[LLVMdev] Possible VirtRegMap Bug

[David Greene]

I've been playing around with spillers and found that the SimpleSpiller fails 
badly on a particular code.

The problem arises because SimpleSpiller does the test 
VRM.isAssignedReg(virtReg) which is implemented as:

00183     bool isAssignedReg(unsigned virtReg) const {
00184       if (getStackSlot(virtReg) == NO_STACK_SLOT &&
00185           getReMatId(virtReg) == NO_STACK_SLOT)
00186         return true;
00187       // Split register can be assigned a physical register as well as a
00188       // stack slot or remat id.
00189       return (Virt2SplitMap[virtReg] && Virt2PhysMap[virtReg] != 
NO_PHYS_REG);
00190     }

VRM::assignVirt2Phys is implemented as:

00147     void assignVirt2Phys(unsigned virtReg, unsigned physReg) {
00148       assert(TargetRegisterInfo::isVirtualRegister(virtReg) &&
00149              TargetRegisterInfo::isPhysicalRegister(physReg));
00150       assert(Virt2PhysMap[virtReg] == NO_PHYS_REG &&
00151              "attempt to assign physical register to already mapped "
00152              "virtual register");
00153       Virt2PhysMap[virtReg] = physReg;
00154     }

Note that this doesn't clear the stack slot or remat mappings.

In the particular code I'm looking at, linear scan spills an interval and
one of the intervals created (%reg1631) gets mark as rematerializable by 
LiveIntervals:

00984     if (CreatedNewVReg) {
00985       if (DefIsReMat) {
00986         vrm.setVirtIsReMaterialized(NewVReg, ReMatDefMI/*, CanDelete*/);
00987         if (ReMatIds[VNI->id] == VirtRegMap::MAX_STACK_SLOT) {
00988           // Each valnum may have its own remat id.
00989           ReMatIds[VNI->id] = vrm.assignVirtReMatId(NewVReg);

Linear scan dutifully assigns %reg1631 to AL.  Then SimpleSpiller runs and 
does this:

00261             unsigned VirtReg = MO.getReg();
00262             unsigned PhysReg = VRM.getPhys(VirtReg);
00263             if (!VRM.isAssignedReg(VirtReg)) {
00264               int StackSlot = VRM.getStackSlot(VirtReg);

Well, according to VRM, %reg1631 is NOT assigned to a register (because it has 
a remat id) even though linear scan assigned it one.  VRM then returns garbage 
as the stack slot and things go downhill from there.

So should VirtRegMap reset the stack slot and remat id maps when it is told to 
assign a phys to a virt?

                                                 -Dave