On Thu, May 20, 2010 at 9:35 PM, Alan Modra <amo...@gmail.com> wrote: > On Thu, May 20, 2010 at 09:40:47AM -0700, Mark Mitchell wrote: >> It is of course a feature much >> less valuable on a workstation/server class operating system than on the >> VxWorks/RTEMS class of RTOS systems. > > Even on servers this option may be quite valuable. I recall seing > figures that showed using fp regs for something like structure copies > could cost thousands of cpu cycles. > > Why? With lazy fpu save and restore, the first use of the fpu in a > given time slice takes an interrupt. So if your task is only using > the fpu occasionally it is a severe misoptimization to choose to use > fp regs rather than gp regs.
If the patch is the one I remember, I believe the consensus is the patch was not safe -- it was substituting RTL patterns in ways that could violate GCC internal semantics. I would gratefully accept a safe and reliable patch, such as scanning the function early and disabling patterns with FP instructions if the function did not contain explicit references to FP operations. No one disagrees with the potential benefit of the feature. David
