Re: [PATCH] Extend -falign-FOO=N to N[,M]: the second number is max padding

[Michael Matz <matz at suse dot de>]

Hi,

On Thu, 11 Aug 2016, Denys Vlasenko wrote:

> This change makes it possible to align function to 64-byte boundaries 
> *IF* this does not introduce huge amount of padding.
> 
> Patch drops forced alignment to 8 if requested alignment is higher than 
> 8: before the patch, -falign-functions=9 was generating
> 
>         .p2align 4,,8
>         .p2align 3
> 
> which means: "align to 16 if the skip is 8 bytes or less; else align to 8".
> After this change, ".p2align 3" is not emitted.
> 
> It is dropped because I ultimately want to do something like 
> -falign-functions=64,8 - IOW, I want to align functions to 64 bytes, but 
> only if that generates padding of less than 8 bytes - otherwise I want 
> *no alignment at all*.

Have you tested the performance impact of your patch?  Note that the macro 
you changed is used for function and code label alignment.  So, unless I 
misunderstand something that means that if the large alignment can't be 
achieved for e.g. a loop start label, you won't align it at all anymore.  
This should be fairly catastrophic for any loopy benchmark, so anything 
like this would have to be checked on a couple benchmarks from cpu2000 
(possibly cpu2006), which has some that are extremely alignment sensitive.

Even for function labels I'd find no alignment at all strange, and I don't 
see why you'd want this.

> The forced ".p2align 3" interferes with that intention.

Well, yes, but _why_ don't you want 8 byte alignment if 64-byte can't be 
achieved?


Ciao,
Michael.