[AMDGPU] Fix illegal agpr use by VALU

[rampitec]

Summary

When SIFixSGPRCopies attempts to fix an illegal copy from vector to
scalar register it calls moveToVALU(). A copy from an agpr to sgpr
becomes a copy from agpr to agpr, which may result in the illegal
register class at a use of this copy.

Solution is to copy it always into a vgpr. This may result in a
subsequent copy into an agpr if that is what really needed, however
should not happen too often and likely will be folded later.

The opposite situation may not happen because an sgpr is always
illegal where agpr is legal, so such user instructions may not
exist.

[arsenm]

LGTM

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

const TargetRegisterClass *SIInstrInfo::getDestEquivalentVGPRClass(
   case AMDGPU::WQM:
   case AMDGPU::SOFT_WQM:
   case AMDGPU::WWM: {
     const TargetRegisterClass *SrcRC = getOpRegClass(Inst, 1);
     if (RI.hasAGPRs(SrcRC)) {
       if (RI.hasAGPRs(NewDstRC))
         return nullptr;
 
+      switch (Inst.getOpcode()) {
+      case AMDGPU::PHI:
+      case AMDGPU::REG_SEQUENCE:
+      case AMDGPU::INSERT_SUBREG:
         NewDstRC = RI.getEquivalentAGPRClass(NewDstRC);
+        break;
+      default:
+        NewDstRC = RI.getEquivalentVGPRClass(NewDstRC);
+      }
+
       if (!NewDstRC)
         return nullptr;
     } else {
        if (RI.hasVGPRs(NewDstRC))
         return nullptr;
 
       NewDstRC = RI.getEquivalentVGPRClass(NewDstRC);
       if (!NewDstRC)

llvm/test/CodeGen/AMDGPU/fold_acc_copy_into_valu.mir

+# RUN: llc -march=amdgcn -mcpu=gfx908 -o - -run-pass si-fix-sgpr-copies -verify-machineinstrs %s | FileCheck -check-prefix=GCN %s
+
+# GCN-LABEL: fold_acc_copy_into_valu
+# GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY %0.sub0
+# GCN: %2:vgpr_32 = V_AND_B32_e32 [[COPY]], undef %3:vgpr_32, implicit $exec
+---
+name:            fold_acc_copy_into_valu
+body:             |
+  bb.0.entry:
+  
+    %0:areg_1024 = IMPLICIT_DEF
+    %1:sreg_32_xm0 = COPY %0.sub0
+    %3:vgpr_32 = V_AND_B32_e32 %1, undef %2:vgpr_32, implicit $exec
+  
+...

llvm/test/CodeGen/AMDGPU/illegal-sgpr-to-vgpr-copy.ll

 ; RUN: not llc -march=amdgcn < %s 2>&1 | FileCheck -check-prefix=ERR %s
-; RUN: not llc -march=amdgcn < %s | FileCheck -check-prefix=GCN %s
+; RUN: not llc -march=amdgcn < %s 2>&1 | FileCheck -check-prefix=GCN %s
 
 ; ERR: error: <unknown>:0:0: in function illegal_vgpr_to_sgpr_copy_i32 void (): illegal SGPR to VGPR copy
 ; GCN: ; illegal copy v1 to s9
 
 define amdgpu_kernel void @illegal_vgpr_to_sgpr_copy_i32() #0 {
   %vgpr = call i32 asm sideeffect "; def $0", "=${v1}"()
   call void asm sideeffect "; use $0", "${s9}"(i32 %vgpr)
   ret void

 ; GCN: ; illegal copy v[0:15] to s[16:31]
 define amdgpu_kernel void @illegal_vgpr_to_sgpr_copy_v16i32() #0 {
   %vgpr = call <16 x i32> asm sideeffect "; def $0", "=${v[0:15]}"()
   call void asm sideeffect "; use $0", "${s[16:31]}"(<16 x i32> %vgpr)
   ret void
 }
 
 ; ERR: error: <unknown>:0:0: in function illegal_agpr_to_sgpr_copy_i32 void (): illegal SGPR to VGPR copy
-; GCN: ; illegal copy a1 to s9
+; GCN: v_accvgpr_read_b32 [[COPY1:v[0-9]+]], a1
+; GCN: ; illegal copy [[COPY1]] to s9
 define amdgpu_kernel void @illegal_agpr_to_sgpr_copy_i32() #1 {
   %agpr = call i32 asm sideeffect "; def $0", "=${a1}"()
   call void asm sideeffect "; use $0", "${s9}"(i32 %agpr)
   ret void
 }
 
 ; ERR: error: <unknown>:0:0: in function illegal_agpr_to_sgpr_copy_v2i32 void (): illegal SGPR to VGPR copy
-; GCN: ; illegal copy a[0:1] to s[10:11]
+; GCN-DAG: v_accvgpr_read_b32 v[[COPY1L:[0-9]+]], a0
+; GCN-DAG: v_accvgpr_read_b32 v[[COPY1H:[0-9]+]], a1
+; GCN: ; illegal copy v{{\[}}[[COPY1L]]:[[COPY1H]]] to s[10:11]
 define amdgpu_kernel void @illegal_agpr_to_sgpr_copy_v2i32() #1 {
   %vgpr = call <2 x i32> asm sideeffect "; def $0", "=${a[0:1]}"()
   call void asm sideeffect "; use $0", "${s[10:11]}"(<2 x i32> %vgpr)
   ret void
 }
 
 attributes #0 = { nounwind }
 attributes #1 = { nounwind "target-cpu"="gfx908" }