Hi,

This patch improves register pressure scheduling (both SCHED_PRESSURE_WEIGHTED and SCHED_PRESSURE_MODEL) to better estimate number of available registers.

At the moment the scheduler does not account for spills in the prologues and restores in the epilogue, which occur from use of call-used registers.  The current state is, essentially, optimized for case when there is a hot loop inside the function, and the loop executes significantly more often than the prologue/epilogue.  However, on the opposite end, we have a case when the function is just a single non-cyclic basic block, which executes just as often as prologue / epilogue, so spills in the prologue hurt performance as much as spills in the basic block itself.  In such a case the scheduler should throttle-down on the number of available registers and try to not go beyond call-clobbered registers.

The patch uses basic block frequencies to balance the cost of using call-used registers for intermediate cases between the two above extremes.

The motivation for this patch was a floating-point testcase on arm-linux-gnueabihf (ARM is one of the few targets that use register pressure scheduling by default).

A "thanks" goes to Richard good discussion of the problem and suggestions on the approach to fix it.

The patch was bootstrapped on x86_64-linux-gnu (which doesn't really exercises the patch), and cross-tested on arm-linux-gnueabihf and aarch64-linux-gnu.

OK to apply?

--
Maxim Kuvyrkov
www.linaro.org