[llvm] r314882 - Recommit [UnreachableBlockElim] Use COPY if PHI input is undef

[Mikael Holmen via llvm-commits]

Author: uabelho
Date: Wed Oct  4 00:42:45 2017
New Revision: 314882

URL: http://llvm.org/viewvc/llvm-project?rev=314882&view=rev
Log:
Recommit [UnreachableBlockElim] Use COPY if PHI input is undef

This time invoking llc with "-march=x86-64" in the testcase, so we don't assume
the default target is x86.

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

Added:
    llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir
Modified:
    llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp

Modified: llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp?rev=314882&r1=314881&r2=314882&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp (original)
+++ llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp Wed Oct  4 00:42:45 2017
@@ -207,11 +207,12 @@ bool UnreachableMachineBlockElim::runOnM
           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();

Added: llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir?rev=314882&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir (added)
+++ llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir Wed Oct  4 00:42:45 2017
@@ -0,0 +1,38 @@
+# RUN: llc -march=x86-64 %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: