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

[Rich Felker <dalias@libc.org>]

On Wed, May 20, 2015 at 02:10:41PM +0200, Michael Matz wrote:
> Hi,
> 
> On Tue, 19 May 2015, Richard Henderson wrote:
> 
> > It is.  The relaxation that HJ is working on requires that the reads 
> > from the got not be hoisted.  I'm not especially convinced that what 
> > he's working on is a win.
> > 
> > With LTO, the compiler can do the same job that he's attempting in the 
> > linker, without an extra nop.  Without LTO, leaving it to the linker 
> > means that you can't hoist the load and hide the memory latency.
> 
> Well, hoisting always needs a register, and if hoisted out of a loop 
> (which you all seem to be after) that register is live through the whole 
> loop body.  You need a register for each different called function in such 
> loop, trading the one GOT pointer with N other registers.  For 
> register-starved machines this is a real problem, even x86-64 doesn't have 
> that many.  I.e. I'm not convinced that this hoisting will really be much 
> 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:
anything where the majority of the callee's data is floating point and
thus kept in xmm registers. In that case register pressure is a lot
lower, and there's also an obvious class of cross-DSO functions calls
you'd be making over and over again: anything from libm.

Rich