Index: llvm/lib/Target/X86/X86InstrInfo.cpp
===================================================================
--- llvm/lib/Target/X86/X86InstrInfo.cpp
+++ llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -3648,8 +3648,15 @@
 
 std::optional<DestSourcePair>
 X86InstrInfo::isCopyInstrImpl(const MachineInstr &MI) const {
-  if (MI.isMoveReg())
+  if (MI.isMoveReg()) {
+    // FIXME: Dirty hack for apparent invariant that doesn't hold when
+    // subreg_to_reg is coalesced with ordinary copies, such that the bits that
+    // were asserted as 0 are now undef.
+    if (MI.getOperand(0).isUndef() && MI.getOperand(0).getSubReg())
+      return std::nullopt;
+
     return DestSourcePair{MI.getOperand(0), MI.getOperand(1)};
+  }
   return std::nullopt;
 }
 
Index: llvm/test/CodeGen/X86/GlobalISel/add-ext.ll
===================================================================
--- llvm/test/CodeGen/X86/GlobalISel/add-ext.ll
+++ llvm/test/CodeGen/X86/GlobalISel/add-ext.ll
@@ -205,8 +205,8 @@
 ; CHECK-NEXT:    addq %rdi, %rcx
 ; CHECK-NEXT:    movl (%rcx), %ecx
 ; CHECK-NEXT:    addl (%rax), %ecx
-; CHECK-NEXT:    movl %esi, %esi
-; CHECK-NEXT:    imulq $4, %rsi, %rax
+; CHECK-NEXT:    movl %esi, %eax
+; CHECK-NEXT:    imulq $4, %rax, %rax
 ; CHECK-NEXT:    addq %rdi, %rax
 ; CHECK-NEXT:    movl %ecx, (%rax)
 ; CHECK-NEXT:    retq
Index: llvm/test/CodeGen/X86/dagcombine-cse.ll
===================================================================
--- llvm/test/CodeGen/X86/dagcombine-cse.ll
+++ llvm/test/CodeGen/X86/dagcombine-cse.ll
@@ -106,24 +106,24 @@
 ;
 ; X64-LABEL: square_high:
 ; X64:       ## %bb.0: ## %entry
-; X64-NEXT:    movl %esi, %esi
-; X64-NEXT:    movq %rsi, %rax
+; X64-NEXT:    movl %esi, %ecx
+; X64-NEXT:    movq %rcx, %rax
 ; X64-NEXT:    mulq %rdi
-; X64-NEXT:    movq %rdx, %rcx
+; X64-NEXT:    movq %rdx, %rsi
 ; X64-NEXT:    movq %rax, %r8
 ; X64-NEXT:    movq %rdi, %rax
 ; X64-NEXT:    mulq %rdi
 ; X64-NEXT:    addq %r8, %rdx
-; X64-NEXT:    movq %rcx, %rax
+; X64-NEXT:    movq %rsi, %rax
 ; X64-NEXT:    adcq $0, %rax
 ; X64-NEXT:    addq %rdx, %r8
-; X64-NEXT:    adcq %rcx, %rax
-; X64-NEXT:    imulq %rsi, %rsi
-; X64-NEXT:    addq %rax, %rsi
-; X64-NEXT:    shrdq $32, %rsi, %r8
-; X64-NEXT:    shrq $32, %rsi
+; X64-NEXT:    adcq %rsi, %rax
+; X64-NEXT:    imulq %rcx, %rcx
+; X64-NEXT:    addq %rax, %rcx
+; X64-NEXT:    shrdq $32, %rcx, %r8
+; X64-NEXT:    shrq $32, %rcx
 ; X64-NEXT:    movq %r8, %rax
-; X64-NEXT:    movq %rsi, %rdx
+; X64-NEXT:    movq %rcx, %rdx
 ; X64-NEXT:    retq
 entry:
   %conv = zext i96 %x to i192
Index: llvm/test/CodeGen/X86/fold-and-shift-x86_64.ll
===================================================================
--- llvm/test/CodeGen/X86/fold-and-shift-x86_64.ll
+++ llvm/test/CodeGen/X86/fold-and-shift-x86_64.ll
@@ -34,8 +34,8 @@
 define i8 @t3(ptr %X, i64 %i) {
 ; CHECK-LABEL: t3:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    movl %esi, %esi
-; CHECK-NEXT:    movzbl (%rdi,%rsi,4), %eax
+; CHECK-NEXT:    movl %esi, %eax
+; CHECK-NEXT:    movzbl (%rdi,%rax,4), %eax
 ; CHECK-NEXT:    retq
 
 entry:
Index: llvm/test/CodeGen/X86/unfold-masked-merge-scalar-constmask-lowhigh.ll
===================================================================
--- llvm/test/CodeGen/X86/unfold-masked-merge-scalar-constmask-lowhigh.ll
+++ llvm/test/CodeGen/X86/unfold-masked-merge-scalar-constmask-lowhigh.ll
@@ -78,18 +78,18 @@
 define i64 @out64_constmask(i64 %x, i64 %y) {
 ; CHECK-NOBMI-LABEL: out64_constmask:
 ; CHECK-NOBMI:       # %bb.0:
-; CHECK-NOBMI-NEXT:    movl %edi, %edi
+; CHECK-NOBMI-NEXT:    movl %edi, %ecx
 ; CHECK-NOBMI-NEXT:    movabsq $-4294967296, %rax # imm = 0xFFFFFFFF00000000
 ; CHECK-NOBMI-NEXT:    andq %rsi, %rax
-; CHECK-NOBMI-NEXT:    orq %rdi, %rax
+; CHECK-NOBMI-NEXT:    orq %rcx, %rax
 ; CHECK-NOBMI-NEXT:    retq
 ;
 ; CHECK-BMI-LABEL: out64_constmask:
 ; CHECK-BMI:       # %bb.0:
-; CHECK-BMI-NEXT:    movl %edi, %edi
+; CHECK-BMI-NEXT:    movl %edi, %ecx
 ; CHECK-BMI-NEXT:    movabsq $-4294967296, %rax # imm = 0xFFFFFFFF00000000
 ; CHECK-BMI-NEXT:    andq %rsi, %rax
-; CHECK-BMI-NEXT:    orq %rdi, %rax
+; CHECK-BMI-NEXT:    orq %rcx, %rax
 ; CHECK-BMI-NEXT:    retq
   %mx = and i64 %x, 4294967295
   %my = and i64 %y, -4294967296