[X86] X86LowerTileCopy: Find dead register to use to prevent save-reload of tile register (llvm#83628)

Author: AZero13

llvm/lib/Target/X86/X86LowerTileCopy.cpp

@@ -20,6 +20,7 @@
 #include "X86InstrBuilder.h"
 #include "X86InstrInfo.h"
 #include "X86Subtarget.h"
+#include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
@@ -72,10 +73,16 @@ FunctionPass *llvm::createX86LowerTileCopyPass() {
 bool X86LowerTileCopy::runOnMachineFunction(MachineFunction &MF) {
   const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
   const X86InstrInfo *TII = ST.getInstrInfo();
+  const TargetRegisterInfo *TRI = ST.getRegisterInfo();
+  BitVector GR64Regs =
+      TRI->getAllocatableSet(MF, TRI->getRegClass(X86::GR64RegClassID));
   bool Changed = false;
 
   for (MachineBasicBlock &MBB : MF) {
-    for (MachineInstr &MI : llvm::make_early_inc_range(MBB)) {
+    LiveRegUnits UsedRegs(*TRI);
+    UsedRegs.addLiveOuts(MBB);
+    for (MachineInstr &MI : llvm::make_early_inc_range(reverse(MBB))) {
+      UsedRegs.stepBackward(MI);
       if (!MI.isCopy())
         continue;
       MachineOperand &DstMO = MI.getOperand(0);
@@ -85,27 +92,41 @@ bool X86LowerTileCopy::runOnMachineFunction(MachineFunction &MF) {
       if (!X86::TILERegClass.contains(DstReg, SrcReg))
         continue;
 
-      const TargetRegisterInfo *TRI = ST.getRegisterInfo();
       // Allocate stack slot for tile register
       unsigned Size = TRI->getSpillSize(X86::TILERegClass);
       Align Alignment = TRI->getSpillAlign(X86::TILERegClass);
       int TileSS = MF.getFrameInfo().CreateSpillStackObject(Size, Alignment);
-      // Allocate stack slot for stride register
-      Size = TRI->getSpillSize(X86::GR64RegClass);
-      Alignment = TRI->getSpillAlign(X86::GR64RegClass);
-      int StrideSS = MF.getFrameInfo().CreateSpillStackObject(Size, Alignment);
 
-      // TODO: Pick a killed regiter to avoid save/reload. There is problem
-      // to get live interval in this stage.
-      Register GR64Cand = X86::RAX;
+      int StrideSS = 0;
+
+      // Pick a killed register to avoid a save/reload.
+      Register GR64Cand = X86::NoRegister;
+      for (auto RegT : GR64Regs.set_bits()) {
+        if (UsedRegs.available(RegT)) {
+          GR64Cand = RegT;
+          break;
+        }
+      }
 
       const DebugLoc &DL = MI.getDebugLoc();
-      // mov %rax (%sp)
-      BuildMI(MBB, MI, DL, TII->get(X86::IMPLICIT_DEF), GR64Cand);
-      addFrameReference(BuildMI(MBB, MI, DL, TII->get(X86::MOV64mr)), StrideSS)
-          .addReg(GR64Cand);
-      // mov 64 %rax
-      BuildMI(MBB, MI, DL, TII->get(X86::MOV64ri), GR64Cand).addImm(64);
+      if (GR64Cand) {
+        // mov 64 %reg
+        BuildMI(MBB, MI, DL, TII->get(X86::MOV64ri), GR64Cand).addImm(64);
+      } else {
+        // No available register? Save RAX and reload it after use.
+
+        // Allocate stack slot for stride register
+        Size = TRI->getSpillSize(X86::GR64RegClass);
+        Alignment = TRI->getSpillAlign(X86::GR64RegClass);
+        StrideSS = MF.getFrameInfo().CreateSpillStackObject(Size, Alignment);
+
+        // mov %reg (%sp)
+        addFrameReference(BuildMI(MBB, MI, DL, TII->get(X86::MOV64mr)),
+                          StrideSS)
+            .addReg(X86::RAX);
+        // mov 64 %reg
+        BuildMI(MBB, MI, DL, TII->get(X86::MOV64ri), X86::RAX).addImm(64);
+      }
       // tilestored %tmm, (%sp, %idx)
 #define GET_EGPR_IF_ENABLED(OPC) (ST.hasEGPR() ? OPC##_EVEX : OPC)
       unsigned Opc = GET_EGPR_IF_ENABLED(X86::TILESTORED);
@@ -120,10 +141,12 @@ bool X86LowerTileCopy::runOnMachineFunction(MachineFunction &MF) {
 #undef GET_EGPR_IF_ENABLED
       NewMI = addFrameReference(BuildMI(MBB, MI, DL, TII->get(Opc), DstReg),
                                 TileSS);
-      // restore %rax
-      // mov (%sp) %rax
-      addFrameReference(BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm), GR64Cand),
-                        StrideSS);
+      if (!GR64Cand) {
+        // restore %rax
+        // mov (%sp) %rax
+        addFrameReference(
+            BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm), GR64Cand), StrideSS);
+      }
       MI.eraseFromParent();
       Changed = true;
     }

llvm/test/CodeGen/X86/AMX/amx-lower-tile-copy.ll

@@ -44,12 +44,8 @@ define dso_local void @test1(ptr%buf) nounwind {
 ; CHECK-NEXT:    tileloadd 3024(%rsp,%rax), %tmm3 # 1024-byte Folded Reload
 ; CHECK-NEXT:    tileloadd (%rbx,%r15), %tmm0
 ; CHECK-NEXT:    tileloadd (%rbx,%r15), %tmm1
-; CHECK-NEXT:    # implicit-def: $rax
-; CHECK-NEXT:    movq %rax, {{[-0-9]+}}(%r{{[sb]}}p) # 8-byte Spill
-; CHECK-NEXT:    movabsq $64, %rax
 ; CHECK-NEXT:    tilestored %tmm3, 1024(%rsp,%rax) # 1024-byte Folded Spill
 ; CHECK-NEXT:    tileloadd {{[-0-9]+}}(%r{{[sb]}}p), %tmm2 # 1024-byte Folded Reload
-; CHECK-NEXT:    movq {{[-0-9]+}}(%r{{[sb]}}p), %rax # 8-byte Reload
 ; CHECK-NEXT:    tdpbssd %tmm1, %tmm0, %tmm2
 ; CHECK-NEXT:    tilestored %tmm2, (%rbx,%r15)
 ; CHECK-NEXT:    incl %r14d
@@ -111,16 +107,10 @@ define dso_local void @test1(ptr%buf) nounwind {
 ; EGPR-NEXT:    # EVEX TO VEX Compression encoding: [0xc4,0xe2,0x7b,0x4b,0x9c,0x04,0xd0,0x0b,0x00,0x00]
 ; EGPR-NEXT:    tileloadd (%rbx,%r15), %tmm0 # EVEX TO VEX Compression encoding: [0xc4,0xa2,0x7b,0x4b,0x04,0x3b]
 ; EGPR-NEXT:    tileloadd (%rbx,%r15), %tmm1 # EVEX TO VEX Compression encoding: [0xc4,0xa2,0x7b,0x4b,0x0c,0x3b]
-; EGPR-NEXT:    # implicit-def: $rax
-; EGPR-NEXT:    movq %rax, {{[-0-9]+}}(%r{{[sb]}}p) # 8-byte Spill
-; EGPR-NEXT:    # encoding: [0x48,0x89,0x84,0x24,0xb8,0x03,0x00,0x00]
-; EGPR-NEXT:    movabsq $64, %rax # encoding: [0x48,0xb8,0x40,0x00,0x00,0x00,0x00,0x00,0x00,0x00]
 ; EGPR-NEXT:    tilestored %tmm3, 1024(%rsp,%rax) # 1024-byte Folded Spill
 ; EGPR-NEXT:    # EVEX TO VEX Compression encoding: [0xc4,0xe2,0x7a,0x4b,0x9c,0x04,0x00,0x04,0x00,0x00]
 ; EGPR-NEXT:    tileloadd {{[-0-9]+}}(%r{{[sb]}}p), %tmm2 # 1024-byte Folded Reload
 ; EGPR-NEXT:    # EVEX TO VEX Compression encoding: [0xc4,0xe2,0x7b,0x4b,0x94,0x24,0x00,0x04,0x00,0x00]
-; EGPR-NEXT:    movq {{[-0-9]+}}(%r{{[sb]}}p), %rax # 8-byte Reload
-; EGPR-NEXT:    # encoding: [0x48,0x8b,0x84,0x24,0xb8,0x03,0x00,0x00]
 ; EGPR-NEXT:    tdpbssd %tmm1, %tmm0, %tmm2 # encoding: [0xc4,0xe2,0x73,0x5e,0xd0]
 ; EGPR-NEXT:    tilestored %tmm2, (%rbx,%r15) # EVEX TO VEX Compression encoding: [0xc4,0xa2,0x7a,0x4b,0x14,0x3b]
 ; EGPR-NEXT:    incl %r14d # encoding: [0x41,0xff,0xc6]