Messages in this thread |  | | From | Mike Harrold <> | Subject | Re: * 4 converted to << 2 for networking code | Date | Wed, 10 Jan 2001 10:18:38 -0500 (EST) |
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> > On Wed, Jan 10, 2001 at 06:03:22PM +0100, antirez wrote: > > On Wed, Jan 10, 2001 at 09:54:04AM -0500, Brian Gerst wrote: > > > This patch isn't really necessary, because GCC will automatically > > > convert multiplications and divisions by powers of two to use shifts. > > > > Sure, but since many << 2 already exists in the net kernel code > > I feel it's better to use just a format, and it seems more appropriate > > to write << 2, just to reflect what we want. > > Also some piece of kernel code may be used with compilers that does not > > optimize power of two. > > On most processors <<2 is slower than *4. It's outright stupid to > write <<2 when we mean *4 in order to optimize for one out of a > gazillion supported architectures - even more so when the compiler > for the one CPU where <<2 is faster, will actually generate a shift > instead of a multiply as a part of the standard optimization. > > One question for the GCC people: Will gcc change <<2 to *4 on other > architectures ? If so, then my case is not quite as strong of course.
Be careful. *4 is not a simple <<2 substitution (by the compiler) if the variable is signed. *4 translates to 3 instructions (on x86) if it's an int.
My feeling is that it shouldn't matter if you use <<2 or *4 even if the compiler optimises - one would hope that the compiler would optimise to the fastest in both directions.
Regards,
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