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
