A problem that's exposed when machine LICM is enabled. Consider this code:

LBB1_3:   # bb
...
        xorl    %ebp, %ebp
        subl    (%ebx), %ebp
...
        incl    %ecx
        cmpl    %edi, %ecx
        jl      LBB1_3  # bb

Whe using machine LICM, LLVM converts it into:

        xorl %esi, %esi
LBB1_3: # bb
...
        movl    %esi, %ebp
        subl    (%ebx), %ebp
...
        incl    %ecx
        cmpl    %edi, %ecx
        jl      LBB1_3  # bb

Two address conversion inserts the copy instruction. However, it's cheaper to
rematerialize it, and remat helps reduce register pressure.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51562 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 40 insertions, 1 deletions

diff --git a/lib/CodeGen/TwoAddressInstructionPass.cpp b/lib/CodeGen/TwoAddressInstructionPass.cpp
index 1e24914..84491a5 100644
--- a/lib/CodeGen/TwoAddressInstructionPass.cpp
+++ b/lib/CodeGen/TwoAddressInstructionPass.cpp
@@ -39,6 +39,7 @@
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/STLExtras.h"
 using namespace llvm;
@@ -200,6 +201,8 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
   DOUT << "********** REWRITING TWO-ADDR INSTRS **********\n";
   DOUT << "********** Function: " << MF.getFunction()->getName() << '\n';
 
+  SmallPtrSet<MachineInstr*, 8> ReMattedInstrs;
+
   for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end();
        mbbi != mbbe; ++mbbi) {
     for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end();
@@ -321,7 +324,14 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
 
         InstructionRearranged:
           const TargetRegisterClass* rc = MF.getRegInfo().getRegClass(regA);
-          TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc);
+          MachineInstr *Orig = MRI->getVRegDef(regB);
+
+          if (Orig && TII->isTriviallyReMaterializable(Orig)) {
+            TII->reMaterialize(*mbbi, mi, regA, Orig);
+            ReMattedInstrs.insert(Orig);
+          } else {
+            TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc);
+          }
 
           MachineBasicBlock::iterator prevMi = prior(mi);
           DOUT << "\t\tprepend:\t"; DEBUG(prevMi->print(*cerr.stream(), &TM));
@@ -357,5 +367,34 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
     }
   }
 
+  SmallPtrSet<MachineInstr*, 8>::iterator I = ReMattedInstrs.begin();
+  SmallPtrSet<MachineInstr*, 8>::iterator E = ReMattedInstrs.end();
+
+  for (; I != E; ++I) {
+    MachineInstr *MI = *I;
+    bool InstrDead = true;
+
+    for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+      const MachineOperand &MO = MI->getOperand(i);
+      if (!MO.isRegister())
+        continue;
+      unsigned MOReg = MO.getReg();
+      if (!MOReg)
+        continue;
+      if (MO.isDef()) {
+        if (MO.isImplicit())
+          continue;
+
+        if (MRI->use_begin(MOReg) != MRI->use_end()) {
+          InstrDead = false;
+          break;
+        }
+      }
+    }
+
+    if (InstrDead && MI->getNumOperands() > 0)
+      MI->eraseFromParent();
+  }
+
   return MadeChange;
 }