This is the mail archive of the mailing list for the glibc project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [RFC][BZ #17943] Use long for int_fast8_t

On 11/02/15 13:34, OndÅej BÃlka wrote:
> 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

Only if the intermediate result (128+128) is assigned directly to a
variable with less precision than int.  Otherwise the whole expression
is calculated with int precision.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]