Re: [PATCH i386] Allow sibcalls in no-PLT PIC

[Michael Matz <matz@suse.de>]

Hi,

On Wed, 20 May 2015, Rich Felker wrote:

> > of a win that often, outside toy examples.  Sure, the compiler can hoist 
> > function addresses trivially, but I think it will lead to spilling more 
> > often than not, or alternatively the hoisting will be undone by the 
> > register allocators rematerialization.  Of course, this would have to be 
> > measured for real not hand-waved, but, well, I'd be surprised if it's not 
> > so.
> 
> The obvious example where it's useful on x86_64 is a major class: 

Yes, I can construct all kinds of examples where it's useful.  That 
doesn't touch the topic of real-world cases or hard numbers actually 
comparing the number of hoisted callee addresses, the number that stay 
hoisted until after register allocation and the number of spills added by 
hoisting, on some relevant code base, like gcc itself, or SPEC.

> anything where the majority of the callee's data is floating point and 
> thus kept in xmm registers.

This code tends to work on multiple arrays in practice, and hence integer 
registers are required for all the addresses and offsets and loop 
counters.

> In that case register pressure is a lot lower,

Register pressure on x86 is never low :)  Yes, x86-64 and others are much 
better in this regard.

> and there's also an obvious class of cross-DSO functions calls you'd be 
> making over and over again: anything from libm.


Ciao,
Michael.