| 1 |
On Thu, Jun 26, 2003 at 12:58:47PM +0100, Dhruba Bandopadhyay wrote: |
| 2 |
> Genflags is looking to be a very promising tool. Has any thought been |
| 3 |
> given to a secondary tier of cflags that would take a little more |
| 4 |
> liberty and be more optimised/aggressive? |
| 5 |
> |
| 6 |
> Can initiative be taken to provide a slightly more aggressive yet safe |
| 7 |
> set of cflags assigned to a command parameter or would that be difficult |
| 8 |
> and time consuming to incorporate into the tool? Just a thought. |
| 9 |
The tool already has everything for adding in special CFLAGS on all levels. |
| 10 |
That is: |
| 11 |
globally - for all arches |
| 12 |
per-arch - eg: only for ppc |
| 13 |
per-subarch - eg: only for athlon-xp |
| 14 |
|
| 15 |
If somebody else is willing to undertake _conclusively_ proving |
| 16 |
optimization flags are 100% stable and _always_ provide correct |
| 17 |
(-ffast-math doesn't) and most importantly, do _NOT_ interfere with |
| 18 |
tracking down bugs, then those flags will be added. |
| 19 |
|
| 20 |
> For example, the following flags were omitted in the case of my |
| 21 |
> machine. |
| 22 |
> |
| 23 |
> -fomit-frame-pointer -fprefetch-loop-arrays -falign-functions=4 |
| 24 |
> -funroll-loops -ffast-math -fforce-addr |
| 25 |
Lets look at this set of CFLAGS for now. |
| 26 |
-fomit-frame-pointer , makes debugging impossible, not going to use |
| 27 |
this. |
| 28 |
|
| 29 |
-falign-functions=4 , This is automatically implied by your -march/-mcpu |
| 30 |
settings. |
| 31 |
|
| 32 |
-funroll-loops , This only slows things down the majority of the time. |
| 33 |
|
| 34 |
-ffast-math , see this from the GCC manpage: |
| 35 |
"This option should never be turned on by any -O option since it can |
| 36 |
result in incorrect output for programs which depend on an exact |
| 37 |
implementation of IEEE or ISO rules/specifications for math functions." |
| 38 |
|
| 39 |
-fprefetch-loop-arrays , The only flag here of much value. Automatically |
| 40 |
implied by -O3 on most systems I believe. |
| 41 |
|
| 42 |
-fforce-addr, from the GCC manpage: |
| 43 |
"Force memory address constants to be copied into registers before doing |
| 44 |
arithmetic on them. This may produce better code just as -fforce-mem |
| 45 |
may." |
| 46 |
Pointer arirthmatic isn't common, so the performance boost here is |
| 47 |
minimal. |
| 48 |
|
| 49 |
|
| 50 |
> P.S. Here is a demonstration of it working on my machine. |
| 51 |
> ==== |
| 52 |
> dhruba@wolf bin $ ./info2flags |
| 53 |
> CHOST="i686-unknown-gnu-linux" |
| 54 |
> CFLAGS="-march=pentium4 -O3 -pipe" |
| 55 |
> CXXFLAGS="-march=pentium4 -O3 -pipe" |
| 56 |
[snip] |
| 57 |
Did you look at the README? It states that the help functions are |
| 58 |
not done yet, and to use the source instead. |
| 59 |
|
| 60 |
-- |
| 61 |
Robin Hugh Johnson |
| 62 |
E-Mail : robbat2@××××××××××××××.net |
| 63 |
Home Page : http://www.orbis-terrarum.net/?l=people.robbat2 |
| 64 |
ICQ# : 30269588 or 41961639 |
| 65 |
GnuPG FP : 11AC BA4F 4778 E3F6 E4ED F38E B27B 944E 3488 4E85 |
Archives