Teach two-address lowering how to unfold a load to open up commuting

opportunities. For example, this lets it emit this:

   movq (%rax), %rcx
   addq %rdx, %rcx

instead of this:

   movq %rdx, %rcx
   addq (%rax), %rcx

in the case where %rdx has subsequent uses. It's the same number
of instructions, and usually the same encoding size on x86, but
it appears faster, and in general, it may allow better scheduling
for the load.

llvm-svn: 106493

1 files changed, 84 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp b/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp
index 14102d1f3dd..6144ecd002e 100644
--- a/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp
+++ b/llvm/lib/CodeGen/TwoAddressInstructionPass.cpp
@@ -898,6 +898,90 @@ TryInstructionTransform(MachineBasicBlock::iterator &mi,
       }
     }
   }
+
+  // If this is an instruction with a load folded into it, try unfolding
+  // the load, e.g. avoid this:
+  //   movq %rdx, %rcx
+  //   addq (%rax), %rcx
+  // in favor of this:
+  //   movq (%rax), %rcx
+  //   addq %rdx, %rcx
+  // because it's preferable to schedule a load than a register copy.
+  if (TID.mayLoad() && !regBKilled) {
+    // Determine if a load can be unfolded.
+    unsigned LoadRegIndex;
+    unsigned NewOpc =
+      TII->getOpcodeAfterMemoryUnfold(mi->getOpcode(),
+                                      /*UnfoldLoad=*/true,
+                                      /*UnfoldStore=*/false,
+                                      &LoadRegIndex);
+    if (NewOpc != 0) {
+      const TargetInstrDesc &UnfoldTID = TII->get(NewOpc);
+      if (UnfoldTID.getNumDefs() == 1) {
+        MachineFunction &MF = *mbbi->getParent();
+
+        // Unfold the load.
+        DEBUG(dbgs() << "2addr:   UNFOLDING: " << *mi);
+        const TargetRegisterClass *RC =
+          UnfoldTID.OpInfo[LoadRegIndex].getRegClass(TRI);
+        unsigned Reg = MRI->createVirtualRegister(RC);
+        SmallVector<MachineInstr *, 2> NewMIs;
+        bool Success =
+          TII->unfoldMemoryOperand(MF, mi, Reg,
+                                   /*UnfoldLoad=*/true, /*UnfoldStore=*/false,
+                                   NewMIs);
+        (void)Success;
+        assert(Success &&
+               "unfoldMemoryOperand failed when getOpcodeAfterMemoryUnfold "
+               "succeeded!");
+        assert(NewMIs.size() == 2 &&
+               "Unfolded a load into multiple instructions!");
+        // The load was previously folded, so this is the only use.
+        NewMIs[1]->addRegisterKilled(Reg, TRI);
+
+        // Tentatively insert the instructions into the block so that they
+        // look "normal" to the transformation logic.
+        mbbi->insert(mi, NewMIs[0]);
+        mbbi->insert(mi, NewMIs[1]);
+
+        DEBUG(dbgs() << "2addr:    NEW LOAD: " << *NewMIs[0]
+                     << "2addr:    NEW INST: " << *NewMIs[1]);
+
+        // Transform the instruction, now that it no longer has a load.
+        unsigned NewDstIdx = NewMIs[1]->findRegisterDefOperandIdx(regA);
+        unsigned NewSrcIdx = NewMIs[1]->findRegisterUseOperandIdx(regB);
+        MachineBasicBlock::iterator NewMI = NewMIs[1];
+        bool TransformSuccess =
+          TryInstructionTransform(NewMI, mi, mbbi,
+                                  NewSrcIdx, NewDstIdx, Dist);
+        if (TransformSuccess ||
+            NewMIs[1]->getOperand(NewSrcIdx).isKill()) {
+          // Success, or at least we made an improvement. Keep the unfolded
+          // instructions and discard the original.
+          if (LV) {
+            for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
+              MachineOperand &MO = mi->getOperand(i);
+              if (MO.isReg() && MO.isUse() && MO.isKill())
+                LV->replaceKillInstruction(Reg, mi, NewMIs[0]);
+            }
+            LV->addVirtualRegisterKilled(Reg, NewMIs[1]);
+          }
+          mi->eraseFromParent();
+          mi = NewMIs[1];
+          if (TransformSuccess)
+            return true;
+        } else {
+          // Transforming didn't eliminate the tie and didn't lead to an
+          // improvement. Clean up the unfolded instructions and keep the
+          // original.
+          DEBUG(dbgs() << "2addr: ABANDONING UNFOLD\n");
+          NewMIs[0]->eraseFromParent();
+          NewMIs[1]->eraseFromParent();
+        }
+      }
+    }
+  }
+
   return false;
 }