[llvm-branch-commits] [llvm] release/20.x: [SystemZ] Replace SELRMux with COPY in case of identical operands. (#125108) (PR #125236)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/125236

Backport eb1a571114a799f532a12b2f062746d3b92fed88

Requested by: @nikic

>From 0ba1e70afa95a91cc60d804a00bcde43b8bae8d2 Mon Sep 17 00:00:00 2001
From: Jonas Paulsson <paulson1 at linux.ibm.com>
Date: Fri, 31 Jan 2025 06:58:01 -0600
Subject: [PATCH] [SystemZ] Replace SELRMux with COPY in case of identical
 operands. (#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.

(cherry picked from commit eb1a571114a799f532a12b2f062746d3b92fed88)
---
 .../lib/Target/SystemZ/SystemZPostRewrite.cpp | 12 +++++++++++
 llvm/test/CodeGen/SystemZ/cond-move-10.mir    | 21 +++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 llvm/test/CodeGen/SystemZ/cond-move-10.mir

diff --git a/llvm/lib/Target/SystemZ/SystemZPostRewrite.cpp b/llvm/lib/Target/SystemZ/SystemZPostRewrite.cpp
index e15f9027cc20956..cf3073f0f209048 100644
--- a/llvm/lib/Target/SystemZ/SystemZPostRewrite.cpp
+++ b/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.
diff --git a/llvm/test/CodeGen/SystemZ/cond-move-10.mir b/llvm/test/CodeGen/SystemZ/cond-move-10.mir
new file mode 100644
index 000000000000000..1db960829729eab
--- /dev/null
+++ b/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
+...