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

[Mikael Holmén via Phabricator via llvm-commits]

uabelho created this revision.
uabelho added reviewers: kparzysz, gberry.

Just like we do with undef flags, we need to drop kill flags if they aren't
present in all merged instructions. Otherwise we would merge

  <instr1> $r0
  <instr2> killed $r0

and

  <instr1> killed $r0
  <instr2> undef $r0

into

  <instr1> killed $r0
  <instr2> $r0

and then the verifier would complain about $r0 not being live at <instr2>.


Repository:
  rL LLVM

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
@@ -866,14 +866,29 @@
     // 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()) {
         const MachineOperand &OtherMO = MBBI->getOperand(I);
         if (!OtherMO.isUndef())
           MO.setIsUndef(false);
       }
+      if (MO.isReg() && MO.isKill()) {
+        const MachineOperand &OtherMO = MBBI->getOperand(I);
+        // An expection 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;


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D51028.161665.patch
Type: text/x-patch
Size: 2989 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180821/52947851/attachment.bin>