> From: David Brownell > Newsgroups: gmane.linux.usb.devel > Subject: Re: PATCH: usb: descriptor structures have to be packed > Date: Tue, 6 Feb 2007 16:02:27 -0800
Hallo. > On Tuesday 06 February 2007 2:48 pm, Pete Zaitcev wrote: >> On Tue, 6 Feb 2007 13:08:19 -0800, Inaky Perez-Gonzalez <[EMAIL PROTECTED]> >> wrote: >> >> > [btw, I truly have little idea about which are those specific costs, >> > out of professional curiosity, got any pointers?] >> >> http://www.digitalvampire.org/blog/articles/2006/07/31/why-you-shouldnt-use-__attribute__-packed > > In fact the executive summary on that page applies nicely: > > "don't add __attribute__((packed)) to your code unless > you know you need it." > > My point [b] explained why/how "we know we need it". You seem > to have ignored that part of the discussion this time, just like > the last several times it's come up. > > > And for anyone who's tempted to shudder at the ASM there, from > two platforms well-known for code bloat: [] Dave, would you like additionally answer some related questions? TIA. Would you clarify "non aligned access issue" on well known platforms (*x86-*). I see on gcc's output, that they are not carry much about it: ,-*- gcc | objdump -S -*- | 40046a: 0f b7 40 05 movzwl 0x5(%rax),%eax | 40046e: 0f be c0 movsbl %al,%eax (memory access shift is odd -- 0x05) | or (one is even -- 0x08) | 400445: 8b 40 08 mov 0x8(%rax),%eax | 400448: 0f be c0 movsbl %al,%eax `-*- some docs are saying about (0)"slightly faster operation", if access is aligned. What it's all about? (1) When i started to understand things about packed/padded structs in gcC, my first ever question was: "WTF that padding is doing there, if i didn't coded it explicitly?" (2) Is it impact of hardware memory bus access issues, like (0) or that bloated code from ia64 and Sparc, showed in proposed URL, that i even can't parse (: ? I've also read in some Intel's docs, that having bigger struct's members first is better, what can you say on this? >> __attribute((__packed__)) is hardly superior to the old style which >> worked fine for a 1000 years before gcc. > > Yes, that "old style" you're promoting is indeed from the Dark Ages. > And you *can* make anything sound bad if you disregard its strengths, > as you've been doing with respect to "packed". Do you mean byte access "magic", probably due to (2)? In scope of (1) i can't understand: structs are padded implicitly, member access is coded explicitly. Thus i think, using "packed" attribute for things like inet-packets, wire, hardware is absolute *must*, because structures are usually copied/moved from memory to devices "as they are". (hope you will understand all this "English" ;) Thanks. -- -o--=O`C info emacs : not found /. .\ ( is there any reason to live? ) #oo'L O info make : not found o ( yes --- R.I.P. FSF+RMS ) <___=E M man gcc : not found `-- ( viva Debian Operating System ) ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier. Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
