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Re: Optimizations for the MIPS target
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David Daney wrote:
> Joe Buck wrote:
>
>> On Thu, May 04, 2006 at 04:31:15PM +0300, Nikolaos Kavvadias
>> wrote:
>>
>>> My question here is: culdn't "-Os" optimization option force
>>> .align 0 alignment? Is there any way to use the minimal data
>>> memory requirements for global symbols possible?
>>
>>
>>
>> While it could, I hope that it does not go so far as to make the
>> reads unaligned (on processors that allow this). -Os currently
>> optimizes well for speed as well as for space, and there's a
>> substantial performance penalty for unaligned reads and writes in
>> many cases.
>>
>
> IIRC, On mips they are trapped and emulated in the Linux/(your OS
> here) kernel. That means that literally hundreds of instructions
> are executed for each unaligned access.
>
> On systems where they are not emulated, you would get a hard
> failure.
>
> David Daney.
That's reasonable. And the trap handling routines, being called really
often in such case, incur a very high performance cost. In a
hypothetical case of a processor, there could be provision for
adjusting the unaligned (byte-wise) addresses to the intended
alignment in hardware But that's probably unecessary complexity.
Nikolaos Kavvadias
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