[X86] Remove redundant test after setzucc (#129506)

Patch #96594 substitutes setcc + zext pair with setzucc, but it results
in redundant test because X86FlagsCopyLowering doesn't recognize it.

This patch removes redundant test by reverting setzucc to setcc
(optimized) + zext.

Author: phoebewang

llvm/lib/Target/X86/X86FlagsCopyLowering.cpp

@@ -615,7 +615,7 @@ bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
           MRI->replaceRegWith(MI.getOperand(0).getReg(),
                               CopyDefI.getOperand(0).getReg());
           MI.eraseFromParent();
-        } else if (X86::isSETCC(Opc)) {
+        } else if (X86::isSETCC(Opc) || X86::isSETZUCC(Opc)) {
           rewriteSetCC(*TestMBB, TestPos, TestLoc, MI, CondRegs);
         } else if (isArithmeticOp(Opc)) {
           rewriteArithmetic(*TestMBB, TestPos, TestLoc, MI, CondRegs);
@@ -781,6 +781,29 @@ void X86FlagsCopyLoweringPass::rewriteSetCC(MachineBasicBlock &MBB,
   if (!CondReg)
     CondReg = promoteCondToReg(MBB, Pos, Loc, Cond);
 
+  if (X86::isSETZUCC(MI.getOpcode())) {
+    // SETZUCC is generated for register only for now.
+    assert(!MI.mayStore() && "Cannot handle memory variants");
+    assert(MI.getOperand(0).isReg() &&
+           "Cannot have a non-register defined operand to SETZUcc!");
+    Register OldReg = MI.getOperand(0).getReg();
+    // Drop Kill flags on the old register before replacing. CondReg may have
+    // a longer live range.
+    MRI->clearKillFlags(OldReg);
+    for (auto &Use : MRI->use_instructions(OldReg)) {
+      assert(Use.getOpcode() == X86::INSERT_SUBREG &&
+             "SETZUCC should be only used by INSERT_SUBREG");
+      Use.getOperand(2).setReg(CondReg);
+      // Recover MOV32r0 before INSERT_SUBREG, which removed by SETZUCC.
+      Register ZeroReg = MRI->createVirtualRegister(&X86::GR32RegClass);
+      BuildMI(*Use.getParent(), &Use, Use.getDebugLoc(), TII->get(X86::MOV32r0),
+              ZeroReg);
+      Use.getOperand(1).setReg(ZeroReg);
+    }
+    MI.eraseFromParent();
+    return;
+  }
+
   // Rewriting a register def is trivial: we just replace the register and
   // remove the setcc.
   if (!MI.mayStore()) {

llvm/test/CodeGen/X86/apx/setzucc.ll

@@ -60,11 +60,11 @@ define i32 @flags_copy_lowering() nounwind {
 ; CHECK-NEXT:    setb %sil
 ; CHECK-NEXT:    adcl $0, %ecx
 ; CHECK-NEXT:    testb %sil, %sil
-; CHECK-NEXT:    setzune %dl
-; CHECK-NEXT:    testb %sil, %sil
 ; CHECK-NEXT:    je .LBB4_3
 ; CHECK-NEXT:  # %bb.2: # %bb1
 ; CHECK-NEXT:    # in Loop: Header=BB4_1 Depth=1
+; CHECK-NEXT:    xorl %edx, %edx
+; CHECK-NEXT:    movb %sil, %dl
 ; CHECK-NEXT:    testb %al, %al
 ; CHECK-NEXT:    jne .LBB4_1
 ; CHECK-NEXT:  .LBB4_3: # %bb2