[AMDGPU][True16] add PreRA hint to improve elimination for 16bit

and 32bit register copy

Author: broxigarchen

llvm/lib/Target/AMDGPU/GCNPreRAOptimizations.cpp

@@ -22,12 +22,22 @@
 /// although the same shall be possible with other register classes and
 /// instructions if necessary.
 ///
+/// This pass also adds register allocation hints to COPY.
+/// The hints will be post-processed by SIRegisterInfo::getRegAllocationHints.
+/// When using True16, we often see COPY moving a 16-bit value between a VGPR_32
+/// This pass also adds register allocation hints to COPY.
+/// The hints will be post-processed by SIRegisterInfo::getRegAllocationHints.
+/// When using True16, we often see COPY moving a 16-bit value between a VGPR_32
+/// and a VGPR_16. If we use the VGPR_16 that corresponds to the lo16 bits of
+/// the VGPR_32, the COPY can be completely eliminated.
+///
 //===----------------------------------------------------------------------===//
 
 #include "GCNPreRAOptimizations.h"
 #include "AMDGPU.h"
 #include "GCNSubtarget.h"
 #include "MCTargetDesc/AMDGPUMCTargetDesc.h"
+#include "SIRegisterInfo.h"
 #include "llvm/CodeGen/LiveIntervals.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/InitializePasses.h"
@@ -253,5 +263,38 @@ bool GCNPreRAOptimizationsImpl::run(MachineFunction &MF) {
     Changed |= processReg(Reg);
   }
 
+  if (!ST.useRealTrue16Insts())
+    return Changed;
+
+  // Add RA hints to improve True16 COPY elimination.
+  for (const MachineBasicBlock &MBB : MF) {
+    for (const MachineInstr &MI : MBB) {
+      if (MI.getOpcode() != AMDGPU::COPY)
+        continue;
+      Register Dst = MI.getOperand(0).getReg();
+      Register Src = MI.getOperand(1).getReg();
+      if (Dst.isVirtual() &&
+          MRI->getRegClass(Dst) == &AMDGPU::VGPR_16RegClass &&
+          Src.isPhysical() &&
+          TRI->getRegClassForReg(*MRI, Src) == &AMDGPU::VGPR_32RegClass)
+        MRI->setRegAllocationHint(Dst, 0, TRI->getSubReg(Src, AMDGPU::lo16));
+      if (Src.isVirtual() &&
+          MRI->getRegClass(Src) == &AMDGPU::VGPR_16RegClass &&
+          Dst.isPhysical() &&
+          TRI->getRegClassForReg(*MRI, Dst) == &AMDGPU::VGPR_32RegClass)
+        MRI->setRegAllocationHint(Src, 0, TRI->getSubReg(Dst, AMDGPU::lo16));
+      if (!Dst.isVirtual() || !Src.isVirtual())
+        continue;
+      if (MRI->getRegClass(Dst) == &AMDGPU::VGPR_32RegClass &&
+          MRI->getRegClass(Src) == &AMDGPU::VGPR_16RegClass) {
+        MRI->setRegAllocationHint(Dst, AMDGPURI::Size32, Src);
+        MRI->setRegAllocationHint(Src, AMDGPURI::Size16, Dst);
+      }
+      if (MRI->getRegClass(Dst) == &AMDGPU::VGPR_16RegClass &&
+          MRI->getRegClass(Src) == &AMDGPU::VGPR_32RegClass)
+        MRI->setRegAllocationHint(Dst, AMDGPURI::Size16, Src);
+    }
+  }
+
   return Changed;
 }

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -3652,6 +3652,73 @@ const int *SIRegisterInfo::getRegUnitPressureSets(unsigned RegUnit) const {
   return AMDGPUGenRegisterInfo::getRegUnitPressureSets(RegUnit);
 }
 
+bool SIRegisterInfo::getRegAllocationHints(Register VirtReg,
+                                           ArrayRef<MCPhysReg> Order,
+                                           SmallVectorImpl<MCPhysReg> &Hints,
+                                           const MachineFunction &MF,
+                                           const VirtRegMap *VRM,
+                                           const LiveRegMatrix *Matrix) const {
+
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  const SIRegisterInfo *TRI = ST.getRegisterInfo();
+
+  std::pair<unsigned, Register> Hint = MRI.getRegAllocationHint(VirtReg);
+
+  switch (Hint.first) {
+  case AMDGPURI::Size32: {
+    Register Paired = Hint.second;
+    assert(Paired);
+    Register PairedPhys;
+    if (Paired.isPhysical()) {
+      PairedPhys =
+          getMatchingSuperReg(Paired, AMDGPU::lo16, &AMDGPU::VGPR_32RegClass);
+    } else if (VRM && VRM->hasPhys(Paired)) {
+      PairedPhys = getMatchingSuperReg(VRM->getPhys(Paired), AMDGPU::lo16,
+                                       &AMDGPU::VGPR_32RegClass);
+    }
+
+    // Prefer the paired physreg.
+    if (PairedPhys)
+      // isLo(Paired) is implicitly true here from the API of
+      // getMatchingSuperReg.
+      Hints.push_back(PairedPhys);
+    return false;
+  }
+  case AMDGPURI::Size16: {
+    Register Paired = Hint.second;
+    assert(Paired);
+    Register PairedPhys;
+    if (Paired.isPhysical()) {
+      PairedPhys = TRI->getSubReg(Paired, AMDGPU::lo16);
+    } else if (VRM && VRM->hasPhys(Paired)) {
+      PairedPhys = TRI->getSubReg(VRM->getPhys(Paired), AMDGPU::lo16);
+    }
+
+    // First prefer the paired physreg.
+    if (PairedPhys)
+      Hints.push_back(PairedPhys);
+    else {
+      // Add all the lo16 physregs.
+      // When the Paired operand has not yet been assigned a physreg it is
+      // better to try putting VirtReg in a lo16 register, because possibly
+      // later Paired can be assigned to the overlapping register and the COPY
+      // can be eliminated.
+      for (MCPhysReg PhysReg : Order) {
+        if (PhysReg == PairedPhys || AMDGPU::isHi16Reg(PhysReg, *this))
+          continue;
+        if (AMDGPU::VGPR_16RegClass.contains(PhysReg) &&
+            !MRI.isReserved(PhysReg))
+          Hints.push_back(PhysReg);
+      }
+    }
+    return false;
+  }
+  default:
+    return TargetRegisterInfo::getRegAllocationHints(VirtReg, Order, Hints, MF,
+                                                     VRM);
+  }
+}
+
 MCRegister SIRegisterInfo::getReturnAddressReg(const MachineFunction &MF) const {
   // Not a callee saved register.
   return AMDGPU::SGPR30_SGPR31;

llvm/lib/Target/AMDGPU/SIRegisterInfo.h

@@ -29,6 +29,13 @@ class LiveRegUnits;
 class RegisterBank;
 struct SGPRSpillBuilder;
 
+/// Register allocation hint types. Helps eliminate unneeded COPY with True16
+namespace AMDGPURI {
+
+enum { Size16 = 1, Size32 = 2 };
+
+} // end namespace AMDGPURI
+
 class SIRegisterInfo final : public AMDGPUGenRegisterInfo {
 private:
   const GCNSubtarget &ST;
@@ -329,6 +336,11 @@ class SIRegisterInfo final : public AMDGPUGenRegisterInfo {
   unsigned getRegPressureSetLimit(const MachineFunction &MF,
                                   unsigned Idx) const override;
 
+  bool getRegAllocationHints(Register VirtReg, ArrayRef<MCPhysReg> Order,
+                             SmallVectorImpl<MCPhysReg> &Hints,
+                             const MachineFunction &MF, const VirtRegMap *VRM,
+                             const LiveRegMatrix *Matrix) const override;
+
   const int *getRegUnitPressureSets(unsigned RegUnit) const override;
 
   MCRegister getReturnAddressReg(const MachineFunction &MF) const;

llvm/test/CodeGen/AMDGPU/fadd.f16.ll

@@ -76,9 +76,7 @@ define amdgpu_kernel void @fadd_f16(
 ; GFX11-SDAG-NEXT:    s_waitcnt vmcnt(0)
 ; GFX11-SDAG-NEXT:    buffer_load_u16 v1, off, s[4:7], 0 glc dlc
 ; GFX11-SDAG-NEXT:    s_waitcnt vmcnt(0)
-; GFX11-SDAG-NEXT:    v_mov_b16_e32 v0.h, v1.l
-; GFX11-SDAG-NEXT:    s_delay_alu instid0(VALU_DEP_1)
-; GFX11-SDAG-NEXT:    v_add_f16_e32 v0.l, v0.l, v0.h
+; GFX11-SDAG-NEXT:    v_add_f16_e32 v0.l, v0.l, v1.l
 ; GFX11-SDAG-NEXT:    buffer_store_b16 v0, off, s[8:11], 0
 ; GFX11-SDAG-NEXT:    s_endpgm
 ;