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Re: [RFC][BZ #17943] Use long for int_fast8_t
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: Rich Felker <dalias at libc dot org>
- Cc: libc-alpha at sourceware dot org
- Date: Wed, 11 Feb 2015 14:34:09 +0100
- Subject: Re: [RFC][BZ #17943] Use long for int_fast8_t
- Authentication-results: sourceware.org; auth=none
- References: <20150208110426 dot GA28729 at domone> <20150209181324 dot GE23507 at brightrain dot aerifal dot cx>
On Mon, Feb 09, 2015 at 01:13:24PM -0500, Rich Felker wrote:
> On Sun, Feb 08, 2015 at 12:04:26PM +0100, OndÅej BÃlka wrote:
> > Hi, as in bugzilla entry what is rationale of using char as int_fast8_t?
> > It is definitely slower with division, following code is 25% slower on
> > haswell with char than when you use long.
> This claim is nonsense. It's a compiler bug. If the 8-bit divide
> instruction is slow, then the compiler should use 32-bit or 64-bit
> divide instructions to divide 8-bit types. (Note: there's actually no
> such thing as a division of 8-byte types; formally, they're promoted
> to int, so it's the compiler being stupid if it generates a slow 8-bit
> divide instruction for operands that are formally int!) There's no
> reason to use a different type for the _storage_.
That is also nonsense, you cannot get same speed as 32bit instruction
without having 8bit instruction with same performance.
Compiler must add extra truncation instructions to get correct result
which slows it down, otherwise it gets wrong result for cases like (128+128)%3