> I disagree, perhaps mostly because the compiler is not clever enough, but
> right
> now the code for solution 1 is (actually I have rewritten the code
> and it reads:
>
> mask = (FM & 1)
> | ((FM << 3) & 0x10)
> | ((FM << 6) & 0x100)
> | ((FM << 9) & 0x1000)
> | ((FM << 12) & 0x10000)
> | ((FM << 15) & 0x100000)
> | ((FM << 18) & 0x1000000)
> | ((FM << 21) & 0x10000000);
> to avoid sequence point in case it hampers the compiler)
>
> and the output is:
>
> rlwinm 10,3,3,27,27 # D.11621, FM,,
> rlwinm 9,3,6,23,23 # D.11621, FM,,
> or 9,10,9 #, D.11621, D.11621, D.11621
> rlwinm 10,3,0,31,31 # D.11621, FM,
> or 9,9,10 #, D.11621, D.11621, D.11621
> rlwinm 10,3,9,19,19 # D.11621, FM,,
> or 9,9,10 #, D.11621, D.11621, D.11621
> rlwinm 10,3,12,15,15 # D.11621, FM,,
> or 9,9,10 #, D.11621, D.11621, D.11621
> rlwinm 10,3,15,11,11 # D.11621, FM,,
> or 9,9,10 #, D.11621, D.11621, D.11621
> rlwinm 10,3,18,7,7 # D.11621, FM,,
> or 9,9,10 #, D.11621, D.11621, D.11621
> rlwinm 3,3,21,3,3 # D.11621, FM,,
> or 9,9,3 #, mask, D.11621, D.11621
> mulli 9,9,15 # mask, mask,
>
> see that r9 is used 7 times as both input and output operand, plus
> once for rlwinm. This gives a dependency length of 8 at least.
>
> In the other case (I've deleted the code) the dependency length
> was significantly shorter. In any case that one is fewer instructions,
> which is good for occasional use.
Hmmm... I hand-counted a dependency length of 8 for the other version.
Maybe there are some ppc instructions that reduce it.
Stupid compiler :-)
Trouble is, I bet that even if you code it as:
mask1 = (FM & 1) | ((FM << 3) & 0x10);
mask2 = ((FM << 6) & 0x100) | ((FM << 9) & 0x1000);
mask3 = ((FM << 12) & 0x10000) | ((FM << 15) & 0x100000);
mask4 = ((FM << 18) & 0x1000000) | ((FM << 21) & 0x10000000);
mask1 |= mask2;
mask3 |= mask4;
mask = mask1 | mask3;
the compiler will 'optimise' it to the above before code generation.
If it doesn't adding () to pair the | might be enough.
Then a new version of the compiler will change the behaviour again.
David
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