Re: [PATCH] MIPS: crypto: Clean up useless assignment operations

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On 2024/6/28 18:12, Herbert Xu wrote:
> On Fri, Jun 28, 2024 at 04:41:17PM +0800, WangYuli wrote:
>> When entering the "len & sizeof(u32)" branch, len must be less than 8.
>> So after one operation, len must be less than 4.
>> At this time, "len -= sizeof(u32)" is not necessary for 64-bit CPUs.
>>
>> A similar issue has been solved at Loongarch.
>>
>> Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v6.10-rc5&id=fea1c949f6ca5059e12de00d0483645debc5b206
>> Signed-off-by: Guan Wentao <guanwentao@xxxxxxxxxxxxx>
>> Signed-off-by: WangYuli <wangyuli@xxxxxxxxxxxxx>
>> ---
>>  arch/mips/crypto/crc32-mips.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/mips/crypto/crc32-mips.c b/arch/mips/crypto/crc32-mips.c
>> index ec6d58008f8e..505d2d897849 100644
>> --- a/arch/mips/crypto/crc32-mips.c
>> +++ b/arch/mips/crypto/crc32-mips.c
>> @@ -94,7 +94,9 @@ static u32 crc32_mips_le_hw(u32 crc_, const u8 *p, unsigned int len)
>>
>>          CRC32(crc, value, w);
>>          p += sizeof(u32);
>> +#ifndef CONFIG_64BIT
>>          len -= sizeof(u32);
>> +#endif
>
> First of all, did you verify that this actually makes a difference?
> Please post the actual assembly output with and without this patch.

Sure.

The left shows the assembly after applying this patch, while the right shows the origin. ( Generated by Clang 17.0.6 )

0000000000000018 <chksum_update>: 0000000000000018 <chksum_update>:
;     ctx->crc = crc32_mips_le_hw(ctx->crc, data, length); ;     ctx->crc = crc32_mips_le_hw(ctx->crc, data, length);       18: 08 00 82 8c      lw    $2, 0x8($4) 18: 08 00 82 8c      lw    $2, 0x8($4)
;     while (len >= sizeof(u64)) { ;     while (len >= sizeof(u64)) {
      1c: 08 00 c1 2c      sltiu    $1, $6, 0x8 <chksum_init+0x8> 1c: 08 00 c1 2c      sltiu    $1, $6, 0x8 <chksum_init+0x8>       20: 06 00 20 f8      bnezc    $1, 0x3c <chksum_update+0x24> 20: 06 00 20 f8      bnezc    $1, 0x3c <chksum_update+0x24> ;     return le64_to_cpu(__get_unaligned_t(__le64, p)); ;     return le64_to_cpu(__get_unaligned_t(__le64, p));       24: 00 00 a3 dc      ld    $3, 0x0($5) 24: 00 00 a3 dc      ld    $3, 0x0($5)
;         CRC32(crc, value, d); ;         CRC32(crc, value, d);
      28: cf 00 62 7c      <unknown> crc32d v0,v1,v0 28: cf 00 62 7c      <unknown> crc32d    v0,v1,v0
;         len -= sizeof(u64); ;         len -= sizeof(u64);
      2c: f8 ff c6 24      addiu    $6, $6, -0x8 <chksumc_digest+0xfffffffffffffce0> 2c: f8 ff c6 24      addiu    $6, $6, -0x8 <chksumc_digest+0xfffffffffffffd48>
;     while (len >= sizeof(u64)) { ;     while (len >= sizeof(u64)) {
      30: 08 00 c1 2c      sltiu    $1, $6, 0x8 <chksum_init+0x8> 30: 08 00 c1 2c      sltiu    $1, $6, 0x8 <chksum_init+0x8>       34: fb ff 20 10      beqz    $1, 0x24 <chksum_update+0xc> 34: fb ff 20 10      beqz    $1, 0x24 <chksum_update+0xc>       38: 08 00 a5 64      daddiu    $5, $5, 0x8 <chksum_init+0x8> 38: 08 00 a5 64      daddiu    $5, $5, 0x8 <chksum_init+0x8>
;     if (len & sizeof(u32)) { ;     if (len & sizeof(u32)) {
      3c: 04 00 c1 2c      sltiu    $1, $6, 0x4 <chksum_init+0x4> 3c: 04 00 c1 2c      sltiu    $1, $6, 0x4 <chksum_init+0x4>       40: 0a 00 20 10      beqz    $1, 0x6c <chksum_update+0x54> 40: 04 00 20 f8      bnezc    $1, 0x54 <chksum_update+0x3c>       44: 03 f8 c3 7c      dext    $3, $6, 0x0, 0x20 <chksum_update+0x8> ;     return le32_to_cpu(__get_unaligned_t(__le32, p));
;     if (len & sizeof(u16)) { 44: 00 00 a3 8c      lw    $3, 0x0($5)
      48: 02 00 61 30      andi    $1, $3, 0x2 <chksum_init+0x2> ;         CRC32(crc, value, w);
      4c: 0c 00 20 f8      bnezc    $1, 0x80 <chksum_update+0x68>
;     if (len & sizeof(u8)) { 48: 8f 00 62 7c      <unknown> crc32w    v0,v1,v0
      50: 01 00 61 30      andi    $1, $3, 0x1 <chksum_init+0x1>
      54: 02 00 20 d8      beqzc    $1, 0x60 <chksum_update+0x48> ;         len -= sizeof(u32); ;         CRC32(crc, value, b); 4c: fc ff c6 24      addiu    $6, $6, -0x4 <chksumc_digest+0xfffffffffffffd4c>
      58: 00 00 a3 90      lbu    $3, 0x0($5) ;         p += sizeof(u32);
50: 04 00 a5 64      daddiu    $5, $5, 0x4 <chksum_init+0x4>
      5c: 0f 00 62 7c      <unknown> crc32b v0,v1,v0 ;     if (len & sizeof(u16)) {
54: 03 f8 c3 7c      dext    $3, $6, 0x0, 0x20 <chksum_update+0x8>
;     ctx->crc = crc32_mips_le_hw(ctx->crc, data, length); 58: 02 00 61 30      andi    $1, $3, 0x2 <chksum_init+0x2>       60: 08 00 82 ac      sw    $2, 0x8($4) 5c: 03 00 20 d8      beqzc    $1, 0x6c <chksum_update+0x54>
;     return 0; ;         CRC32(crc, value, h);
      64: 09 00 e0 03      jr $ra 60: 00 00 a6 94      lhu    $6, 0x0($5)
      68: 00 00 02 64      daddiu    $2, $zero, 0x0 <chksum_init>
;     return le32_to_cpu(__get_unaligned_t(__le32, p)); 64: 4f 00 c2 7c      <unknown> crc32h    v0,a2,v0
      6c: 00 00 a6 8c      lw    $6, 0x0($5)
;         CRC32(crc, value, w); ;         p += sizeof(u16);
      70: 8f 00 c2 7c      <unknown> crc32w v0,a2,v0 68: 02 00 a5 64      daddiu    $5, $5, 0x2 <chksum_init+0x2>
;     if (len & sizeof(u16)) { ;     if (len & sizeof(u8)) {
      74: 02 00 61 30      andi    $1, $3, 0x2 <chksum_init+0x2> 6c: 01 00 61 30      andi    $1, $3, 0x1 <chksum_init+0x1>       78: f5 ff 20 10      beqz    $1, 0x50 <chksum_update+0x38> 70: 02 00 20 d8      beqzc    $1, 0x7c <chksum_update+0x64>
      7c: 04 00 a5 64      daddiu    $5, $5, 0x4 <chksum_init+0x4>
;         CRC32(crc, value, h); ;         CRC32(crc, value, b);
      80: 00 00 a6 94      lhu    $6, 0x0($5) 74: 00 00 a3 90      lbu    $3, 0x0($5)

      84: 4f 00 c2 7c      <unknown> crc32h v0,a2,v0 78: 0f 00 62 7c      <unknown> crc32b    v0,v1,v0

;     if (len & sizeof(u8)) { ;     ctx->crc = crc32_mips_le_hw(ctx->crc, data, length);       88: 01 00 61 30      andi    $1, $3, 0x1 <chksum_init+0x1> 7c: 08 00 82 ac      sw    $2, 0x8($4)       8c: f4 ff 20 10      beqz    $1, 0x60 <chksum_update+0x48> ;     return 0;       90: 02 00 a5 64      daddiu    $5, $5, 0x2 <chksum_init+0x2> 80: 09 00 e0 03      jr    $ra       94: 00 00 00 08      j    0x0 <chksum_init> 84: 00 00 02 64      daddiu    $2, $zero, 0x0 <chksum_init>


In our testing, this assignment operation affects Clang's code expansion and instruction reordering.

This redundant assignment operation confuses Clang and prevents us from obtaining optimized

assembly code.


I extracted the 'crc32_mips_le_hw()' function as a user-mode demo to analyze the assembly code

generated for it on MIPS64.

Link: https://godbolt.org/z/r4dGbhTGf



As you can see, regardless of the Clang or GCC version, this redundant operation affects the generated

assembly code.
>
> If it does make a difference, you should avoid doing ifdefs as they
> are more likely to cause build failures.  Instead do something like
>
>         if (!IS_ENABLED(CONFIG_64BIT))
>             len -= sizeof(u32);
Okay, I'll send a Patch V2 to fix this and update the commit message based on above.
>
>
> Cheers,

--
WangYuli <wangyuli@xxxxxxxxxxxxx>






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