[PATCH] D51028: [BranchFolder] Drop kill flags if they aren't present in all merged instructions

[Mikael Holmén via Phabricator via llvm-commits]

uabelho updated this revision to Diff 197926.
uabelho added a comment.

Rebased, fixed a spelling mistake.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D51028/new/

https://reviews.llvm.org/D51028

Files:
  lib/CodeGen/BranchFolding.cpp
  test/CodeGen/Hexagon/branchfolder-clear-kill.mir


Index: test/CodeGen/Hexagon/branchfolder-clear-kill.mir
===================================================================
--- /dev/null
+++ test/CodeGen/Hexagon/branchfolder-clear-kill.mir
@@ -0,0 +1,47 @@
+# RUN: llc -march=hexagon -run-pass branch-folder %s -o - -verify-machineinstrs | FileCheck %s
+
+# When the branchfolder merges common tails, it needs to clear both undef and
+# killed flags if they differ between the merged blocks.
+
+# In the below example, if the killed flag is not cleared we will be left with
+#  A2_nop 0, killed $r0
+#  A2_nop 0, $r0
+# and then the verifier will complain about use of an undefined physical
+# register.
+
+---
+# CHECK-LABEL: name: func0
+# CHECK-LABEL: bb.0:
+# CHECK: liveins: $r0, $r31
+# CHECK: A2_nop implicit $r0
+# CHECK: A2_nop implicit $r0
+# CHECK: PS_jmpret
+
+name: func0
+tracksRegLiveness: true
+
+body: |
+  bb.0:
+    liveins: $r0, $r31
+    successors: %bb.1, %bb.2
+      J2_jumpt undef $p0, %bb.2, implicit-def $pc
+      J2_jump %bb.1, implicit-def $pc
+
+  bb.1:
+    liveins: $r0, $r31
+    successors: %bb.3
+      A2_nop implicit $r0
+      A2_nop implicit killed $r0
+      J2_jump %bb.3, implicit-def $pc
+
+  bb.2:
+    liveins: $r0, $r31
+    successors: %bb.3
+      A2_nop implicit killed $r0
+      A2_nop implicit undef $r0
+      J2_jump %bb.3, implicit-def $pc
+
+  bb.3:
+    liveins: $r31
+      PS_jmpret killed $r31, implicit-def $pc
+...
Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -865,7 +865,7 @@
     // Merge MMOs from memory operations in the common block.
     if (MBBICommon->mayLoad() || MBBICommon->mayStore())
       MBBICommon->cloneMergedMemRefs(*MBB->getParent(), {&*MBBICommon, &*MBBI});
-    // Drop undef flags if they aren't present in all merged instructions.
+    // Drop undef/kill flags if they aren't present in all merged instructions.
     for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) {
       MachineOperand &MO = MBBICommon->getOperand(I);
       if (MO.isReg() && MO.isUndef()) {
@@ -873,6 +873,21 @@
         if (!OtherMO.isUndef())
           MO.setIsUndef(false);
       }
+      if (MO.isReg() && MO.isKill()) {
+        const MachineOperand &OtherMO = MBBI->getOperand(I);
+        // An exception to the clearing of the kill flag is if we merge
+        // something like:
+        //  <instr1> undef $r0
+        //  $r0 = <instr2>
+        // and
+        //  <instr1> killed $r0
+        //  $r0 = <instr2>
+        // Here we should keep the kill flag even if it's only set in one of the
+        // merged paths since it's undef in the other. So the only real value
+        // in $r0 that actually reaches <instr1> will indeed be killed there.
+        if (!OtherMO.isKill() && !OtherMO.isUndef())
+          MO.setIsKill(false);
+      }
     }
 
     ++MBBI;


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