Re: [PATCH 1/1] LZ4: Port LZ4 1.9.x FAST_DEC_LOOP and enable it on x86 and ARM64

[Gao Xiang]

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 Ã 09:56, Cyan <yann.collet.73@xxxxxxxxx> a Ãcrit :
>>
>> 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 Ã 20:28, Mao, Chenxi <Chenxi.Mao@xxxxxxxx> a Ãcrit :
>>>
>>> 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@xxxxxxxxxx]
>>> Sent: Wednesday, May 15, 2019 10:21 AM
>>> To: Mao, Chenxi <Chenxi.Mao@xxxxxxxx>
>>> Cc: akpm@xxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Feng, Roy <Roy.Feng@xxxxxxxx>; Xu, Yuanli 2 <Yuanli.Xu@xxxxxxxx>; Alm, Robert 2 <Robert.Alm@xxxxxxxx>; Takahashi, Masaya A (Sony Mobile) <Masaya.A.Takahashi@xxxxxxxx>; Miao Xie <miaoxie@xxxxxxxxxx>
>>> 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@xxxxxxxx>
>>>> ---
>>>>  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)) {
>>>>