IfConversion: Add implicit uses for redefined regs with live subregisters

Normally, if conversion would add implicit uses for redefined registers,
e.g. R0<def> = add_if ..., R0<imp-use>. However, if only subregisters of
R0 are known to be live but not R0 itself, such implicit uses will not be
added, causing prior definitions of such subregisters and R0 itself to
become dead.

llvm-svn: 282626

lib/CodeGen/IfConversion.cpp

@@ -1453,6 +1453,17 @@ static void UpdatePredRedefs(MachineInstr &MI, LivePhysRegs &Redefs) {
     }
     if (LiveBeforeMI.count(Reg))
       MIB.addReg(Reg, RegState::Implicit);
+    else {
+      bool HasLiveSubReg = false;
+      for (MCSubRegIterator S(Reg, TRI); S.isValid(); ++S) {
+        if (!LiveBeforeMI.count(*S))
+          continue;
+        HasLiveSubReg = true;
+        break;
+      }
+      if (HasLiveSubReg)
+        MIB.addReg(Reg, RegState::Implicit);
+    }
   }
 }
 

test/CodeGen/Hexagon/ifcvt-live-subreg.mir

@@ -0,0 +1,50 @@
+# RUN: llc -march=hexagon -run-pass if-converter -o - %s | FileCheck %s
+# Check that an implicit use is generated for a predicated instruction
+# when a subregister of the redefined register is live.
+
+# CHECK-LABEL: name: foo
+
+# Verify the predicated block:
+# CHECK-LABEL: bb.0:
+# CHECK: liveins: %r0, %r1, %p0, %d8
+# CHECK: %d8 = A2_combinew killed %r0, killed %r1
+# CHECK: %d8 = L2_ploadrdf_io %p0, %r29, 0, implicit %d8
+# CHECK: J2_jumprf %p0, killed %r31, implicit-def %pc, implicit-def %pc, implicit killed %d8
+
+--- |
+  define void @foo() {
+    ret void
+  }
+...
+
+
+---
+name:            foo
+alignment:       4
+tracksRegLiveness: true
+liveins:
+  - { reg: '%r0' }
+  - { reg: '%r1' }
+  - { reg: '%p0' }
+  - { reg: '%d8' }
+body:             |
+  bb.0:
+    successors: %bb.1, %bb.2
+    liveins: %r0, %r1, %p0, %d8
+    %d8 = A2_combinew killed %r0, killed %r1
+    J2_jumpf killed %p0, %bb.2, implicit-def %pc
+
+  bb.1:
+    liveins: %d0, %r17
+    %r0 = A2_tfrsi 0
+    %r1 = A2_tfrsi 0
+    A2_nop ; non-predicable
+    J2_jumpr killed %r31, implicit-def dead %pc, implicit killed %d0
+
+  bb.2:
+    ; Predicate this block.
+    %d8 = L2_loadrd_io %r29, 0
+    J2_jumpr killed %r31, implicit-def dead %pc, implicit killed %d8
+
+...
+