RegisterCoalescer: Do not introduce uses of empty register classes

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -2051,6 +2051,12 @@ bool RegisterCoalescer::joinCopy(
   }
 
   if (CP.getNewRC()) {
+    if (RegClassInfo.getNumAllocatableRegs(CP.getNewRC()) == 0) {
+      LLVM_DEBUG(dbgs() << "\tNo " << TRI->getRegClassName(CP.getNewRC())
+                        << "are available for allocation\n");
+      return false;
+    }
+
     auto SrcRC = MRI->getRegClass(CP.getSrcReg());
     auto DstRC = MRI->getRegClass(CP.getDstReg());
     unsigned SrcIdx = CP.getSrcIdx();

llvm/test/CodeGen/AMDGPU/coalescer-avoid-coalesce-class-with-no-registers.ll

@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 < %s | FileCheck %s
+
+; Make sure the coalescer doesn't introduce any uses of
+; vreg_1024. None are available to allocate with the register budget
+; of this function.
+
+define void @no_introduce_vreg_1024() #0 {
+; CHECK-LABEL: no_introduce_vreg_1024:
+; CHECK:       ; %bb.0:
+; CHECK-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
+; CHECK-NEXT:    ;;#ASMSTART
+; CHECK-NEXT:    ; def v[0:7]
+; CHECK-NEXT:    ;;#ASMEND
+; CHECK-NEXT:    v_mov_b32_e32 v9, v0
+; CHECK-NEXT:    ;;#ASMSTART
+; CHECK-NEXT:    ; use v[0:15]
+; CHECK-NEXT:    ;;#ASMEND
+; CHECK-NEXT:    s_setpc_b64 s[30:31]
+  %tuple = call <8 x i32> asm sideeffect "; def $0","=v"()
+  %sub0 = extractelement <8 x i32> %tuple, i32 0
+  %insert = insertelement <16 x i32> poison, i32 %sub0, i32 9
+  call void asm sideeffect "; use $0","v"(<16 x i32> %insert)
+  ret void
+}
+
+attributes #0 = { nounwind "amdgpu-waves-per-eu"="10,10" }

llvm/test/CodeGen/AMDGPU/coalescer-avoid-coalesce-class-with-no-registers.mir

@@ -0,0 +1,34 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -run-pass=register-coalescer -o - %s | FileCheck %s
+
+# The register budget for this function does not permit using 1024-bit
+# registers. The coalescer should not introduce a 1024-bit virtual
+# register which will fail to allocate.
+
+--- |
+  define void @no_introduce_vreg_1024() #0 {
+    ret void
+  }
+
+  attributes #0 = { "amdgpu-waves-per-eu"="10,10" }
+...
+---
+name:            no_introduce_vreg_1024
+tracksRegLiveness: true
+machineFunctionInfo:
+  occupancy:       10
+body:             |
+  bb.0:
+    liveins: $vgpr0_vgpr1_vgpr2_vgpr3_vgpr4_vgpr5_vgpr6_vgpr7
+
+    ; CHECK-LABEL: name: no_introduce_vreg_1024
+    ; CHECK: liveins: $vgpr0_vgpr1_vgpr2_vgpr3_vgpr4_vgpr5_vgpr6_vgpr7
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:vreg_256 = COPY $vgpr0_vgpr1_vgpr2_vgpr3_vgpr4_vgpr5_vgpr6_vgpr7
+    ; CHECK-NEXT: undef [[COPY1:%[0-9]+]].sub9:vreg_512 = COPY [[COPY]].sub0
+    ; CHECK-NEXT: SI_RETURN implicit [[COPY1]]
+    %0:vreg_256 = COPY $vgpr0_vgpr1_vgpr2_vgpr3_vgpr4_vgpr5_vgpr6_vgpr7
+    undef %1.sub9:vreg_512 = COPY %0.sub0
+    SI_RETURN implicit %1
+
+...