Could an Intel 486 wake up with FC3

[James Wilkinson]

Jean Francois Martinez wrote:
> The problem is that GCC is not (or at least wasn't when I tested it
> on gcc 2.96) smart enough to optimize for processor X while restricting
> to Y instruction set when Y is different from both X 
> and 386.  Ie if you use "-mcpu=i686 -march=i586" (ie optimize for 686
> but restrict to Pentium instruction) it generates code
> who runs at exactly the same speed that if you had typed "-mcpu=i686
> -march=i386".  When I say exactly I mean within 0.1% or 0.2% margin (ie
> within noise limits) and for EVERY test.  The usual thing when 
> playing with compiler parms was that on at least one of my tests
> one of the flags providing at least a 2 or 3% difference.  Not
> there.  Complete equality.  At this point the logical deduction is that
> gcc "downgraded" to 386 mode because it was unable to generate
> code using the 686 timetable and 586 instructions.
> 
> As I said I tested the above on gcc 2.96 and haven't repeated the
> experience on gcc 3.x
> 
> 
> So except for marketing purposes it doesn't make sense to try
> to compile "normal" packages with 586 as the minimal processor.
> The packages where there will be a difference are those who
> rely either on hand-written assembler like the kernel or glibc.

Um. You seem to be assuming that the extra instructions in the Pentium
will improve performance, and that therefore gcc is deficient. Might it
not simply be that the (few) extra instructions really aren't that
useful in improving performance?

(Incidentally: which CPU were you using? I understand that not all of
the new instructions provide a performance benefit on later processors:
they tend to get broken back down to the same micro-ops.)

The logical conclusion that Red Hat's engineers came to (and they're far
more knowledgable about the whole thing than I am) is simply that none
of the extra instructions that the Pentium makes available (and there
aren't that many) are very useful in improving performance. The major
exception comes in atomic locking instructions for NTPL. And that (for
very good binary and source level compatibility reasons) is done in
glibc, which *is* compiled to i586 or i686.

See, for example,
https://www.redhat.com/archives/fedora-devel-list/2004-June/msg00232.html
and linked posts.

James.

-- 
James Wilkinson       | "Luck is my middle name," said Rincewind,
Exeter    Devon    UK | indistinctly. "Mind you, my first name is Bad."
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