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From: Sunil Pandey <skpgkp2@sourceware.org> To: glibc-cvs@sourceware.org Subject: [glibc/release/2.35/master] x86: Cleanup bounds checking in large memcpy case Date: Mon, 18 Jul 2022 20:58:37 +0000 (GMT) [thread overview] Message-ID: <20220718205837.C29453852776@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=ce32ad91eb7b50442d033e3ef37d9800f8fcfdde commit ce32ad91eb7b50442d033e3ef37d9800f8fcfdde 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 af51177d5d..d1518b8bab 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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