6 Jun
2020
6 Jun
'20
11:49 p.m.
Steve Johnson's position paper on optimising
compilers may amuse you:
https://dl.acm.org/doi/abs/10.1145/567532.567542
Indeed. This passage struck a particular chord:
"I contend that the class of applications that depend on, for example, loop
optimization and dead code elimination for their efficient solution is of
modest size, growing smaller, and often very susceptible to expression in
applicative languages where the optimization is built into the individual
applicative operators."
I don't know whether I saw that note at the time, but since then I've
come to believe, particularly in regard to C, that one case of dead-code
elmination should be guaranteed. That case is if(0), where 0 is the
value of a constant expression.
This guarantee would take the place of many--possibly even
most--ifdefs. Every ifdef is an ugly intrusion and a pain to read.
Syntactically it occurs at top level completely out of sync with the
indentation and flow of text. Conversion to if would be a big win.
Doug
6 Jun
6 Jun
11:55 p.m.
New subject: History of popularity of C
On Sat, Jun 6, 2020 at 3:50 PM Doug McIlroy <doug(a)cs.dartmouth.edu> wrote:
I'd love something like this to work, but the semantic interpretation would
need to also somehow be omitted, otherwise how do you replace
#ifdef AIX
ioct(fd, AIX_SPECIFIC_IOCTL, ...)
#endif
on a HUPX system that doesn't define AIX_SPECIFIC_IOCTL...
I remember hearing that BLISS could cope because it deferred the semantic
interpretation of the identifiers until after a round of dead code
elimination so it didn't need a pre-processor...
Warner
Steve
Johnson's position paper on optimising compilers may amuse you:
https://dl.acm.org/doi/abs/10.1145/567532.567542
Indeed. This passage struck a particular chord:
"I contend that the class of applications that depend on, for example, loop
optimization and dead code elimination for their efficient solution is of
modest size, growing smaller, and often very susceptible to expression in
applicative languages where the optimization is built into the individual
applicative operators."
I don't know whether I saw that note at the time, but since then I've
come to believe, particularly in regard to C, that one case of dead-code
elmination should be guaranteed. That case is if(0), where 0 is the
value of a constant expression.
This guarantee would take the place of many--possibly even
most--ifdefs. Every ifdef is an ugly intrusion and a pain to read.
Syntactically it occurs at top level completely out of sync with the
indentation and flow of text. Conversion to if would be a big win.
8 Jun
8 Jun
3:56 p.m.
New subject: History of popularity of C
On Sat, Jun 06, 2020 at 05:49:23PM -0400, Doug McIlroy wrote:
Syntactically it occurs at top level completely out of
sync with the
indentation and flow of text. Conversion to if would be a big win.
Well, doesn't that rather depend upon how one indents it?
As I recall, that freedom came with ANSI C.
But yeah, I'd prefer the if(0) type of elimination.
DF
$ cc yes-no.c; ./a.out; cc -DYES yes-no.c; ./a.out; cat yes-no.c
No
Yes
#include <stdio.h>
int main(void)
{
#ifdef YES
puts("Yes");
#else
puts("No");
#endif
return 0;
}
5:20 p.m.
New subject: History of popularity of C
But yeah, I'd prefer the if(0) type of
elimination.
Some parts of OpenSSL do this, and it seems to work well across a pretty
wide variety of platforms/toolchains.
A really important benefit is that you get code syntax-checked as opposed
to unused #ifdef branches rotting. A minor benefit is you don't have ifdef
spaghetti around local variables, to avoid "xxx is not used" warnings.
5:30 p.m.
New subject: History of popularity of C
On Sat, Jun 6, 2020 at 5:50 PM Doug McIlroy <doug(a)cs.dartmouth.edu> wrote:
This came up on this list back in 2017 and I posted something about this at
the time:
https://minnie.tuhs.org/pipermail/tuhs/2017-January/009325.html
If the language had been extended to support a "compile-type conditional"
syntax ("if #(LINUX)", for example) it would have obviated the need for
most preprocessor conditionals. As long as, syntactically, the compiler
could tokenize a block in absence of e.g. type definitions, dead-code
elimination could do the job.
- Dan C.
Steve
Johnson's position paper on optimising compilers may amuse you:
https://dl.acm.org/doi/abs/10.1145/567532.567542
Indeed. This passage struck a particular chord:
"I contend that the class of applications that depend on, for example, loop
optimization and dead code elimination for their efficient solution is of
modest size, growing smaller, and often very susceptible to expression in
applicative languages where the optimization is built into the individual
applicative operators."
I don't know whether I saw that note at the time, but since then I've
come to believe, particularly in regard to C, that one case of dead-code
elmination should be guaranteed. That case is if(0), where 0 is the
value of a constant expression.
This guarantee would take the place of many--possibly even
most--ifdefs. Every ifdef is an ugly intrusion and a pain to read.
Syntactically it occurs at top level completely out of sync with the
indentation and flow of text. Conversion to if would be a big win.
6:32 p.m.
New subject: History of popularity of C
Doug McIlroy <doug(a)cs.dartmouth.edu> wrote:
Every ifdef is an ugly intrusion and a pain to read.
I maintain `unifdef`, which is not a complicated program to port but it
does require some platform-specific stuff on Windows (partly because of
the poor standards conformance of the Windows C library). There's a fun
rule that `unifdef` must contain no #ifdef (or #if etc.) so the
portability stuff is handled by the build scripts adjusting the include
path and list of object files. Not a very scalable solution :-)
Tony.
--
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participants (6)
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Dan Cross -
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Doug McIlroy -
Richard Salz -
Tony Finch -
Warner Losh