AMDGPU: Reuse carry out register during FI elimination

[kerbowa]

Summary

Pre gfx9 we need to scavenge a 64-bit SGPR to use as the carry out for an Add.
If only one SGPR was available this crashed when trying to scavenge another
32bit SGPR to materialize the offset.

Instead, reuse a 32-bit SGPR from the carry out as the offset register.

Also prefer to use vcc for the unused carry out when it is available.

[rampitec]

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

6216

RI.getVCC(), even though right now we cannot hit this with wave32.

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

1130

It ignores wave32 where carry is 32 bit. Granted right now we have no carry adds on all GFX10, but we cannot be sure this will be the same in the future. At least assert if needed the register is 64 bit.

[kerbowa]

Don't assume 64bit carry out.

[rampitec]

LGTM

[arsenm]

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

1136

It would be slightly safer to directly check wave32 or not than relying on getting the physreg class and directly checking it. Sometimes the exact subclass changes

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

 MachineInstrBuilder SIInstrInfo::getAddNoCarry(MachineBasicBlock &MBB,
                                                MachineBasicBlock::iterator I,
                                                const DebugLoc &DL,
                                                Register DestReg,
                                                RegScavenger &RS) const {
   if (ST.hasAddNoCarry())
     return BuildMI(MBB, I, DL, get(AMDGPU::V_ADD_U32_e32), DestReg);
 
-  Register UnusedCarry = RS.scavengeRegister(RI.getBoolRC(), I, 0, false);
+  // If available, prefer to use vcc.
+  Register UnusedCarry = !RS.isRegUsed(AMDGPU::VCC)
+                             ? AMDGPU::VCC
+                             : RS.scavengeRegister(RI.getBoolRC(), I, 0, false);
+
   // TODO: Users need to deal with this.
   if (!UnusedCarry.isValid())
     return MachineInstrBuilder();
 
   return BuildMI(MBB, I, DL, get(AMDGPU::V_ADD_I32_e64), DestReg)
            .addReg(UnusedCarry, RegState::Define | RegState::Dead);
 }
 

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

default: {
             // TODO: Fold if use instruction is another add of a constant.
             if (IsVOP2 || AMDGPU::isInlinableLiteral32(Offset, ST.hasInv2PiInlineImm())) {
               // FIXME: This can fail
               MIB.addImm(Offset);
               MIB.addReg(ScaledReg, RegState::Kill);
               if (!IsVOP2)
                 MIB.addImm(0); // clamp bit
             } else {
-              Register ConstOffsetReg =
-                RS->scavengeRegister(&AMDGPU::SReg_32_XM0RegClass, MIB, 0, false);
+              assert(MIB->getOpcode() == AMDGPU::V_ADD_I32_e64 &&
+                     "Need to reuse carry out subreg");
 
-              // This should always be able to use the unused carry out.
-              assert(ConstOffsetReg && "this scavenge should not be able to fail");
+              // Use scavenged unused carry out as offset register.
+              Register ConstOffsetReg = getSubReg(MIB.getReg(1), AMDGPU::sub0);
 
               BuildMI(*MBB, *MIB, DL, TII->get(AMDGPU::S_MOV_B32), ConstOffsetReg)
                 .addImm(Offset);
               MIB.addReg(ConstOffsetReg, RegState::Kill);
               MIB.addReg(ScaledReg, RegState::Kill);
               MIB.addImm(0); // clamp bit
             }
           } else {
             // We have to produce a carry out, and we there isn't a free SGPR
             // pair for it. We can keep the whole computation on the SALU to
             // avoid clobbering an additional register at the cost of an extra
             // mov.
 
             // We may have 1 free scratch SGPR even though a carry out is

llvm/test/CodeGen/AMDGPU/pei-scavenge-sgpr-carry-out.mir

bb.0:
     ; CHECK: $vgpr0 = V_OR_B32_e32 killed $vgpr2, $vgpr1, implicit $exec, implicit $sgpr4, implicit $sgpr5, implicit $sgpr6, implicit $sgpr7, implicit $sgpr8, implicit $sgpr9, implicit $sgpr10, implicit $sgpr11, implicit $sgpr12, implicit $sgpr13, implicit $sgpr14, implicit $sgpr15, implicit $sgpr16, implicit $sgpr17, implicit $sgpr18, implicit $sgpr19, implicit $sgpr20, implicit $sgpr21, implicit $sgpr22, implicit $sgpr23, implicit $sgpr24, implicit $sgpr25, implicit $sgpr26, implicit $sgpr17, implicit $sgpr28, implicit $sgpr31
     ; CHECK: $sgpr32 = frame-destroy S_SUB_U32 $sgpr32, 1572864, implicit-def $scc
     ; CHECK: $sgpr33 = frame-setup COPY $sgpr27
     ; CHECK: S_ENDPGM 0, implicit $vcc
     S_NOP 0, implicit-def $sgpr4, implicit-def $sgpr5, implicit-def $sgpr6, implicit-def $sgpr7, implicit-def $sgpr8, implicit-def $sgpr9, implicit-def $sgpr10, implicit-def $sgpr11, implicit-def $sgpr12, implicit-def $sgpr13, implicit-def $sgpr14, implicit-def $sgpr15, implicit-def $sgpr16, implicit-def $sgpr17, implicit-def $sgpr18, implicit-def $sgpr19, implicit-def $sgpr20, implicit-def $sgpr21, implicit-def $sgpr22, implicit-def $sgpr23, implicit-def $sgpr24, implicit-def $sgpr25, implicit-def $sgpr26, implicit-def $sgpr17, implicit-def $sgpr28, implicit-def $sgpr30, implicit-def $sgpr31, implicit-def $vcc
     $vgpr0 = V_OR_B32_e32 %stack.1, $vgpr1, implicit $exec, implicit $sgpr4, implicit $sgpr5, implicit $sgpr6, implicit $sgpr7, implicit $sgpr8, implicit $sgpr9, implicit $sgpr10, implicit $sgpr11, implicit $sgpr12, implicit $sgpr13, implicit $sgpr14, implicit $sgpr15, implicit $sgpr16, implicit $sgpr17, implicit $sgpr18, implicit $sgpr19, implicit $sgpr20, implicit $sgpr21, implicit $sgpr22, implicit $sgpr23, implicit $sgpr24, implicit $sgpr25, implicit $sgpr26, implicit $sgpr17, implicit $sgpr28, implicit $sgpr31
     S_ENDPGM 0, implicit $vcc
 ...
+
+# When only one 64-bit SGPR is available for the unused carry out pre gfx9,
+# we must reuse one of the 32-bit SGPR sub-regs to materialize the offset.
+
+---
+name: scavenge_sgpr_pei_one_sgpr_64
+tracksRegLiveness: true
+
+stack:
+  - { id: 0, type: default, offset: 0, size: 4, alignment: 8192 }
+  - { id: 1, type: default, offset: 0, size: 4, alignment: 8192 }
+
+machineFunctionInfo:
+  isEntryFunction: false
+  scratchRSrcReg:  $sgpr0_sgpr1_sgpr2_sgpr3
+  scratchWaveOffsetReg: $sgpr34
+  frameOffsetReg:  $sgpr33
+  stackPtrOffsetReg:  $sgpr32
+
+body:             |
+  bb.0:
+    liveins: $vgpr1
+
+    ; CHECK-LABEL: name: scavenge_sgpr_pei_one_sgpr_64
+    ; CHECK: liveins: $vgpr1
+    ; CHECK: $sgpr27 = frame-setup COPY $sgpr33
+    ; CHECK: $sgpr4 = frame-setup S_ADD_U32 $sgpr32, 524224, implicit-def $scc
+    ; CHECK: $sgpr33 = frame-setup S_AND_B32 killed $sgpr4, 4294443008, implicit-def $scc
+    ; CHECK: $sgpr32 = frame-setup S_ADD_U32 $sgpr32, 1572864, implicit-def $scc
+    ; CHECK: S_NOP 0, implicit-def $sgpr4, implicit-def $sgpr5, implicit-def $sgpr6, implicit-def $sgpr7, implicit-def $sgpr8, implicit-def $sgpr9, implicit-def $sgpr10, implicit-def $sgpr11, implicit-def $sgpr12, implicit-def $sgpr13, implicit-def $sgpr14, implicit-def $sgpr15, implicit-def $sgpr16, implicit-def $sgpr17, implicit-def $sgpr18, implicit-def $sgpr19, implicit-def $sgpr20, implicit-def $sgpr21, implicit-def $sgpr22, implicit-def $sgpr23, implicit-def $sgpr24, implicit-def $sgpr25, implicit-def $sgpr26, implicit-def $sgpr17, implicit-def $sgpr30, implicit-def $sgpr31, implicit-def $vcc
+    ; CHECK: $sgpr28 = S_SUB_U32 $sgpr33, $sgpr34, implicit-def $scc
+    ; CHECK: $vgpr3 = V_LSHRREV_B32_e64 6, killed $sgpr28, implicit $exec
+    ; CHECK: $sgpr28 = S_MOV_B32 8192
+    ; CHECK: $vgpr2, dead $sgpr28_sgpr29 = V_ADD_I32_e64 killed $sgpr28, killed $vgpr3, 0, implicit $exec
+    ; CHECK: $vgpr0 = V_OR_B32_e32 killed $vgpr2, $vgpr1, implicit $exec, implicit $sgpr4, implicit $sgpr5, implicit $sgpr6, implicit $sgpr7, implicit $sgpr8, implicit $sgpr9, implicit $sgpr10, implicit $sgpr11, implicit $sgpr12, implicit $sgpr13, implicit $sgpr14, implicit $sgpr15, implicit $sgpr16, implicit $sgpr17, implicit $sgpr18, implicit $sgpr19, implicit $sgpr20, implicit $sgpr21, implicit $sgpr22, implicit $sgpr23, implicit $sgpr24, implicit $sgpr25, implicit $sgpr26, implicit $sgpr17, implicit $sgpr31
+    ; CHECK: $sgpr32 = frame-destroy S_SUB_U32 $sgpr32, 1572864, implicit-def $scc
+    ; CHECK: $sgpr33 = frame-setup COPY $sgpr27
+    ; CHECK: S_ENDPGM 0, implicit $vcc
+    S_NOP 0, implicit-def $sgpr4, implicit-def $sgpr5, implicit-def $sgpr6, implicit-def $sgpr7, implicit-def $sgpr8, implicit-def $sgpr9, implicit-def $sgpr10, implicit-def $sgpr11, implicit-def $sgpr12, implicit-def $sgpr13, implicit-def $sgpr14, implicit-def $sgpr15, implicit-def $sgpr16, implicit-def $sgpr17, implicit-def $sgpr18, implicit-def $sgpr19, implicit-def $sgpr20, implicit-def $sgpr21, implicit-def $sgpr22, implicit-def $sgpr23, implicit-def $sgpr24, implicit-def $sgpr25, implicit-def $sgpr26, implicit-def $sgpr17, implicit-def $sgpr30, implicit-def $sgpr31, implicit-def $vcc
+    $vgpr0 = V_OR_B32_e32 %stack.1, $vgpr1, implicit $exec, implicit $sgpr4, implicit $sgpr5, implicit $sgpr6, implicit $sgpr7, implicit $sgpr8, implicit $sgpr9, implicit $sgpr10, implicit $sgpr11, implicit $sgpr12, implicit $sgpr13, implicit $sgpr14, implicit $sgpr15, implicit $sgpr16, implicit $sgpr17, implicit $sgpr18, implicit $sgpr19, implicit $sgpr20, implicit $sgpr21, implicit $sgpr22, implicit $sgpr23, implicit $sgpr24, implicit $sgpr25, implicit $sgpr26, implicit $sgpr17, implicit $sgpr31
+    S_ENDPGM 0, implicit $vcc
+...
+
+# Prefer to use vcc as unused carry out.
+
+---
+name: scavenge_sgpr_pei_prefer_vcc
+tracksRegLiveness: true
+
+stack:
+  - { id: 0, type: default, offset: 0, size: 4, alignment: 8192 }
+  - { id: 1, type: default, offset: 0, size: 4, alignment: 8192 }
+
+machineFunctionInfo:
+  isEntryFunction: false
+  scratchRSrcReg:  $sgpr0_sgpr1_sgpr2_sgpr3
+  scratchWaveOffsetReg: $sgpr34
+  frameOffsetReg:  $sgpr33
+  stackPtrOffsetReg:  $sgpr32
+
+body:             |
+  bb.0:
+    liveins: $vgpr1
+
+    ; CHECK-LABEL: name: scavenge_sgpr_pei_prefer_vcc
+    ; CHECK: liveins: $vgpr1
+    ; CHECK: $sgpr27 = frame-setup COPY $sgpr33
+    ; CHECK: $sgpr4 = frame-setup S_ADD_U32 $sgpr32, 524224, implicit-def $scc
+    ; CHECK: $sgpr33 = frame-setup S_AND_B32 killed $sgpr4, 4294443008, implicit-def $scc
+    ; CHECK: $sgpr32 = frame-setup S_ADD_U32 $sgpr32, 1572864, implicit-def $scc
+    ; CHECK: S_NOP 0, implicit-def $sgpr4, implicit-def $sgpr5, implicit-def $sgpr6, implicit-def $sgpr7, implicit-def $sgpr8, implicit-def $sgpr9, implicit-def $sgpr10, implicit-def $sgpr11, implicit-def $sgpr12, implicit-def $sgpr13, implicit-def $sgpr14, implicit-def $sgpr15, implicit-def $sgpr16, implicit-def $sgpr17, implicit-def $sgpr18, implicit-def $sgpr19, implicit-def $sgpr20, implicit-def $sgpr21, implicit-def $sgpr22, implicit-def $sgpr23, implicit-def $sgpr24, implicit-def $sgpr25, implicit-def $sgpr26, implicit-def $sgpr17, implicit-def $sgpr30, implicit-def $sgpr31
+    ; CHECK: $vcc_hi = S_SUB_U32 $sgpr33, $sgpr34, implicit-def $scc
+    ; CHECK: $vgpr3 = V_LSHRREV_B32_e64 6, killed $vcc_hi, implicit $exec
+    ; CHECK: $vcc_lo = S_MOV_B32 8192
+    ; CHECK: $vgpr2, dead $vcc = V_ADD_I32_e64 killed $vcc_lo, killed $vgpr3, 0, implicit $exec
+    ; CHECK: $vgpr0 = V_OR_B32_e32 killed $vgpr2, $vgpr1, implicit $exec, implicit $sgpr4, implicit $sgpr5, implicit $sgpr6, implicit $sgpr7, implicit $sgpr8, implicit $sgpr9, implicit $sgpr10, implicit $sgpr11, implicit $sgpr12, implicit $sgpr13, implicit $sgpr14, implicit $sgpr15, implicit $sgpr16, implicit $sgpr17, implicit $sgpr18, implicit $sgpr19, implicit $sgpr20, implicit $sgpr21, implicit $sgpr22, implicit $sgpr23, implicit $sgpr24, implicit $sgpr25, implicit $sgpr26, implicit $sgpr17, implicit $sgpr31
+    ; CHECK: $sgpr32 = frame-destroy S_SUB_U32 $sgpr32, 1572864, implicit-def $scc
+    ; CHECK: $sgpr33 = frame-setup COPY $sgpr27
+    ; CHECK: S_ENDPGM 0
+    S_NOP 0, implicit-def $sgpr4, implicit-def $sgpr5, implicit-def $sgpr6, implicit-def $sgpr7, implicit-def $sgpr8, implicit-def $sgpr9, implicit-def $sgpr10, implicit-def $sgpr11, implicit-def $sgpr12, implicit-def $sgpr13, implicit-def $sgpr14, implicit-def $sgpr15, implicit-def $sgpr16, implicit-def $sgpr17, implicit-def $sgpr18, implicit-def $sgpr19, implicit-def $sgpr20, implicit-def $sgpr21, implicit-def $sgpr22, implicit-def $sgpr23, implicit-def $sgpr24, implicit-def $sgpr25, implicit-def $sgpr26, implicit-def $sgpr17, implicit-def $sgpr30, implicit-def $sgpr31
+    $vgpr0 = V_OR_B32_e32 %stack.1, $vgpr1, implicit $exec, implicit $sgpr4, implicit $sgpr5, implicit $sgpr6, implicit $sgpr7, implicit $sgpr8, implicit $sgpr9, implicit $sgpr10, implicit $sgpr11, implicit $sgpr12, implicit $sgpr13, implicit $sgpr14, implicit $sgpr15, implicit $sgpr16, implicit $sgpr17, implicit $sgpr18, implicit $sgpr19, implicit $sgpr20, implicit $sgpr21, implicit $sgpr22, implicit $sgpr23, implicit $sgpr24, implicit $sgpr25, implicit $sgpr26, implicit $sgpr17, implicit $sgpr31
+    S_ENDPGM 0
+...