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Date: Wed, 12 Jul 2017 05:38:06 +0200
From: Adam Borowski <kilobyte@angband.pl>
To: Nick Terrell <terrelln@fb.com>
Cc: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>,
        Kernel Team <Kernel-team@fb.com>, Chris Mason <clm@fb.com>,
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Subject: Re: [PATCH v2 3/4] btrfs: Add zstd support
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On Tue, Jul 11, 2017 at 06:01:38AM +0000, Nick Terrell wrote:
> On 7/10/17, 9:57 PM, "Nick Terrell" <terrelln@fb.com> wrote:
> > The problem is caused by a gcc-7 bug [1]. It miscompiles
> > ZSTD_wildcopy(void *dst, void const *src, ptrdiff_t len) when len is 0.
>
> Sorry, my patch still triggered the gcc bug, I used the wrong compiler.
> This patch works, and runs about the same speed as before the patch for
> small inputs, and slightly faster for larger inputs (100+ bytes). I'll
> look for a faster workaround if benchmarks show it matters.

Confirmed, the fix stops the crash for me.  Yay!

> --- a/lib/zstd/zstd_internal.h
> +++ b/lib/zstd/zstd_internal.h
> @@ -139,12 +139,8 @@ static void ZSTD_copy8(void *dst, const void *src) { memcpy(dst, src, 8); }
>  #define WILDCOPY_OVERLENGTH 8
>  ZSTD_STATIC void ZSTD_wildcopy(void *dst, const void *src, ptrdiff_t length)
>  {
> -	const BYTE *ip = (const BYTE *)src;
> -	BYTE *op = (BYTE *)dst;
> -	BYTE *const oend = op + length;
> -	do
> -		COPY8(op, ip)
> -	while (op < oend);
> +	if (length > 0)
> +		memcpy(dst, src, length);
>  }
>  
>  ZSTD_STATIC void ZSTD_wildcopy_e(void *dst, const void *src, void *dstEnd) /* should be faster for decoding, but strangely, not verified on all platform */

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