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Date:   Thu, 16 May 2019 07:48:41 +0800
From:   "Mao, Chenxi" <Chenxi.Mao@...y.com>
To:     Gao Xiang <gaoxiang25@...wei.com>, Cyan <yann.collet.73@...il.com>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Feng, Roy" <Roy.Feng@...y.com>,
        "Xu, Yuanli 2" <Yuanli.Xu@...y.com>,
        "Alm, Robert 2" <Robert.Alm@...y.com>,
        "Takahashi, Masaya A (Sony Mobile)" <Masaya.A.Takahashi@...y.com>,
        Miao Xie <miaoxie@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 1/1] LZ4: Port LZ4 1.9.x FAST_DEC_LOOP and enable it on
 x86 and ARM64

Hi Yann and Xiang:
For this FAST_DEC_LOOP change, I only pick up decompress related patches to current kernel LZ4 implementation(based on 1.8.3).
Here is my cherry-pick list:
2589c44 created LZ4_FAST_DEC_LOOP build macro
605d811 enable LZ4_FAST_DEC_LOOP build macro on aarch64/GCC by default
5d7d116 decompress_generic: Limit fastpath to x86
75fb878 decompress_generic: Add fastpath for small offsets
faac110 decompress_generic: Unroll loops a bit more
1fbaf84 decompress_generic: remove msan write
28b8249 decompress_generic: re-add fastpath
232f1e2 decompress_generic: drop partial copy check in fast loop
59332a3 decompress_generic: Optimize literal copies
5dfa7d4 decompress_generic: optimize match copy
28356e0 decompress_generic: Add a loop fastpath
4da3360 decompress_generic: Refactor variable length fields

Only cherry-pick these changes to LZ4 1.8.3 would not be introduce risks.

@Xiang:
Do you prefer to upgrade kernel lz4 to 1.9.1 directly or only pick some optimization patches?

Chenxi


On 5/16/19 1:47 AM, Gao Xiang wrote:
> Hi Yann,
> 
> On 2019/5/16 1:03, Cyan wrote:
>> Re-posted,
>> it seems the previous message was rejected by the linux-kernel server
>> due to some kind of format limitation (no html).
>>
>>
>> Le mer. 15 mai 2019 à 09:56, Cyan <yann.collet.73@...il.com> a écrit :
>>>
>>> The v1.9.0 version has a bug which makes it read a few bytes out of bound in certain cases.
>>> This was fixed in v1.9.1.
>>> Therefore, if you plan upgrading version, skip directly to v1.9.1.
>>>
>>> Note that, in v1.9.1, LZ4_decompress_fast() is now deprecated.
>>> LZ4_decompress_fast() is a security liability, since it's unable to cope with malicious inputs.
>>> While this is not a problem when the producer is under direct control (local production and consumption, no intermediate storage),
>>> this entry point seems misused in multiple scenarios where input cannot be decently trusted.
>>> The deprecation warning is meant to "scare away" such usages.
>>> It's still possible to intentionally remove the warning, for users who know what they are doing.
>>>
>>> Note that, on top of that, the _fast() variant is now slower that LZ4_decompress_safe(), removing its most important positive differentiator.
>>> That's because LZ4_decompress_fast() is "blind", it doesn't know the input size.
>>> It just guesses it, from the requested output size, and by relying on end-of-block conditions.
>>> This effectively limits its capability to copy up to 8 bytes at a time,
>>> which proves slower than the 32-bytes at a time used in LZ4_decompress_safe().
>>>
>>> LZ4_decompress_fast() still has some possible usage :
>>> it doesn't need the compressed size.
>>> Therefore, in a scenario where decompressed size is known, but compressed size isn't,
>>> it cannot be replaced by `LZ4_decompress_safe()`.
>>> This is pretty rare, but can happen in some cases, erofs for example.
>>> `LZ4_decompress_safe_partial()` *might* be able to replace it for such scenario,
>>> but I haven't thoroughly tested it, so can't yet vouch for it.
> 
> Yes, that is my real concern...
> 
> In fact, we have shipped erofs with LZ4_decompress_safe_partial() of lz4-1.8.3
> for our HUAWEI P30/P30 Pro and other HUAWEI EMUI 9.1 phones on the market...
> 
> It seemd lz4-1.8.3 is rather stable since no stability report from our internal
> beta test and real consumers...
> 
> I haven't tested lz4-1.9.0 yet since I am working on the erofs paper [1]...
> 
> But anyway, the patch looks good to me since it has obvious performance gain...
> 
>>>
>>> If it doesn't, then it will be necessary to create a new entry point.
>>> The main difference is that this new entry point will need a bound for the input size, in order to guarantee it never reads beyond input buffer.
>>> But as said, for the time being, such dedicated entry point doesn't exist.
> 
> I agree, but current erofs still have to choose LZ4_decompress_fast...
> I will test these 2 apis later and will report if something is wrong as well.
> 
> [1] https://www.usenix.org/conference/atc19/presentation/gao
>     https://www.usenix.org/conference/atc19/technical-sessions
> 
> Thanks,
> Gao Xiang
> 
> 
>>>
>>> The new FAST_DEC_LOOP setting works well on x86 and x64.
>>> Recently, it has been extended to gcc+arm64 (`dev` branch).
>>> The combination of clang+arm64 is less successful though (impacts vary, depending on version and exact hardware), so it's still disabled by default.
>>> But anyone can test it by setting FAST_DEC_LOOP definition to 1, thus enabling the new decoder loop.
>>>
>>>
>>> Y.
>>>
>>> Le mar. 14 mai 2019 à 20:28, Mao, Chenxi <Chenxi.Mao@...y.com> a écrit :
>>>>
>>>> Hi Xiang:
>>>>
>>>> Thanks for your reply, I will have a stress test on my device later.
>>>> I didn't have chance to test LZ4 with clang build because of device limitation. I think I could do it later.
>>>> I guess the clang performance downgrade is caused by some compiler optimization options.
>>>> I will double check it later if I got the device which can build kernel with clang.
>>>>
>>>> BTW, the FAST_DEC_LOOP leverage LZ4_wildCopy8 instead of LZ4_wildCopy,
>>>> however based on my test, original LZ4_wildCopy has the better performance.
>>>> Original LZ4_wildCopy API still invoked by FAST_DEC_LOOP apis.
>>>> That might be a problem for X86 devices for current patch.
>>>>
>>>> Chenxi
>>>>
>>>> -----Original Message-----
>>>> From: Gao Xiang [mailto:gaoxiang25@...wei.com]
>>>> Sent: Wednesday, May 15, 2019 10:21 AM
>>>> To: Mao, Chenxi <Chenxi.Mao@...y.com>
>>>> Cc: akpm@...ux-foundation.org; linux-kernel@...r.kernel.org; Feng, Roy <Roy.Feng@...y.com>; Xu, Yuanli 2 <Yuanli.Xu@...y.com>; Alm, Robert 2 <Robert.Alm@...y.com>; Takahashi, Masaya A (Sony Mobile) <Masaya.A.Takahashi@...y.com>; Miao Xie <miaoxie@...wei.com>
>>>> Subject: Re: [PATCH 1/1] LZ4: Port LZ4 1.9.x FAST_DEC_LOOP and enable it on x86 and ARM64
>>>>
>>>> Hi Chenxi,
>>>>
>>>> On 2019/5/15 8:43, Chenxi Mao wrote:
>>>>> FAST_DEC_LOOP was introduced from LZ4 1.9.
>>>>> This change would be introduce 10% on decompress operation according
>>>>> to LZ4 benchmark result on X86 devices.
>>>>> Meanwhile, LZ4 with FAST_DEC_LOOP could get improvements, however
>>>>> clang compiler has downgrade if FAST_DEC_LOOP enabled.
>>>>
>>>> I noticed this optimization and lz4-1.9.0 [1] was just released month ago with this big change therefore I'm a little afraid of its current stability (especially some rare used decompression apis...)
>>>>
>>>> Could you Cc more people (e.g. lz4 original author Yann Collet) in order to get some more ideas about this if possible?
>>>>
>>>> Anyway, I like this optimization as well since FAST_DEC_LOOP improves decompression speed a lot :)
>>>>
>>>> [1] https://github.com/lz4/lz4/releases/tag/v1.9.0
>>>>
>>>> Thanks,
>>>> Gao Xiang
>>>>
>>>>>
>>>>> So FAST_DEC_LOOP only enabled on X86/X86-64 or ARM64 with GCC build.
>>>>>
>>>>> Here is the test result on ARM64(cortex-A53)
>>>>>
>>>>> Benchmark via ZRAM:
>>>>>
>>>>> Test case:
>>>>> fio --bs=32k --randrepeat=1 --randseed=100 --refill_buffers \
>>>>> --buffer_compress_percentage=75 \
>>>>> --scramble_buffers=1 --direct=1 --loops=100 --numjobs=8 \
>>>>> --filename=/dev/block/zram0 --name=seq-write --rw=write --stonewall \
>>>>> --name=seq-read --rw=read --stonewall --name=seq-readwrite \ --rw=rw
>>>>> --stonewall --name=rand-readwrite --rw=randrw --stonewall
>>>>>
>>>>> Patched:
>>>>>    READ: bw=7077MiB/s (7421MB/s)
>>>>> Vanilla:
>>>>>    READ: bw=5134MiB/s (5384MB/s)
>>>>>
>>>>> Reference:
>>>>> 1. https://github.com/lz4/lz4/pull/645
>>>>> 2. https://github.com/lz4/lz4/pull/707
>>>>>
>>>>> Signed-off-by: chenxi.mao <chenxi.mao@...y.com>
>>>>> ---
>>>>>  lib/lz4/lz4_decompress.c | 425
>>>>> +++++++++++++++++++++++++++++++++------
>>>>>  1 file changed, 361 insertions(+), 64 deletions(-)
>>>>>
>>>>> diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c index
>>>>> 0c9d3ad17e0f..a4c87e32b3c0 100644
>>>>> --- a/lib/lz4/lz4_decompress.c
>>>>> +++ b/lib/lz4/lz4_decompress.c
>>>>> @@ -50,6 +50,131 @@
>>>>>  #define assert(condition) ((void)0)
>>>>>  #endif
>>>>>
>>>>> +#ifndef LZ4_FAST_DEC_LOOP
>>>>> +#if defined(__i386__) || defined(__x86_64__) #define
>>>>> +LZ4_FAST_DEC_LOOP 1 #elif defined(__aarch64__) && !defined(__clang__)
>>>>> +     /* On aarch64, we disable this optimization for clang because on certain
>>>>> +      * mobile chipsets and clang, it reduces performance. For more information
>>>>> +      * refer to https://github.com/lz4/lz4/pull/707. */ #define
>>>>> +LZ4_FAST_DEC_LOOP 1 #else #define LZ4_FAST_DEC_LOOP 0 #endif #endif
>>>>> +
>>>>> +static const unsigned inc32table[8] = { 0, 1, 2, 1, 0, 4, 4, 4 };
>>>>> +static const int dec64table[8] = { 0, 0, 0, -1, -4, 1, 2, 3 };
>>>>> +
>>>>> +#if LZ4_FAST_DEC_LOOP
>>>>> +#define FASTLOOP_SAFE_DISTANCE 64
>>>>> +FORCE_INLINE void
>>>>> +LZ4_memcpy_using_offset_base(BYTE * dstPtr, const BYTE * srcPtr, BYTE * dstEnd,
>>>>> +                          const size_t offset)
>>>>> +{
>>>>> +     if (offset < 8) {
>>>>> +             dstPtr[0] = srcPtr[0];
>>>>> +
>>>>> +             dstPtr[1] = srcPtr[1];
>>>>> +             dstPtr[2] = srcPtr[2];
>>>>> +             dstPtr[3] = srcPtr[3];
>>>>> +             srcPtr += inc32table[offset];
>>>>> +             memcpy(dstPtr + 4, srcPtr, 4);
>>>>> +             srcPtr -= dec64table[offset];
>>>>> +             dstPtr += 8;
>>>>> +     } else {
>>>>> +             memcpy(dstPtr, srcPtr, 8);
>>>>> +             dstPtr += 8;
>>>>> +             srcPtr += 8;
>>>>> +     }
>>>>> +
>>>>> +     LZ4_wildCopy(dstPtr, srcPtr, dstEnd); }
>>>>> +
>>>>> +/* customized variant of memcpy, which can overwrite up to 32 bytes
>>>>> +beyond dstEnd
>>>>> + * this version copies two times 16 bytes (instead of one time 32
>>>>> +bytes)
>>>>> + * because it must be compatible with offsets >= 16. */ FORCE_INLINE
>>>>> +void LZ4_wildCopy32(void *dstPtr, const void *srcPtr, void *dstEnd) {
>>>>> +     BYTE *d = (BYTE *) dstPtr;
>>>>> +     const BYTE *s = (const BYTE *)srcPtr;
>>>>> +     BYTE *const e = (BYTE *) dstEnd;
>>>>> +
>>>>> +     do {
>>>>> +             memcpy(d, s, 16);
>>>>> +             memcpy(d + 16, s + 16, 16);
>>>>> +             d += 32;
>>>>> +             s += 32;
>>>>> +     } while (d < e);
>>>>> +}
>>>>> +
>>>>> +FORCE_INLINE void
>>>>> +LZ4_memcpy_using_offset(BYTE *dstPtr, const BYTE *srcPtr, BYTE *dstEnd,
>>>>> +                     const size_t offset)
>>>>> +{
>>>>> +     BYTE v[8];
>>>>> +     switch (offset) {
>>>>> +
>>>>> +     case 1:
>>>>> +             memset(v, *srcPtr, 8);
>>>>> +             goto copy_loop;
>>>>> +     case 2:
>>>>> +             memcpy(v, srcPtr, 2);
>>>>> +             memcpy(&v[2], srcPtr, 2);
>>>>> +             memcpy(&v[4], &v[0], 4);
>>>>> +             goto copy_loop;
>>>>> +     case 4:
>>>>> +             memcpy(v, srcPtr, 4);
>>>>> +             memcpy(&v[4], srcPtr, 4);
>>>>> +             goto copy_loop;
>>>>> +     default:
>>>>> +             LZ4_memcpy_using_offset_base(dstPtr, srcPtr, dstEnd, offset);
>>>>> +             return;
>>>>> +     }
>>>>> +
>>>>> +      copy_loop:
>>>>> +     memcpy(dstPtr, v, 8);
>>>>> +     dstPtr += 8;
>>>>> +     while (dstPtr < dstEnd) {
>>>>> +             memcpy(dstPtr, v, 8);
>>>>> +             dstPtr += 8;
>>>>> +     }
>>>>> +}
>>>>> +
>>>>> +/* Read the variable-length literal or match length.
>>>>> + *
>>>>> + * ip - pointer to use as input.
>>>>> + * lencheck - end ip.  Return an error if ip advances >= lencheck.
>>>>> + * loop_check - check ip >= lencheck in body of loop.  Returns loop_error if so.
>>>>> + * initial_check - check ip >= lencheck before start of loop.  Returns initial_error if so.
>>>>> + * error (output) - error code.  Should be set to 0 before call.
>>>>> + */
>>>>> +typedef enum { loop_error = -2, initial_error = -1, ok = 0}
>>>>> +variable_length_error; FORCE_INLINE unsigned read_variable_length(const BYTE **ip,
>>>>> +                                        const BYTE *lencheck,
>>>>> +                                        int loop_check, int initial_check,
>>>>> +                                        variable_length_error *error) {
>>>>> +     unsigned length = 0;
>>>>> +     unsigned s;
>>>>> +     if (initial_check && unlikely((*ip) >= lencheck)) {     /* overflow detection */
>>>>> +             *error = initial_error;
>>>>> +             return length;
>>>>> +     }
>>>>> +     do {
>>>>> +             s = **ip;
>>>>> +             (*ip)++;
>>>>> +             length += s;
>>>>> +             if (loop_check && unlikely((*ip) >= lencheck)) {        /* overflow detection */
>>>>> +                     *error = loop_error;
>>>>> +                     return length;
>>>>> +             }
>>>>> +     } while (s == 255);
>>>>> +
>>>>> +     return length;
>>>>> +}
>>>>> +#endif
>>>>> +
>>>>>  /*
>>>>>   * LZ4_decompress_generic() :
>>>>>   * This generic decompression function covers all use cases.
>>>>> @@ -80,25 +205,28 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>        const size_t dictSize
>>>>>        )
>>>>>  {
>>>>> -     const BYTE *ip = (const BYTE *) src;
>>>>> -     const BYTE * const iend = ip + srcSize;
>>>>> +     const BYTE *ip = (const BYTE *)src;
>>>>> +     const BYTE *const iend = ip + srcSize;
>>>>>
>>>>>       BYTE *op = (BYTE *) dst;
>>>>> -     BYTE * const oend = op + outputSize;
>>>>> +     BYTE *const oend = op + outputSize;
>>>>>       BYTE *cpy;
>>>>>
>>>>> -     const BYTE * const dictEnd = (const BYTE *)dictStart + dictSize;
>>>>> -     static const unsigned int inc32table[8] = {0, 1, 2, 1, 0, 4, 4, 4};
>>>>> -     static const int dec64table[8] = {0, 0, 0, -1, -4, 1, 2, 3};
>>>>> +     const BYTE *const dictEnd = (const BYTE *)dictStart + dictSize;
>>>>>
>>>>>       const int safeDecode = (endOnInput == endOnInputSize);
>>>>>       const int checkOffset = ((safeDecode) && (dictSize < (int)(64 *
>>>>> KB)));
>>>>>
>>>>>       /* Set up the "end" pointers for the shortcut. */
>>>>>       const BYTE *const shortiend = iend -
>>>>> -             (endOnInput ? 14 : 8) /*maxLL*/ - 2 /*offset*/;
>>>>> +         (endOnInput ? 14 : 8) /*maxLL*/ - 2 /*offset*/;
>>>>>       const BYTE *const shortoend = oend -
>>>>> -             (endOnInput ? 14 : 8) /*maxLL*/ - 18 /*maxML*/;
>>>>> +         (endOnInput ? 14 : 8) /*maxLL*/ - 18 /*maxML*/;
>>>>> +
>>>>> +     const BYTE *match;
>>>>> +     size_t offset;
>>>>> +     unsigned int token;
>>>>> +     size_t length;
>>>>>
>>>>>       DEBUGLOG(5, "%s (srcSize:%i, dstSize:%i)", __func__,
>>>>>                srcSize, outputSize);
>>>>> @@ -117,15 +245,194 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>       if ((endOnInput) && unlikely(srcSize == 0))
>>>>>               return -1;
>>>>>
>>>>> -     /* Main Loop : decode sequences */
>>>>> +#if LZ4_FAST_DEC_LOOP
>>>>> +     if ((oend - op) < FASTLOOP_SAFE_DISTANCE) {
>>>>> +             DEBUGLOG(6, "skip fast decode loop");
>>>>> +             goto safe_decode;
>>>>> +     }
>>>>> +
>>>>> +     /* Fast loop : decode sequences as long as output <
>>>>> +iend-FASTLOOP_SAFE_DISTANCE */
>>>>>       while (1) {
>>>>> -             size_t length;
>>>>> -             const BYTE *match;
>>>>> -             size_t offset;
>>>>> +             /* Main fastloop assertion: We can always wildcopy FASTLOOP_SAFE_DISTANCE */
>>>>> +             assert(oend - op >= FASTLOOP_SAFE_DISTANCE);
>>>>> +             if (endOnInput) {
>>>>> +                     assert(ip < iend);
>>>>> +             }
>>>>> +             token = *ip++;
>>>>> +             length = token >> ML_BITS;      /* literal length */
>>>>> +
>>>>> +             assert(!endOnInput || ip <= iend);      /* ip < iend before the increment */
>>>>> +
>>>>> +             /* decode literal length */
>>>>> +             if (length == RUN_MASK) {
>>>>> +                     variable_length_error error = ok;
>>>>> +                     length +=
>>>>> +                         read_variable_length(&ip, iend - RUN_MASK,
>>>>> +                                              endOnInput, endOnInput,
>>>>> +                                              &error);
>>>>> +                     if (error == initial_error) {
>>>>> +                             goto _output_error;
>>>>> +                     }
>>>>> +                     if ((safeDecode)
>>>>> +                         && unlikely((uptrval) (op) + length <
>>>>> +                                     (uptrval) (op))) {
>>>>> +                             goto _output_error;
>>>>> +                     }       /* overflow detection */
>>>>> +                     if ((safeDecode)
>>>>> +                         && unlikely((uptrval) (ip) + length <
>>>>> +                                     (uptrval) (ip))) {
>>>>> +                             goto _output_error;
>>>>> +                     }
>>>>> +
>>>>> +                     /* overflow detection */
>>>>> +                     /* copy literals */
>>>>> +                     cpy = op + length;
>>>>> +                     LZ4_STATIC_ASSERT(MFLIMIT >= WILDCOPYLENGTH);
>>>>> +                     if (endOnInput) {       /* LZ4_decompress_safe() */
>>>>> +                             if ((cpy > oend - 32)
>>>>> +                                 || (ip + length > iend - 32)) {
>>>>> +                                     goto safe_literal_copy;
>>>>> +                             }
>>>>> +                             LZ4_wildCopy32(op, ip, cpy);
>>>>> +                     } else {        /* LZ4_decompress_fast() */
>>>>> +                             if (cpy > oend - 8) {
>>>>> +                                     goto safe_literal_copy;
>>>>> +                             }
>>>>> +                             LZ4_wildCopy(op, ip, cpy);
>>>>> +                             /* LZ4_decompress_fast() cannot copy more than 8 bytes at a time */
>>>>> +                             /* it doesn't know input length, and only relies on end-of-block */
>>>>> +                             /* properties */
>>>>> +                     }
>>>>> +                     ip += length;
>>>>> +                     op = cpy;
>>>>> +             } else {
>>>>> +                     cpy = op + length;
>>>>> +                     if (endOnInput) {       /* LZ4_decompress_safe() */
>>>>> +                             DEBUGLOG(7,
>>>>> +                                      "copy %u bytes in a 16-bytes stripe",
>>>>> +                                      (unsigned)length);
>>>>> +                             /* We don't need to check oend */
>>>>> +                             /* since we check it once for each loop below */
>>>>> +                             if (ip > iend - (16 + 1)) {     /*max lit + offset + nextToken */
>>>>> +                                     goto safe_literal_copy;
>>>>> +                             }
>>>>> +                             /* Literals can only be 14, but hope compilers optimize */
>>>>> +                             /*if we copy by a register size */
>>>>> +                             memcpy(op, ip, 16);
>>>>> +                     } else {
>>>>> +                             /* LZ4_decompress_fast() cannot copy more than 8 bytes at a time */
>>>>> +                             /* it doesn't know input length, and relies on end-of-block */
>>>>> +                             /* properties */
>>>>> +                             memcpy(op, ip, 8);
>>>>> +                             if (length > 8) {
>>>>> +                                     memcpy(op + 8, ip + 8, 8);
>>>>> +                             }
>>>>> +                     }
>>>>> +                     ip += length;
>>>>> +                     op = cpy;
>>>>> +             }
>>>>> +
>>>>> +             /* get offset */
>>>>> +             offset = LZ4_readLE16(ip);
>>>>> +             ip += 2;        /* end-of-block condition violated */
>>>>> +             match = op - offset;
>>>>> +
>>>>> +             /* get matchlength */
>>>>> +             length = token & ML_MASK;
>>>>>
>>>>> -             /* get literal length */
>>>>> -             unsigned int const token = *ip++;
>>>>> -             length = token>>ML_BITS;
>>>>> +             if ((checkOffset) && (unlikely(match + dictSize < lowPrefix))) {
>>>>> +                     goto _output_error;
>>>>> +             }
>>>>> +             /* Error : offset outside buffers */
>>>>> +             if (length == ML_MASK) {
>>>>> +                     variable_length_error error = ok;
>>>>> +                     length +=
>>>>> +                         read_variable_length(&ip, iend - LASTLITERALS + 1,
>>>>> +                                              endOnInput, 0, &error);
>>>>> +                     if (error != ok) {
>>>>> +                             goto _output_error;
>>>>> +                     }
>>>>> +                     if ((safeDecode)
>>>>> +                         && unlikely((uptrval) (op) + length < (uptrval) op)) {
>>>>> +                             goto _output_error;
>>>>> +                     }       /* overflow detection */
>>>>> +                     length += MINMATCH;
>>>>> +                     if (op + length >= oend - FASTLOOP_SAFE_DISTANCE) {
>>>>> +                             goto safe_match_copy;
>>>>> +                     }
>>>>> +             } else {
>>>>> +                     length += MINMATCH;
>>>>> +                     if (op + length >= oend - FASTLOOP_SAFE_DISTANCE) {
>>>>> +                             goto safe_match_copy;
>>>>> +                     }
>>>>> +
>>>>> +                     /* Fastpath check: Avoids a branch in LZ4_wildCopy32 if true */
>>>>> +                     if (!(dict == usingExtDict) || (match >= lowPrefix)) {
>>>>> +                             if (offset >= 8) {
>>>>> +                                     memcpy(op, match, 8);
>>>>> +                                     memcpy(op + 8, match + 8, 8);
>>>>> +                                     memcpy(op + 16, match + 16, 2);
>>>>> +                                     op += length;
>>>>> +                                     continue;
>>>>> +                             }
>>>>> +                     }
>>>>> +             }
>>>>> +
>>>>> +             /* match starting within external dictionary */
>>>>> +             if ((dict == usingExtDict) && (match < lowPrefix)) {
>>>>> +                     if (unlikely(op + length > oend - LASTLITERALS)) {
>>>>> +                             if (partialDecoding) {
>>>>> +                                     /* reach end of buffer */
>>>>> +                                     length =
>>>>> +                                         min(length, (size_t) (oend - op));
>>>>> +                             } else {
>>>>> +                                     /* end-of-block condition violated */
>>>>> +                                     goto _output_error;
>>>>> +                             }
>>>>> +                     }
>>>>> +
>>>>> +                     if (length <= (size_t) (lowPrefix - match)) {
>>>>> +                             /* match fits entirely within external dictionary : just copy */
>>>>> +                             memmove(op, dictEnd - (lowPrefix - match), length);
>>>>> +                             op += length;
>>>>> +                     } else {
>>>>> +                             /* match stretches into both external dict and current block */
>>>>> +                             size_t const copySize =
>>>>> +                                 (size_t) (lowPrefix - match);
>>>>> +                             size_t const restSize = length - copySize;
>>>>> +                             memcpy(op, dictEnd - copySize, copySize);
>>>>> +                             op += copySize;
>>>>> +                             if (restSize > (size_t) (op - lowPrefix)) {     /* overlap copy */
>>>>> +                                     BYTE *const endOfMatch = op + restSize;
>>>>> +                                     const BYTE *copyFrom = lowPrefix;
>>>>> +                                     while (op < endOfMatch) {
>>>>> +                                             *op++ = *copyFrom++;
>>>>> +                                     }
>>>>> +                             } else {
>>>>> +                                     memcpy(op, lowPrefix, restSize);
>>>>> +                                     op += restSize;
>>>>> +                             }
>>>>> +                     }
>>>>> +                     continue;
>>>>> +             }
>>>>> +
>>>>> +             /* copy match within block */
>>>>> +             cpy = op + length;
>>>>> +
>>>>> +             assert((op <= oend) && (oend - op >= 32));
>>>>> +             if (unlikely(offset < 16)) {
>>>>> +                     LZ4_memcpy_using_offset(op, match, cpy, offset);
>>>>> +             } else {
>>>>> +                     LZ4_wildCopy32(op, match, cpy);
>>>>> +             }
>>>>> +
>>>>> +             op = cpy;       /* wildcopy correction */
>>>>> +     }
>>>>> +      safe_decode:
>>>>> +#endif
>>>>> +     /* Main Loop : decode sequences */
>>>>> +     while (1) {
>>>>> +             length = token >> ML_BITS;
>>>>>
>>>>>               /* ip < iend before the increment */
>>>>>               assert(!endOnInput || ip <= iend);
>>>>> @@ -143,26 +450,27 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>                * combined check for both stages).
>>>>>                */
>>>>>               if ((endOnInput ? length != RUN_MASK : length <= 8)
>>>>> -                /*
>>>>> -                 * strictly "less than" on input, to re-enter
>>>>> -                 * the loop with at least one byte
>>>>> -                 */
>>>>> -                && likely((endOnInput ? ip < shortiend : 1) &
>>>>> -                          (op <= shortoend))) {
>>>>> +                 /*
>>>>> +                  * strictly "less than" on input, to re-enter
>>>>> +                  * the loop with at least one byte
>>>>> +                  */
>>>>> +                 && likely((endOnInput ? ip < shortiend : 1) &
>>>>> +                           (op <= shortoend))) {
>>>>>                       /* Copy the literals */
>>>>>                       memcpy(op, ip, endOnInput ? 16 : 8);
>>>>> -                     op += length; ip += length;
>>>>> +                     op += length;
>>>>> +                     ip += length;
>>>>>
>>>>>                       /*
>>>>>                        * The second stage:
>>>>>                        * prepare for match copying, decode full info.
>>>>>                        * If it doesn't work out, the info won't be wasted.
>>>>>                        */
>>>>> -                     length = token & ML_MASK; /* match length */
>>>>> +                     length = token & ML_MASK;       /* match length */
>>>>>                       offset = LZ4_readLE16(ip);
>>>>>                       ip += 2;
>>>>>                       match = op - offset;
>>>>> -                     assert(match <= op); /* check overflow */
>>>>> +                     assert(match <= op);    /* check overflow */
>>>>>
>>>>>                       /* Do not deal with overlapping matches. */
>>>>>                       if ((length != ML_MASK) &&
>>>>> @@ -187,28 +495,24 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>
>>>>>               /* decode literal length */
>>>>>               if (length == RUN_MASK) {
>>>>> -                     unsigned int s;
>>>>>
>>>>> -                     if (unlikely(endOnInput ? ip >= iend - RUN_MASK : 0)) {
>>>>> -                             /* overflow detection */
>>>>> +                     variable_length_error error = ok;
>>>>> +                     length +=
>>>>> +                         read_variable_length(&ip, iend - RUN_MASK,
>>>>> +                                              endOnInput, endOnInput,
>>>>> +                                              &error);
>>>>> +                     if (error == initial_error)
>>>>>                               goto _output_error;
>>>>> -                     }
>>>>> -                     do {
>>>>> -                             s = *ip++;
>>>>> -                             length += s;
>>>>> -                     } while (likely(endOnInput
>>>>> -                             ? ip < iend - RUN_MASK
>>>>> -                             : 1) & (s == 255));
>>>>>
>>>>>                       if ((safeDecode)
>>>>> -                         && unlikely((uptrval)(op) +
>>>>> -                                     length < (uptrval)(op))) {
>>>>> +                         && unlikely((uptrval) (op) +
>>>>> +                                     length < (uptrval) (op))) {
>>>>>                               /* overflow detection */
>>>>>                               goto _output_error;
>>>>>                       }
>>>>>                       if ((safeDecode)
>>>>> -                         && unlikely((uptrval)(ip) +
>>>>> -                                     length < (uptrval)(ip))) {
>>>>> +                         && unlikely((uptrval) (ip) +
>>>>> +                                     length < (uptrval) (ip))) {
>>>>>                               /* overflow detection */
>>>>>                               goto _output_error;
>>>>>                       }
>>>>> @@ -216,11 +520,15 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>
>>>>>               /* copy literals */
>>>>>               cpy = op + length;
>>>>> +#if LZ4_FAST_DEC_LOOP
>>>>> +           safe_literal_copy:
>>>>> +#endif
>>>>>               LZ4_STATIC_ASSERT(MFLIMIT >= WILDCOPYLENGTH);
>>>>>
>>>>>               if (((endOnInput) && ((cpy > oend - MFLIMIT)
>>>>> -                     || (ip + length > iend - (2 + 1 + LASTLITERALS))))
>>>>> -                     || ((!endOnInput) && (cpy > oend - WILDCOPYLENGTH))) {
>>>>> +                                   || (ip + length >
>>>>> +                                       iend - (2 + 1 + LASTLITERALS))))
>>>>> +                 || ((!endOnInput) && (cpy > oend - WILDCOPYLENGTH))) {
>>>>>                       if (partialDecoding) {
>>>>>                               if (cpy > oend) {
>>>>>                                       /*
>>>>> @@ -231,7 +539,7 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>                                       length = oend - op;
>>>>>                               }
>>>>>                               if ((endOnInput)
>>>>> -                                     && (ip + length > iend)) {
>>>>> +                                 && (ip + length > iend)) {
>>>>>                                       /*
>>>>>                                        * Error :
>>>>>                                        * read attempt beyond
>>>>> @@ -241,7 +549,7 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>                               }
>>>>>                       } else {
>>>>>                               if ((!endOnInput)
>>>>> -                                     && (cpy != oend)) {
>>>>> +                                 && (cpy != oend)) {
>>>>>                                       /*
>>>>>                                        * Error :
>>>>>                                        * block decoding must
>>>>> @@ -250,7 +558,7 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>                                       goto _output_error;
>>>>>                               }
>>>>>                               if ((endOnInput)
>>>>> -                                     && ((ip + length != iend)
>>>>> +                                 && ((ip + length != iend)
>>>>>                                       || (cpy > oend))) {
>>>>>                                       /*
>>>>>                                        * Error :
>>>>> @@ -288,29 +596,14 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>                       goto _output_error;
>>>>>               }
>>>>>
>>>>> -             /* costs ~1%; silence an msan warning when offset == 0 */
>>>>> -             /*
>>>>> -              * note : when partialDecoding, there is no guarantee that
>>>>> -              * at least 4 bytes remain available in output buffer
>>>>> -              */
>>>>> -             if (!partialDecoding) {
>>>>> -                     assert(oend > op);
>>>>> -                     assert(oend - op >= 4);
>>>>> -
>>>>> -                     LZ4_write32(op, (U32)offset);
>>>>> -             }
>>>>> -
>>>>>               if (length == ML_MASK) {
>>>>> -                     unsigned int s;
>>>>> -
>>>>> -                     do {
>>>>> -                             s = *ip++;
>>>>> -
>>>>> -                             if ((endOnInput) && (ip > iend - LASTLITERALS))
>>>>> -                                     goto _output_error;
>>>>>
>>>>> -                             length += s;
>>>>> -                     } while (s == 255);
>>>>> +                     variable_length_error error = ok;
>>>>> +                     length +=
>>>>> +                         read_variable_length(&ip, iend - LASTLITERALS + 1,
>>>>> +                                              endOnInput, 0, &error);
>>>>> +                     if (error != ok)
>>>>> +                             goto _output_error;
>>>>>
>>>>>                       if ((safeDecode)
>>>>>                               && unlikely(
>>>>> @@ -322,6 +615,10 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>>>>
>>>>>               length += MINMATCH;
>>>>>
>>>>> +#if LZ4_FAST_DEC_LOOP
>>>>> +safe_match_copy:
>>>>> +#endif
>>>>> +
>>>>>               /* match starting within external dictionary */
>>>>>               if ((dict == usingExtDict) && (match < lowPrefix)) {
>>>>>                       if (unlikely(op + length > oend - LASTLITERALS)) {
>>>>>

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