From: Trevor Saunders <tbsaunde@tbsaunde.org>
To: Jeff Law <law@redhat.com>
Cc: Richard Biener <richard.guenther@gmail.com>,
Aldy Hernandez <aldyh@redhat.com>,
Oleg Endo <oleg.endo@t-online.de>,
Martin Sebor <msebor@gmail.com>,
gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: protected alloca class for malloc fallback
Date: Mon, 08 Aug 2016 17:32:00 -0000 [thread overview]
Message-ID: <20160808173939.GA13790@ball> (raw)
In-Reply-To: <21bcbebe-28a8-58a7-68e8-af9abcb03dce@redhat.com>
On Mon, Aug 08, 2016 at 11:00:28AM -0600, Jeff Law wrote:
> On 08/06/2016 09:08 AM, Richard Biener wrote:
> > On August 6, 2016 12:15:26 PM GMT+02:00, Aldy Hernandez <aldyh@redhat.com> wrote:
> > > On 08/05/2016 04:07 PM, Richard Biener wrote:
> > > > On August 5, 2016 8:15:54 PM GMT+02:00, Oleg Endo
> > > <oleg.endo@t-online.de> wrote:
> > > > > On Fri, 2016-08-05 at 19:55 +0200, Richard Biener wrote:
> > > > >
> > > > > >
> > > > > > Please don't use std::string. For string building you can use
> > > > > > obstacks.
> > > > > >
> > > > >
> > > > > Just out of curiosity ... why? I remember there was some discussion
> > > > > about it, what was the conclusion? Is that now a general rule or
> > > does
> > > > > it depend on the context where strings are used?
> > > >
> > > > Because you make a messy mix of string handling variants.
> > > Std::string is not powerful enough to capture all uses, it is vastly
> > > more expensive to embed into structs and it pulls in too much headers.
> > > > (Oh, and I hate I/o streams even more)
> > >
> > > Oh, and there is prior use in ipa-chkp.c, although I suppose it
> > > could've
> > > crept in:
> >
> > Definitely.
> >
> > >
> > > std::string s;
> > >
> > > /* called_as_built_in checks DECL_NAME to identify calls to
> > > builtins. We want instrumented calls to builtins to be
> > > recognized by called_as_built_in. Therefore use original
> > > DECL_NAME for cloning with no prefixes. */
> > > s = IDENTIFIER_POINTER (DECL_NAME (fndecl));
> > > s += ".chkp";
> > > DECL_NAME (new_decl) = get_identifier (s.c_str ());
> > >
> > > You can't tell me obstacks are easier on the eyes for this ;-).
> >
> > Even strcat is shorter and cheaper.
> ISTM that unless the code is performance critical we should be writing code
> that is easy to understand and hard to get wrong.
it seems hard to disagree with that ;)
> Thus when we have something that is non-critical and can be handled by std::
> routines we should be using them.
>
> If performance is important in a particular piece of code an obstack or
> explicit malloc/free seems better.
I'm not totally convinced we have to trade off performance against a
good interface. It seems to me it wouldn't be that complicated to build
a string class on top of auto_vec that has a similar interface to
std::string.
I'd really rather not have to write a string class of our own, but I can
see some significant advantages to it.
First sizeof std::string is 32 on x86_64, a char *, a size_t for the
length, and a 16 byte union of a size_t for allocated size and a 16 byte
buffer for short strings. I suppose some of this is required by the C++
standard, but I doubt we really need to care about strings longer than
2^32 in gcc. If we put the length and allocated size in the buffer, and
have a separate class for stack allocated buffers I think we can have a
string that is just sizeof void *.
Second it would be useful performance wise to have a std::string_view
type class, but that is c++14 or 17? only so we'd need to import it into
gcc or something.
Trev
next prev parent reply other threads:[~2016-08-08 17:32 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-04 11:30 Aldy Hernandez
2016-08-04 12:58 ` Richard Biener
2016-08-04 15:19 ` Aldy Hernandez
2016-08-04 19:24 ` Jeff Law
2016-08-05 14:37 ` Aldy Hernandez
2016-08-05 15:15 ` Pedro Alves
2016-08-05 16:23 ` Jeff Law
2016-08-05 17:48 ` Richard Biener
2016-08-05 8:17 ` Richard Biener
2016-08-04 19:06 ` Pedro Alves
2016-08-04 19:16 ` Jeff Law
2016-08-04 19:22 ` Pedro Alves
2016-08-04 19:26 ` Jeff Law
2016-08-04 19:31 ` Pedro Alves
2016-08-05 2:10 ` Martin Sebor
2016-08-05 14:42 ` Aldy Hernandez
2016-08-05 17:56 ` Richard Biener
2016-08-05 18:16 ` Oleg Endo
2016-08-05 20:07 ` Richard Biener
2016-08-06 10:09 ` Aldy Hernandez
2016-08-06 10:15 ` Aldy Hernandez
2016-08-06 15:08 ` Richard Biener
2016-08-08 17:00 ` Jeff Law
2016-08-08 17:32 ` Trevor Saunders [this message]
2016-08-08 19:03 ` Richard Biener
2016-08-09 11:34 ` Oleg Endo
2016-08-09 17:34 ` Trevor Saunders
2016-08-10 17:03 ` Oleg Endo
2016-08-11 1:23 ` Trevor Saunders
2016-08-11 12:18 ` Oleg Endo
2016-08-11 17:55 ` Trevor Saunders
2016-08-20 2:29 ` Mike Stump
2016-08-21 20:00 ` C++11? (Re: protected alloca class for malloc fallback) Pedro Alves
2016-08-22 7:10 ` Trevor Saunders
2016-08-22 7:28 ` Richard Biener
2016-08-22 12:02 ` Eric Gallager
2016-08-22 12:58 ` Manuel López-Ibáñez
2016-08-22 22:08 ` Mike Stump
2016-08-23 23:17 ` Eric Gallager
2016-08-09 13:17 ` protected alloca class for malloc fallback Aldy Hernandez
2016-08-09 13:21 ` Bernd Schmidt
2016-08-10 10:04 ` Richard Biener
2016-08-10 10:12 ` Aldy Hernandez
2016-08-10 10:39 ` Richard Biener
2016-08-10 18:00 ` Jeff Law
2016-08-10 18:33 ` Richard Biener
2016-08-16 16:28 ` Jeff Law
2016-08-16 16:44 ` Jakub Jelinek
2016-08-16 16:47 ` Jeff Law
2016-08-16 17:54 ` Martin Sebor
2016-08-17 8:27 ` Richard Biener
2016-08-17 13:39 ` Martin Sebor
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