remainder size for copying tail. */
andl $(PAGE_SIZE * 4 - 1), %edx
/* r10 stores outer loop counter. */
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