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Re: compiler-options?

On Sat, 3 Feb 2001, Kerstin Hoef-Emden wrote:

> when I received my Alpha, I was told to use the compiler-option
> -Wa,-m21164a. Reading a little bit in the gcc info, I didnīt find this
> option (since gcc never complains, when I use it, it seems to exist),
> but I found -mcpu=CPU-type. What is the difference between the two
> options? Is one more recommendable than the other?

I would use -mcpu= before -Wa,-m21164a in general, but that really depends
on what you're looking to do.  The -Wa option passes the -m21164a option
to the assembler, which can help schedule the code better for a particular
CPU variant, but -mcpu= can actually allow gcc to generate/omit
instructions depending on the CPU variant (as well as tell the assembler
to schedule the code better).  Basically, any -Wa, option just passes
whatever follows the comma to the assembler without the compiler looking
at it, but -mcpu= lets the compiler target it's output towards that CPU

> One thing I was told to use as well, was -mieee. What are the comments
> on this item? Does it slow down the execution of the code?

The -mieee option is a special case, IMO.  If you have code that needs it
(ie, anything that needs IEEE math compliance or anything that will
generate a SIGFPE), use it.  If not, it does slow down the code and bloats
it a bit.  There have been frequent discussions about compiling the entire
Alpha deb tree with -mieee, but so far, nobody has made a convincing
enough argument for it to justify the general slow-down that we would
see.  In reality, though, for small binaries like 'ls', it won't make a
noticable difference, but larger software would make it very apparent.


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