[PATCH 3/3] crypto: x86/crc32c - eliminate jump table and excessive unrolling
From: Eric Biggers
Date: Mon Oct 14 2024 - 00:26:00 EST
From: Eric Biggers <ebiggers@xxxxxxxxxx>
crc32c-pcl-intel-asm_64.S has a loop with 1 to 127 iterations fully
unrolled and uses a jump table to jump into the correct location. This
optimization is misguided, as it bloats the binary code size and
introduces an indirect call. x86_64 CPUs can predict loops well, so it
is fine to just use a loop instead. Loop bookkeeping instructions can
compete with the crc instructions for the ALUs, but this is easily
mitigated by unrolling the loop by a smaller amount, such as 4 times.
Therefore, re-roll the loop and make related tweaks to the code.
This reduces the binary code size of crc_pclmul() from 4546 bytes to 418
bytes, a 91% reduction. In general it also makes the code faster, with
some large improvements seen when retpoline is enabled.
More detailed performance results are shown below. They are given as
percent improvement in throughput (negative means regressed) for CPU
microarchitecture vs. input length in bytes. E.g. an improvement from
40 GB/s to 50 GB/s would be listed as 25%.
Table 1: Results with retpoline enabled (the default):
| 512 | 833 | 1024 | 2000 | 3173 | 4096 |
---------------------+-------+-------+-------+------ +-------+-------+
Intel Haswell | 35.0% | 20.7% | 17.8% | 9.7% | -0.2% | 4.4% |
Intel Emerald Rapids | 66.8% | 45.2% | 36.3% | 19.3% | 0.0% | 5.4% |
AMD Zen 2 | 29.5% | 17.2% | 13.5% | 8.6% | -0.5% | 2.8% |
Table 2: Results with retpoline disabled:
| 512 | 833 | 1024 | 2000 | 3173 | 4096 |
---------------------+-------+-------+-------+------ +-------+-------+
Intel Haswell | 3.3% | 4.8% | 4.5% | 0.9% | -2.9% | 0.3% |
Intel Emerald Rapids | 7.5% | 6.4% | 5.2% | 2.3% | -0.0% | 0.6% |
AMD Zen 2 | 11.8% | 1.4% | 0.2% | 1.3% | -0.9% | -0.2% |
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
arch/x86/crypto/crc32c-pcl-intel-asm_64.S | 233 +++++++++-------------
1 file changed, 92 insertions(+), 141 deletions(-)
diff --git a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
index bbf860e90951d..752812bc4991d 100644
--- a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
+++ b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
@@ -5,10 +5,11 @@
* downloaded from:
* http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/crc-iscsi-polynomial-crc32-instruction-paper.pdf
* http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/fast-crc-computation-paper.pdf
*
* Copyright (C) 2012 Intel Corporation.
+ * Copyright 2024 Google LLC
*
* Authors:
* Wajdi Feghali <wajdi.k.feghali@xxxxxxxxx>
* James Guilford <james.guilford@xxxxxxxxx>
* David Cote <david.m.cote@xxxxxxxxx>
@@ -42,186 +43,153 @@
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include <linux/linkage.h>
-#include <asm/nospec-branch.h>
## ISCSI CRC 32 Implementation with crc32 and pclmulqdq Instruction
-.macro LABEL prefix n
-.L\prefix\n\():
-.endm
-
-.macro JMPTBL_ENTRY i
-.quad .Lcrc_\i
-.endm
-
# Define threshold below which buffers are considered "small" and routed to
# regular CRC code that does not interleave the CRC instructions.
#define SMALL_SIZE 200
# unsigned int crc_pcl(const u8 *buffer, unsigned int len, unsigned int crc_init);
.text
SYM_FUNC_START(crc_pcl)
-#define bufp rdi
-#define bufp_dw %edi
-#define bufp_w %di
-#define bufp_b %dil
-#define bufptmp %rcx
-#define block_0 %rcx
-#define block_1 %rdx
-#define block_2 %r11
-#define len %esi
-#define crc_init_arg %edx
-#define tmp %rbx
-#define crc_init %r8d
-#define crc_init_q %r8
-#define crc1 %r9
-#define crc2 %r10
-
- pushq %rbx
- pushq %rdi
- pushq %rsi
+#define bufp %rdi
+#define bufp_d %edi
+#define len %esi
+#define crc_init %edx
+#define crc_init_q %rdx
+#define n_misaligned %ecx /* overlaps chunk_bytes! */
+#define n_misaligned_q %rcx
+#define chunk_bytes %ecx /* overlaps n_misaligned! */
+#define chunk_bytes_q %rcx
+#define crc1 %r8
+#define crc2 %r9
- ## Move crc_init for Linux to a different
- mov crc_init_arg, crc_init
-
- mov %bufp, bufptmp # rdi = *buf
cmp $SMALL_SIZE, len
jb .Lsmall
################################################################
## 1) ALIGN:
################################################################
- neg %bufp
- and $7, %bufp # calculate the unalignment amount of
+ mov bufp_d, n_misaligned
+ neg n_misaligned
+ and $7, n_misaligned # calculate the misalignment amount of
# the address
- je .Lproc_block # Skip if aligned
+ je .Laligned # Skip if aligned
+ # Process 1 <= n_misaligned <= 7 bytes individually in order to align
+ # the remaining data to an 8-byte boundary.
.Ldo_align:
- #### Calculate CRC of unaligned bytes of the buffer (if any)
- movq (bufptmp), tmp # load a quadward from the buffer
- add %bufp, bufptmp # align buffer pointer for quadword
- # processing
- sub bufp_dw, len # update buffer length
+ movq (bufp), %rax
+ add n_misaligned_q, bufp
+ sub n_misaligned, len
.Lalign_loop:
- crc32b %bl, crc_init # compute crc32 of 1-byte
- shr $8, tmp # get next byte
- dec %bufp
+ crc32b %al, crc_init # compute crc32 of 1-byte
+ shr $8, %rax # get next byte
+ dec n_misaligned
jne .Lalign_loop
-
-.Lproc_block:
+.Laligned:
################################################################
- ## 2) PROCESS BLOCKS:
+ ## 2) PROCESS BLOCK:
################################################################
- ## compute num of bytes to be processed
-
cmp $128*24, len
jae .Lfull_block
-.Lcontinue_block:
- ## len < 128*24
- movq $2731, %rax # 2731 = ceil(2^16 / 24)
- mul len
- shrq $16, %rax
-
- ## eax contains floor(bytes / 24) = num 24-byte chunks to do
-
- ## process rax 24-byte chunks (128 >= rax >= 0)
-
- ## compute end address of each block
- ## block 0 (base addr + RAX * 8)
- ## block 1 (base addr + RAX * 16)
- ## block 2 (base addr + RAX * 24)
- lea (bufptmp, %rax, 8), block_0
- lea (block_0, %rax, 8), block_1
- lea (block_1, %rax, 8), block_2
-
- xor crc1, crc1
- xor crc2, crc2
-
- ## branch into array
- leaq jump_table(%rip), %bufp
- mov (%bufp,%rax,8), %bufp
- JMP_NOSPEC bufp
+.Lpartial_block:
+ # Compute floor(len / 24) to get num qwords to process from each lane.
+ imul $2731, len, %eax # 2731 = ceil(2^16 / 24)
+ shr $16, %eax
+ jmp .Lcrc_3lanes
- ################################################################
- ## 2a) PROCESS FULL BLOCKS:
- ################################################################
.Lfull_block:
- movl $128,%eax
- lea 128*8*2(block_0), block_1
- lea 128*8*3(block_0), block_2
- add $128*8*1, block_0
-
- xor crc1,crc1
- xor crc2,crc2
-
- # Fall through into top of crc array (crc_128)
+ # Processing 128 qwords from each lane.
+ mov $128, %eax
################################################################
- ## 3) CRC Array:
+ ## 3) CRC each of three lanes:
################################################################
- i=128
-.rept 128-1
-.altmacro
-LABEL crc_ %i
-.noaltmacro
- ENDBR
- crc32q -i*8(block_0), crc_init_q
- crc32q -i*8(block_1), crc1
- crc32q -i*8(block_2), crc2
- i=(i-1)
-.endr
-
-.altmacro
-LABEL crc_ %i
-.noaltmacro
- ENDBR
- crc32q -i*8(block_0), crc_init_q
- crc32q -i*8(block_1), crc1
-# SKIP crc32 -i*8(block_2), crc2 ; Don't do this one yet
+.Lcrc_3lanes:
+ xor crc1,crc1
+ xor crc2,crc2
+ mov %eax, chunk_bytes
+ shl $3, chunk_bytes # num bytes to process from each lane
+ sub $5, %eax # 4 for 4x_loop, 1 for special last iter
+ jl .Lcrc_3lanes_4x_done
+
+ # Unroll the loop by a factor of 4 to reduce the overhead of the loop
+ # bookkeeping instructions, which can compete with crc32q for the ALUs.
+.Lcrc_3lanes_4x_loop:
+ crc32q (bufp), crc_init_q
+ crc32q (bufp,chunk_bytes_q), crc1
+ crc32q (bufp,chunk_bytes_q,2), crc2
+ crc32q 8(bufp), crc_init_q
+ crc32q 8(bufp,chunk_bytes_q), crc1
+ crc32q 8(bufp,chunk_bytes_q,2), crc2
+ crc32q 16(bufp), crc_init_q
+ crc32q 16(bufp,chunk_bytes_q), crc1
+ crc32q 16(bufp,chunk_bytes_q,2), crc2
+ crc32q 24(bufp), crc_init_q
+ crc32q 24(bufp,chunk_bytes_q), crc1
+ crc32q 24(bufp,chunk_bytes_q,2), crc2
+ add $32, bufp
+ sub $4, %eax
+ jge .Lcrc_3lanes_4x_loop
+
+.Lcrc_3lanes_4x_done:
+ add $4, %eax
+ jz .Lcrc_3lanes_last_qword
+
+.Lcrc_3lanes_1x_loop:
+ crc32q (bufp), crc_init_q
+ crc32q (bufp,chunk_bytes_q), crc1
+ crc32q (bufp,chunk_bytes_q,2), crc2
+ add $8, bufp
+ dec %eax
+ jnz .Lcrc_3lanes_1x_loop
- mov block_2, block_0
+.Lcrc_3lanes_last_qword:
+ crc32q (bufp), crc_init_q
+ crc32q (bufp,chunk_bytes_q), crc1
+# SKIP crc32q (bufp,chunk_bytes_q,2), crc2 ; Don't do this one yet
################################################################
## 4) Combine three results:
################################################################
- lea (K_table-8)(%rip), %bufp # first entry is for idx 1
- shlq $3, %rax # rax *= 8
- pmovzxdq (%bufp,%rax), %xmm0 # 2 consts: K1:K2
- leal (%eax,%eax,2), %eax # rax *= 3 (total *24)
- sub %eax, len # len -= rax*24
+ lea (K_table-8)(%rip), %rax # first entry is for idx 1
+ pmovzxdq (%rax,chunk_bytes_q), %xmm0 # 2 consts: K1:K2
+ lea (chunk_bytes,chunk_bytes,2), %eax # chunk_bytes * 3
+ sub %eax, len # len -= chunk_bytes * 3
movq crc_init_q, %xmm1 # CRC for block 1
pclmulqdq $0x00, %xmm0, %xmm1 # Multiply by K2
movq crc1, %xmm2 # CRC for block 2
pclmulqdq $0x10, %xmm0, %xmm2 # Multiply by K1
pxor %xmm2,%xmm1
movq %xmm1, %rax
- xor -i*8(block_2), %rax
+ xor (bufp,chunk_bytes_q,2), %rax
mov crc2, crc_init_q
crc32 %rax, crc_init_q
+ lea 8(bufp,chunk_bytes_q,2), bufp
################################################################
- ## 5) Check for end:
+ ## 5) If more blocks remain, goto (2):
################################################################
-LABEL crc_ 0
- ENDBR
cmp $128*24, len
- jae .Lfull_block
+ jae .Lfull_block
cmp $SMALL_SIZE, len
- jae .Lcontinue_block
+ jae .Lpartial_block
#######################################################################
## 6) Process any remainder without interleaving:
#######################################################################
.Lsmall:
@@ -229,51 +197,34 @@ LABEL crc_ 0
jz .Ldone
mov len, %eax
shr $3, %eax
jz .Ldo_dword
.Ldo_qwords:
- crc32q (bufptmp), crc_init_q
- add $8, bufptmp
+ crc32q (bufp), crc_init_q
+ add $8, bufp
dec %eax
jnz .Ldo_qwords
.Ldo_dword:
test $4, len
jz .Ldo_word
- crc32l (bufptmp), crc_init
- add $4, bufptmp
+ crc32l (bufp), crc_init
+ add $4, bufp
.Ldo_word:
test $2, len
jz .Ldo_byte
- crc32w (bufptmp), crc_init
- add $2, bufptmp
+ crc32w (bufp), crc_init
+ add $2, bufp
.Ldo_byte:
test $1, len
jz .Ldone
- crc32b (bufptmp), crc_init
+ crc32b (bufp), crc_init
.Ldone:
mov crc_init, %eax
- popq %rsi
- popq %rdi
- popq %rbx
RET
SYM_FUNC_END(crc_pcl)
.section .rodata, "a", @progbits
- ################################################################
- ## jump table Table is 129 entries x 2 bytes each
- ################################################################
-.align 4
-jump_table:
- i=0
-.rept 129
-.altmacro
-JMPTBL_ENTRY %i
-.noaltmacro
- i=i+1
-.endr
-
-
################################################################
## PCLMULQDQ tables
## Table is 128 entries x 2 words (8 bytes) each
################################################################
.align 8
--
2.47.0