[CodeGen] Enhance machine PHIs optimization

Summary:
Make machine PHIs optimization to work for single value register taken from
several different copies. This is the first step to fix PR38917. This change
allows to get rid of redundant PHIs (see opt_phis2.mir test) to make
the subsequent optimizations (like CSE) possible and simpler.

For instance, before this patch the code like this:

%b = COPY %z
...
%a = PHI %bb1, %a; %bb2, %b
could be optimized to:

%a = %b
but the code like this:

%c = COPY %z
...
%b = COPY %z
...
%a = PHI %bb1, %a; %bb2, %b; %bb3, %c
would remain unchanged.
With this patch the latter case will be optimized:

%a = %z```.

Committed on behalf of: Anton Afanasyev anton.a.afanasyev@gmail.com

Reviewers: RKSimon, MatzeB

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54839

llvm-svn: 349271

1 files changed, 11 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/OptimizePHIs.cpp b/llvm/lib/CodeGen/OptimizePHIs.cpp
index befa842..770f6c5 100644
--- a/llvm/lib/CodeGen/OptimizePHIs.cpp
+++ b/llvm/lib/CodeGen/OptimizePHIs.cpp
@@ -90,10 +90,10 @@ bool OptimizePHIs::runOnMachineFunction(MachineFunction &Fn) {
 }
 
 /// IsSingleValuePHICycle - Check if MI is a PHI where all the source operands
-/// are copies of SingleValReg, possibly via copies through other PHIs.  If
+/// are copies of SingleValReg, possibly via copies through other PHIs. If
 /// SingleValReg is zero on entry, it is set to the register with the single
-/// non-copy value.  PHIsInCycle is a set used to keep track of the PHIs that
-/// have been scanned.
+/// non-copy value. PHIsInCycle is a set used to keep track of the PHIs that
+/// have been scanned. PHIs may be grouped by cycle, several cycles or chains.
 bool OptimizePHIs::IsSingleValuePHICycle(MachineInstr *MI,
                                          unsigned &SingleValReg,
                                          InstrSet &PHIsInCycle) {
@@ -119,8 +119,10 @@ bool OptimizePHIs::IsSingleValuePHICycle(MachineInstr *MI,
     if (SrcMI && SrcMI->isCopy() &&
         !SrcMI->getOperand(0).getSubReg() &&
         !SrcMI->getOperand(1).getSubReg() &&
-        TargetRegisterInfo::isVirtualRegister(SrcMI->getOperand(1).getReg()))
-      SrcMI = MRI->getVRegDef(SrcMI->getOperand(1).getReg());
+        TargetRegisterInfo::isVirtualRegister(SrcMI->getOperand(1).getReg())) {
+      SrcReg = SrcMI->getOperand(1).getReg();
+      SrcMI = MRI->getVRegDef(SrcReg);
+    }
     if (!SrcMI)
       return false;
 
@@ -129,7 +131,7 @@ bool OptimizePHIs::IsSingleValuePHICycle(MachineInstr *MI,
         return false;
     } else {
       // Fail if there is more than one non-phi/non-move register.
-      if (SingleValReg != 0)
+      if (SingleValReg != 0 && SingleValReg != SrcReg)
         return false;
       SingleValReg = SrcReg;
     }
@@ -180,6 +182,9 @@ bool OptimizePHIs::OptimizeBB(MachineBasicBlock &MBB) {
       if (!MRI->constrainRegClass(SingleValReg, MRI->getRegClass(OldReg)))
         continue;
 
+      // for the case SingleValReg taken from copy instr
+      MRI->clearKillFlags(SingleValReg);
+
       MRI->replaceRegWith(OldReg, SingleValReg);
       MI->eraseFromParent();
       ++NumPHICycles;