linux-kernel - Re: [PATCH v2 3/4] btrfs: Add zstd support

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Message-ID: <20170712033806.xlkzyo56gmtrr6f5@angband.pl>
Date:   Wed, 12 Jul 2017 05:38:06 +0200
From:   Adam Borowski <kilobyte@...band.pl>
To:     Nick Terrell <terrelln@...com>
Cc:     "Austin S. Hemmelgarn" <ahferroin7@...il.com>,
        Kernel Team <Kernel-team@...com>, Chris Mason <clm@...com>,
        Yann Collet <cyan@...com>, David Sterba <dsterba@...e.cz>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] btrfs: Add zstd support

On Tue, Jul 11, 2017 at 06:01:38AM +0000, Nick Terrell wrote:
> On 7/10/17, 9:57 PM, "Nick Terrell" <terrelln@...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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