SystemZ (z13) has a register file of 16 fp registers and 32 vector registers, where the fp registers are subregs of the first 16 vector registers.
The mischeduler updates current register pressure as it schedules an instruction. If register pressure in the fp reg class increases, so should the vector reg class' regressure value. However, virtual fp registers - no matter how many - could never use the upper 16 vector registers. So I am then thinking that the vector registers should not feel they are under pressure just because there might be many fp registers.
Checking the debug dump of mischeduler:
Scheduling SU(28) %vreg505<def,tied1> = SEBR %vreg505<tied0>, %vreg498, %CC<imp-def,dead>; FP32Bit:%vreg505,%vreg498
The VR32Bit value keeps going up, and from what I will see it might very well affect scheduling of vector instructions poorly, when the VR32Bit regpressure set goes over its limit. It seems to me there should be some mechanism for how a subregclass influences a superregclass in this scenario.
The PressureSets for a FP32Bit virtual register are
FP32Bit, Limit: 16
VR32Bit, Limit: 32
A vector register should update both these pressure sets normally, but an fp register should somehow never make the VR32Bit pset go over the limit of 32.
Or am I missing something that will make this work anyway?