Index: lib/Target/AMDGPU/SIFoldOperands.cpp
===================================================================
--- lib/Target/AMDGPU/SIFoldOperands.cpp
+++ lib/Target/AMDGPU/SIFoldOperands.cpp
@@ -1349,6 +1349,8 @@
 
   for (MachineBasicBlock *MBB : depth_first(&MF)) {
     MachineBasicBlock::iterator I, Next;
+
+    MachineOperand *CurrentKnownM0Val = nullptr;
     for (I = MBB->begin(); I != MBB->end(); I = Next) {
       Next = std::next(I);
       MachineInstr &MI = *I;
@@ -1361,6 +1363,25 @@
         if (IsIEEEMode || (!HasNSZ && !MI.getFlag(MachineInstr::FmNsz)) ||
             !tryFoldOMod(MI))
           tryFoldClamp(MI);
+
+        // Saw an unknown clobber of m0, so we no longer know what it is.
+        if (CurrentKnownM0Val && MI.modifiesRegister(AMDGPU::M0, TRI))
+          CurrentKnownM0Val = nullptr;
+        continue;
+      }
+
+      // Specially track simple redefs of m0 to the same value in a block, so we
+      // can erase the later ones.
+      if (MI.getOperand(0).getReg() == AMDGPU::M0) {
+        MachineOperand &NewM0Val = MI.getOperand(1);
+        if (CurrentKnownM0Val && CurrentKnownM0Val->isIdenticalTo(NewM0Val)) {
+          MI.eraseFromParent();
+          continue;
+        }
+
+        // We aren't tracking other physical registers
+        CurrentKnownM0Val = (NewM0Val.isReg() && NewM0Val.getReg().isPhysical()) ?
+          nullptr : &NewM0Val;
         continue;
       }
 
Index: test/CodeGen/AMDGPU/fold-operands-remove-m0-redef.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/fold-operands-remove-m0-redef.mir
@@ -0,0 +1,352 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -mcpu=fiji -verify-machineinstrs -run-pass=si-fold-operands  %s -o - | FileCheck -check-prefix=GCN %s
+
+--- |
+  define amdgpu_kernel void @redef_m0_same_copy() { ret void }
+  define amdgpu_kernel void @multi_redef_m0_same_copy() { ret void }
+  define amdgpu_kernel void @redef_m0_different_copy() { ret void }
+  define amdgpu_kernel void @redef_m0_mixed_copy0() { ret void }
+  define amdgpu_kernel void @redef_m0_mixed_copy1() { ret void }
+  define amdgpu_kernel void @redef_m0_same_mov_imm() { ret void }
+  define amdgpu_kernel void @redef_m0_different_inst0() { ret void }
+  define amdgpu_kernel void @redef_m0_different_inst1() { ret void }
+  define amdgpu_kernel void @redef_m0_mixed_read_m0() { ret void }
+  define amdgpu_kernel void @redef_m0_same_copy_call() { ret void }
+  define amdgpu_kernel void @redef_m0_same_copy_multi_block() { ret void }
+  define amdgpu_kernel void @redef_m0_copy_self() { ret void }
+  define amdgpu_kernel void @redef_m0_copy_physreg() { ret void }
+
+  declare void @func()
+...
+
+---
+name:            redef_m0_same_copy
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: redef_m0_same_copy
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY %1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            multi_redef_m0_same_copy
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: multi_redef_m0_same_copy
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY %1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY %1
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_different_copy
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0, $sgpr1
+
+    ; GCN-LABEL: name: redef_m0_different_copy
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: [[COPY2:%[0-9]+]]:sgpr_32 = COPY $sgpr1
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: $m0 = COPY [[COPY2]]
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    %2:sgpr_32 = COPY $sgpr1
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY %2
+    %4:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_mixed_copy0
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0, $sgpr1
+
+    ; GCN-LABEL: name: redef_m0_mixed_copy0
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: [[COPY2:%[0-9]+]]:sgpr_32 = COPY $sgpr1
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: $m0 = COPY [[COPY2]]
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    %2:sgpr_32 = COPY $sgpr1
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY %1
+    $m0 = COPY %2
+    %4:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_mixed_copy1
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0, $sgpr1
+
+    ; GCN-LABEL: name: redef_m0_mixed_copy1
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: [[COPY2:%[0-9]+]]:sgpr_32 = COPY $sgpr1
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: $m0 = COPY [[COPY2]]
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    %2:sgpr_32 = COPY $sgpr1
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY %2
+    $m0 = COPY %1
+    %4:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_same_mov_imm
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: redef_m0_same_mov_imm
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = S_MOV_B32 -1
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = S_MOV_B32 -1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = S_MOV_B32 -1
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_different_inst0
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: redef_m0_different_inst0
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: $m0 = IMPLICIT_DEF
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY %1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = IMPLICIT_DEF
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_different_inst1
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: redef_m0_different_inst1
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: S_NOP 0, implicit-def $m0
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY %1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    S_NOP 0, implicit-def $m0
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_mixed_read_m0
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0, $sgpr1
+
+    ; GCN-LABEL: name: redef_m0_mixed_read_m0
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: [[COPY2:%[0-9]+]]:sgpr_32 = COPY $sgpr1
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: $m0 = COPY [[COPY2]]
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: [[DS_READ_B32_2:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 128, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    %2:sgpr_32 = COPY $sgpr1
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY %2
+    %4:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY %2
+    %5:vgpr_32 = DS_READ_B32 %0, 128, 0, implicit $m0, implicit $exec :: (load 4)
+...
+
+---
+name:            redef_m0_same_copy_call
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: redef_m0_same_copy_call
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: dead $sgpr30_sgpr31 = SI_CALL undef $sgpr6_sgpr7, @func, csr_amdgpu_highregs
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY %1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    dead $sgpr30_sgpr31 = SI_CALL undef $sgpr6_sgpr7, @func, csr_amdgpu_highregs
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_same_copy_multi_block
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  ; GCN-LABEL: name: redef_m0_same_copy_multi_block
+  ; GCN: bb.0:
+  ; GCN:   successors: %bb.1(0x80000000)
+  ; GCN:   [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+  ; GCN:   [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+  ; GCN:   $m0 = COPY [[COPY1]]
+  ; GCN:   [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+  ; GCN: bb.1:
+  ; GCN:   $m0 = COPY [[COPY1]]
+  ; GCN:   [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY %1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+
+  bb.1:
+    $m0 = COPY %1
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_copy_self
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: redef_m0_copy_self
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = COPY [[COPY1]]
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: $m0 = COPY $m0
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY %1
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $m0 = COPY $m0
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...
+
+---
+name:            redef_m0_copy_physreg
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+
+    ; GCN-LABEL: name: redef_m0_copy_physreg
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN: [[COPY1:%[0-9]+]]:sgpr_32 = COPY $sgpr0
+    ; GCN: $m0 = COPY $sgpr0
+    ; GCN: [[DS_READ_B32_:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 0, 0, implicit $m0, implicit $exec :: (load 4)
+    ; GCN: $sgpr0 = S_MOV_B32 0
+    ; GCN: $m0 = COPY $sgpr0
+    ; GCN: [[DS_READ_B32_1:%[0-9]+]]:vgpr_32 = DS_READ_B32 [[COPY]], 64, 0, implicit $m0, implicit $exec :: (load 4)
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    $m0 = COPY $sgpr0
+    %2:vgpr_32 = DS_READ_B32 %0, 0, 0, implicit $m0, implicit $exec :: (load 4)
+    $sgpr0 = S_MOV_B32 0
+    $m0 = COPY $sgpr0
+    %3:vgpr_32 = DS_READ_B32 %0, 64, 0, implicit $m0, implicit $exec :: (load 4)
+
+...