[UnreachableBlockElim] Use COPY if PHI input is undef

mikaelholmen · mikaelholmen · commit 65eb2f394cd8 · 2017-10-04T06:06:31.000Z
Summary: If we have %vreg0<def> = PHI %vreg2<undef>, <BB#0>, %vreg3, <BB#2>; GR32:%vreg0,%vreg2,%vreg3 %vreg3<def,tied1> = ADD32ri8 %vreg0<kill,tied0>, 1, %EFLAGS<imp-def>; GR32:%vreg3,%vreg0 then we can't just change %vreg0 into %vreg3, since %vreg2 is actually undef. We would have to also copy the undef flag to be able to change the register. Instead we deal with this case like other cases where we can't just replace the register: we insert a COPY. The code creating the COPY already copied all flags from the PHI input, so the undef flag will be transferred as it should. Reviewers: kparzysz Reviewed By: kparzysz Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D38235 llvm-svn: 314879
diff --git a/llvm/lib/CodeGen/UnreachableBlockElim.cpp b/llvm/lib/CodeGen/UnreachableBlockElim.cpp
@@ -207,11 +207,12 @@ bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
           MachineRegisterInfo &MRI = F.getRegInfo();
           unsigned InputSub = Input.getSubReg();
           if (InputSub == 0 &&
-              MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg))) {
+              MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
+              !Input.isUndef()) {
             MRI.replaceRegWith(OutputReg, InputReg);
           } else {
             // The input register to the PHI has a subregister or it can't be
-            // constrained to the proper register class:
+            // constrained to the proper register class or it is undef:
             // insert a COPY instead of simply replacing the output
             // with the input.
             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
diff --git a/llvm/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir b/llvm/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir
@@ -0,0 +1,38 @@
+# RUN: llc %s -o - -run-pass=processimpdefs -run-pass=unreachable-mbb-elimination | FileCheck %s
+---
+name:            f
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+  - { id: 1, class: gr32, preferred-register: '' }
+  - { id: 2, class: gr32, preferred-register: '' }
+body:             |
+  bb.0:
+    %0 = IMPLICIT_DEF
+    JMP_1 %bb.1
+
+  bb.1:
+    %1 = PHI %0, %bb.0, %2, %bb.2
+    %2 = ADD32ri8 killed %1, 1, implicit-def %eflags
+    JMP_1 %bb.3
+
+  bb.2:
+    JMP_1 %bb.1
+
+  bb.3:
+...
+
+# bb2 above is dead and should be removed and the PHI should be replaced with a
+# COPY from an undef value since the bb0 value in the PHI is undef.
+
+# CHECK:  bb.0:
+# CHECK:    successors: %bb.1
+# CHECK:    JMP_1 %bb.1
+
+# CHECK:  bb.1:
+# CHECK:    successors: %bb.2
+# CHECK:    [[TMP1:%[0-9]+]] = COPY undef %{{[0-9]+}}
+# CHECK:    %{{[0-9]+}} = ADD32ri8 killed [[TMP1]], 1
+# CHECK:    JMP_1 %bb.2
+
+# CHECK:  bb.2:
