AMDGPU: Duplicate instead of COPY constants from VGPR to SGPR (#66882)

rovka · web-flow · commit a04603993b43 · 2023-09-25T13:20:08.000+02:00
Teach the si-fix-sgpr-copies pass to deal with REG_SEQUENCE, PHI or
INSERT_SUBREG where the result is an SGPR, but some of the inputs are
constants materialized into VGPRs. This may happen in cases where for
instance several instructions use an immediate zero and SelectionDAG
chooses to put it in a VGPR to satisfy all of them. This however causes
the si-fix-sgpr-copies to try to switch the whole chain to VGPR and may
lead to illegal VGPR-to-SGPR copies. Rematerializing the constant into
an SGPR fixes the issue.
diff --git a/llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp b/llvm/lib/Target/AMDGPU/SIFixSGPRCopies.cpp
@@ -152,6 +152,13 @@ class SIFixSGPRCopies : public MachineFunctionPass {
 
   void processPHINode(MachineInstr &MI);
 
+  // Check if MO is an immediate materialized into a VGPR, and if so replace it
+  // with an SGPR immediate. The VGPR immediate is also deleted if it does not
+  // have any other uses.
+  bool tryMoveVGPRConstToSGPR(MachineOperand &MO, Register NewDst,
+                              MachineBasicBlock *BlockToInsertTo,
+                              MachineBasicBlock::iterator PointToInsertTo);
+
   StringRef getPassName() const override { return "SI Fix SGPR copies"; }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
@@ -662,13 +669,17 @@ bool SIFixSGPRCopies::runOnMachineFunction(MachineFunction &MF) {
                              : MBB;
               MachineBasicBlock::iterator PointToInsertCopy =
                   MI.isPHI() ? BlockToInsertCopy->getFirstInstrTerminator() : I;
-              MachineInstr *NewCopy =
-                  BuildMI(*BlockToInsertCopy, PointToInsertCopy,
-                          PointToInsertCopy->getDebugLoc(),
-                          TII->get(AMDGPU::COPY), NewDst)
-                      .addReg(MO.getReg());
-              MO.setReg(NewDst);
-              analyzeVGPRToSGPRCopy(NewCopy);
+
+              if (!tryMoveVGPRConstToSGPR(MO, NewDst, BlockToInsertCopy,
+                                          PointToInsertCopy)) {
+                MachineInstr *NewCopy =
+                    BuildMI(*BlockToInsertCopy, PointToInsertCopy,
+                            PointToInsertCopy->getDebugLoc(),
+                            TII->get(AMDGPU::COPY), NewDst)
+                        .addReg(MO.getReg());
+                MO.setReg(NewDst);
+                analyzeVGPRToSGPRCopy(NewCopy);
+              }
             }
           }
         }
@@ -829,6 +840,32 @@ void SIFixSGPRCopies::processPHINode(MachineInstr &MI) {
   }
 }
 
+bool SIFixSGPRCopies::tryMoveVGPRConstToSGPR(
+    MachineOperand &MaybeVGPRConstMO, Register DstReg,
+    MachineBasicBlock *BlockToInsertTo,
+    MachineBasicBlock::iterator PointToInsertTo) {
+
+  MachineInstr *DefMI = MRI->getVRegDef(MaybeVGPRConstMO.getReg());
+  if (!DefMI || !DefMI->isMoveImmediate())
+    return false;
+
+  MachineOperand *SrcConst = TII->getNamedOperand(*DefMI, AMDGPU::OpName::src0);
+  if (SrcConst->isReg())
+    return false;
+
+  const TargetRegisterClass *SrcRC =
+      MRI->getRegClass(MaybeVGPRConstMO.getReg());
+  unsigned MoveSize = TRI->getRegSizeInBits(*SrcRC);
+  unsigned MoveOp = MoveSize == 64 ? AMDGPU::S_MOV_B64 : AMDGPU::S_MOV_B32;
+  BuildMI(*BlockToInsertTo, PointToInsertTo, PointToInsertTo->getDebugLoc(),
+          TII->get(MoveOp), DstReg)
+      .add(*SrcConst);
+  if (MRI->hasOneUse(MaybeVGPRConstMO.getReg()))
+    DefMI->eraseFromParent();
+  MaybeVGPRConstMO.setReg(DstReg);
+  return true;
+}
+
 bool SIFixSGPRCopies::lowerSpecialCase(MachineInstr &MI,
                                        MachineBasicBlock::iterator &I) {
   Register DstReg = MI.getOperand(0).getReg();
@@ -846,25 +883,10 @@ bool SIFixSGPRCopies::lowerSpecialCase(MachineInstr &MI,
               TII->get(AMDGPU::V_READFIRSTLANE_B32), TmpReg)
           .add(MI.getOperand(1));
       MI.getOperand(1).setReg(TmpReg);
-    } else {
-      MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
-      if (DefMI && DefMI->isMoveImmediate()) {
-        MachineOperand SrcConst = DefMI->getOperand(AMDGPU::getNamedOperandIdx(
-            DefMI->getOpcode(), AMDGPU::OpName::src0));
-        if (!SrcConst.isReg()) {
-          const TargetRegisterClass *SrcRC = MRI->getRegClass(SrcReg);
-          unsigned MoveSize = TRI->getRegSizeInBits(*SrcRC);
-          unsigned MoveOp =
-              MoveSize == 64 ? AMDGPU::S_MOV_B64 : AMDGPU::S_MOV_B32;
-          BuildMI(*MI.getParent(), MI, MI.getDebugLoc(), TII->get(MoveOp),
-                  DstReg)
-              .add(SrcConst);
-          I = std::next(I);
-          if (MRI->hasOneUse(SrcReg))
-            DefMI->eraseFromParent();
-          MI.eraseFromParent();
-        }
-      }
+    } else if (tryMoveVGPRConstToSGPR(MI.getOperand(1), DstReg, MI.getParent(),
+                                      MI)) {
+      I = std::next(I);
+      MI.eraseFromParent();
     }
     return true;
   }
diff --git a/llvm/test/CodeGen/AMDGPU/fix-sgpr-copies.mir b/llvm/test/CodeGen/AMDGPU/fix-sgpr-copies.mir
@@ -125,7 +125,62 @@ body:               |
   %2:sreg_32 = S_MOV_B32 1
   %3:sreg_32 = COPY %1:vgpr_32
   %4:sreg_32 = S_CSELECT_B32 killed %2:sreg_32, killed %3:sreg_32, implicit undef $scc
+...
+
+---
+# Test that the VGPR immediate is replaced with an SGPR one.
+# GCN-LABEL: name: reg_sequence_vgpr_immediate
+# GCN: [[A_SGPR:%[0-9]+]]:sreg_32 = IMPLICIT_DEF
+# GCN-NEXT: [[VGPR_CONST:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 37
+# GCN-NEXT: [[SGPR_CONST:%[0-9]+]]:sgpr_32 = S_MOV_B32 37
+# GCN-NEXT: {{%[0-9]+}}:sreg_64 = REG_SEQUENCE [[SGPR_CONST]], %subreg.sub0, [[A_SGPR]], %subreg.sub1
+name: reg_sequence_vgpr_immediate
+body:             |
+  bb.0:
+    %0:sreg_32 = IMPLICIT_DEF
+    %1:vgpr_32 = V_MOV_B32_e32 37, implicit $exec
+    %2:sreg_64 = REG_SEQUENCE %1:vgpr_32, %subreg.sub0, %0:sreg_32, %subreg.sub1
+
+    %3:vgpr_32 = V_ADD_U32_e32 %1:vgpr_32, %1:vgpr_32, implicit $exec
+...
+
+---
+# GCN-LABEL: name: insert_subreg_vgpr_immediate
+# GCN: [[DST:%[0-9]+]]:sgpr_128 = REG_SEQUENCE $sgpr0, %subreg.sub0, $sgpr0, %subreg.sub2
+# GCN-NEXT: [[SGPR_CONST:%[0-9]+]]:sgpr_32 = S_MOV_B32 43
+# GCN-NEXT: {{%[0-9]+}}:sgpr_128 = INSERT_SUBREG [[DST]], [[SGPR_CONST]], %subreg.sub3
+name: insert_subreg_vgpr_immediate
+body:             |
+  bb.0:
+    %0:sgpr_128 = REG_SEQUENCE $sgpr0, %subreg.sub0, $sgpr0, %subreg.sub2
+    %1:vgpr_32 = V_MOV_B32_e32 43, implicit $exec
+    %2:sgpr_128 = INSERT_SUBREG %0, %1, %subreg.sub3
+...
+
 ---
+# GCN-LABEL: name: phi_vgpr_immediate
+# GCN: bb.1:
+# GCN: [[SGPR:%[0-9]+]]:sgpr_32 = S_MOV_B32 51
+# GCN: bb.2:
+# GCN: IMPLICIT_DEF
+# GCN: bb.3:
+# GCN: sreg_32 = PHI [[SGPR]], %bb.1
+name: phi_vgpr_immediate
+tracksRegLiveness: true
+body:               |
+  bb.0:
+    S_CBRANCH_SCC1 %bb.2, implicit undef $scc
+
+  bb.1:
+    %0:vgpr_32 = V_MOV_B32_e32 51, implicit $exec
+    S_BRANCH %bb.3
+
+  bb.2:
+    %1:sreg_32 = IMPLICIT_DEF
+    S_BRANCH %bb.3
+
+  bb.3:
+    %2:sreg_32 = PHI %0:vgpr_32, %bb.1, %1:sreg_32, %bb.2
 
 ---
 name:            cmp_f32
