[PATCH] D78159: [MachineDCE] Make sure MachineDCE considers subregs when adding liveins.

[Marcello Maggioni via Phabricator via llvm-commits]

kariddi created this revision.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

MachineDCE adds to the live physical registers the live in registers
of the successor blocks, but it doesn't really add the subregisters
of those, which are also live, causing potentially instructions subregs
only to be deleted.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D78159

Files:
  llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
  llvm/test/CodeGen/Hexagon/dead-code-elim-sub-reg-livein.mir


Index: llvm/test/CodeGen/Hexagon/dead-code-elim-sub-reg-livein.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/Hexagon/dead-code-elim-sub-reg-livein.mir
@@ -0,0 +1,35 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=hexagon -run-pass dead-mi-elimination -verify-machineinstrs %s -o - | FileCheck %s
+
+# DeadMachineCodeElimination needs to add the subregisters of the live-ins of a block to avoid
+# deleting instructions writing to parts of a super register.
+# This test is making sure that this happens.
+
+---
+name: livein_sub_reg_not_preserved
+tracksRegLiveness: true
+
+body: |
+  ; CHECK-LABEL: name: livein_sub_reg_not_preserved
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   liveins: $d0, $r2
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   liveins: $d0, $r2
+  ; CHECK:   $r0 = A2_tfrsi 1
+  ; CHECK:   A2_nop implicit-def $p0
+  ; CHECK:   J2_jumpt killed $p0, %bb.1, implicit-def dead $pc
+  ; CHECK:   J2_jumpr $r31, implicit-def dead $pc
+  bb.0:
+    liveins: $d0, $r2
+    successors: %bb.1
+
+  bb.1:
+    successors: %bb.1
+    liveins: $d0, $r2
+    $r0 = A2_tfrsi 1
+    A2_nop implicit-def $p0
+    J2_jumpt killed $p0, %bb.1, implicit-def dead $pc
+    J2_jumpr $r31, implicit-def dead $pc
+...
Index: llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
===================================================================
--- llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
+++ llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
@@ -126,7 +126,9 @@
     for (MachineBasicBlock::succ_iterator S = MBB.succ_begin(),
            E = MBB.succ_end(); S != E; S++)
       for (const auto &LI : (*S)->liveins())
-        LivePhysRegs.set(LI.PhysReg);
+        for (MCSubRegIterator SR(LI.PhysReg, TRI, /*IncludeSelf=*/true);
+             SR.isValid(); ++SR)
+          LivePhysRegs.set(*SR);
 
     // Now scan the instructions and delete dead ones, tracking physreg
     // liveness as we go.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D78159.257512.patch
Type: text/x-patch
Size: 2074 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20200414/4e76157a/attachment.bin>