Dax Kelson wrote:
On Sat, 2004-09-25 at 18:09, Alan Cox wrote:I don't think that -mtune=i686 is a lot faster in average (as I remeber the difference was 1-2% for SPECInt2000). Although some benchmarks (with few hot spots in loops) could be a lot faster. E.g. the difference between -mtune=nocona (em64t) and -mtune=k8 (amd64) on Xeon (nocona core) is really big (up to 20% for SPECInt2000).
On Sat, Sep 25, 2004 at 11:09:42PM +0200, Arjan van de Ven wrote:
in theory we compile all of fedora core with -mtune=i686 (orI noticed a curiosity playing with this that perhaps the gcc folks can
-mtune=pentium4 nowadays). however that only actually happens if
packages propagate the RPM_OPT_FLAGS into the CFLAGS properly; almost
all but not quite not all packages actually do this.
explain the right choice for - on Dothan -mtune=i686 is a lot faster than
tuning for pentium4.
-mtune=i686 means tunning for pentiumpro core (core of pentium2 and pentium3 processors). As I know pentium-m design was based on this core (although intel used some implementations from P4 cores too). Of course the code tunned with -mtune=i686 will work better for pentium-m than code tunned with -mtune=pentium4. The factor which has the biggest impact is that Pentium-M is more sensitive to code alignment (function, loop, label alignment) than pentium4. Therefore code generated with -mtune=i686 has bigger alignments and as consequence the generated code with this option is bigger. Also latency time of pentium4 and pentium-m are differrent. Penium-M has more constraints in issuing several insns on the same cycle than pentium4 does. Therefore insn scheduling is more important for pentium-m than pentium4. There are a few other different parameters which gcc takes into account when it tunnes code for pentium-m.
AFAIK there's a -march=pentium-m in gcc 3.4.x. Does that imply thatThere is no difference between tunning for pentium-m and i686 (or pentiumpro) in gcc-3.4. It is just another alias.
-mtune=pentium-m exists?
Vlad
