[SystemZ] Replace SELRMux with COPY in case of identical operands. (llvm#125108)

If both operands of a SELRMux use the same register which is killed, and
the SELRMux is expanded to a jump sequence, a broken MIR results if the
kill flag is not removed. 

This patch replaces the SELRMux with a COPY in these cases.

Author: JonPsson1

llvm/lib/Target/SystemZ/SystemZPostRewrite.cpp

@@ -107,6 +107,18 @@ void SystemZPostRewrite::selectSELRMux(MachineBasicBlock &MBB,
   bool Src1IsHigh = SystemZ::isHighReg(Src1Reg);
   bool Src2IsHigh = SystemZ::isHighReg(Src2Reg);
 
+  // In rare cases both sources are the same register (after
+  // machine-cse). This must be handled as it may lead to wrong-code (after
+  // machine-cp) if the kill flag on Src1 isn't cleared (with
+  // expandCondMove()).
+  if (Src1Reg == Src2Reg) {
+    BuildMI(*MBBI->getParent(), MBBI, MBBI->getDebugLoc(),
+            TII->get(SystemZ::COPY), DestReg)
+        .addReg(MBBI->getOperand(1).getReg(), getRegState(MBBI->getOperand(1)));
+    MBBI->eraseFromParent();
+    return;
+  }
+
   // If sources and destination aren't all high or all low, we may be able to
   // simplify the operation by moving one of the sources to the destination
   // first.  But only if this doesn't clobber the other source.

llvm/test/CodeGen/SystemZ/cond-move-10.mir

@@ -0,0 +1,21 @@
+# RUN: llc -o - %s -mtriple=s390x-linux-gnu -mcpu=z15 -run-pass=systemz-post-rewrite \
+# RUN:   2>&1 | FileCheck %s
+
+# The SELRMux has two identical sources - replace with a copy instruction.
+# CHECK:      name: fun0
+# CHECK:      renamable $r1l = AHIMuxK killed renamable $r1l, -1, implicit-def dead $cc
+# CHECK-NEXT: CHIMux renamable $r5h, 9, implicit-def $cc
+# CHECK-NEXT: $r14h = COPY killed renamable $r1l
+---
+name:            fun0
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $r1l, $r5h
+    renamable $r1l = AHIMuxK killed renamable $r1l, -1, implicit-def dead $cc
+    CHIMux renamable $r5h, 9, implicit-def $cc
+    renamable $r14h = SELRMux killed renamable $r1l, renamable $r1l, 14, 8, implicit $cc
+    $r14l = COPY killed renamable $r14h
+    $r14d = LGFR $r14l
+    Return implicit $r14d
+...