Prioritize smaller register classes for urgent evictions.

It helps compile exotic inline asm. In the test case, normal GR32
virtual registers use up eax-edx so the final GR32_ABCD live range has
no registers left. Since all the live ranges were tiny, we had no way of
prioritizing the smaller register class.

This patch allows tiny unspillable live ranges to be evicted by tiny
unspillable live ranges from a smaller register class.

<rdar://problem/11542429>

llvm-svn: 157715

llvm/lib/CodeGen/RegAllocGreedy.cpp

@@ -558,7 +558,13 @@ bool RAGreedy::canEvictInterference(LiveInterval &VirtReg, unsigned PhysReg,
       // Once a live range becomes small enough, it is urgent that we find a
       // register for it. This is indicated by an infinite spill weight. These
       // urgent live ranges get to evict almost anything.
-      bool Urgent = !VirtReg.isSpillable() && Intf->isSpillable();
+      //
+      // Also allow urgent evictions of unspillable ranges from a strictly
+      // larger allocation order.
+      bool Urgent = !VirtReg.isSpillable() &&
+        (Intf->isSpillable() ||
+         RegClassInfo.getNumAllocatableRegs(MRI->getRegClass(VirtReg.reg)) <
+         RegClassInfo.getNumAllocatableRegs(MRI->getRegClass(Intf->reg)));
       // Only evict older cascades or live ranges without a cascade.
       unsigned IntfCascade = ExtraRegInfo[Intf->reg].Cascade;
       if (Cascade <= IntfCascade) {

llvm/test/CodeGen/X86/inline-asm.ll

@@ -43,3 +43,12 @@ entry:
   %0 = tail call i8 asm sideeffect "xchg $0, $1", "=r,*m,0,~{memory},~{dirflag},~{fpsr},~{flags}"(i32* %p, i1 %desired) nounwind
   ret void
 }
+
+; <rdar://problem/11542429>
+; The constrained GR32_ABCD register class of the 'q' constraint requires
+; special handling after the preceding outputs used up eax-edx.
+define void @constrain_abcd(i8* %h) nounwind ssp {
+entry:
+  %0 = call { i32, i32, i32, i32, i32 } asm sideeffect "", "=&r,=&r,=&r,=&r,=&q,r,~{ecx},~{memory},~{dirflag},~{fpsr},~{flags}"(i8* %h) nounwind
+  ret void
+}