[PATCH] D11696: [x86] machine combiner reassociation: mark EFLAGS operand as 'dead'

[Sanjay Patel]

spatel updated this revision to Diff 31274.
spatel added a comment.

Patch updated: test case now uses '-stop-after machine-combiner' to be more efficient.


http://reviews.llvm.org/D11696

Files:
  lib/Target/X86/X86InstrInfo.cpp
  test/CodeGen/X86/machine-combiner-int.ll

Index: test/CodeGen/X86/machine-combiner-int.ll
===================================================================
--- test/CodeGen/X86/machine-combiner-int.ll
+++ test/CodeGen/X86/machine-combiner-int.ll
@@ -1,4 +1,5 @@
-; RUN: llc -mtriple=x86_64-unknown-unknown -mcpu=x86-64 < %s | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mcpu=x86-64 | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mcpu=x86-64 -stop-after machine-combiner -o /dev/null 2>&1 | FileCheck %s --check-prefix=DEAD
 
 ; Verify that integer multiplies are reassociated. The first multiply in 
 ; each test should be independent of the result of the preceding add (lea).
@@ -23,6 +24,11 @@
 ; CHECK-NEXT:    imull  %ecx, %edx
 ; CHECK-NEXT:    imull  %edx, %eax
 ; CHECK-NEXT:    retq
+
+; DEAD:       ADD32rr
+; DEAD-NEXT:  IMUL32rr{{.*}}implicit-def dead %eflags
+; DEAD-NEXT:  IMUL32rr{{.*}}implicit-def dead %eflags
+
   %t0 = add i32 %x0, %x1
   %t1 = mul i32 %x2, %t0
   %t2 = mul i32 %x3, %t1
Index: lib/Target/X86/X86InstrInfo.cpp
===================================================================
--- lib/Target/X86/X86InstrInfo.cpp
+++ lib/Target/X86/X86InstrInfo.cpp
@@ -6437,6 +6437,45 @@
   return false;
 }
 
+/// This is an architecture-specific helper function of reassociateOps. Set the
+/// appropriate flags for any extra operands of instructions that are being
+/// reassociated.
+static void setExtraReassOperands(MachineInstr &OldMI1, MachineInstr &OldMI2,
+                                  MachineInstr &NewMI1, MachineInstr &NewMI2) {
+  // Integer instructions define an implicit EFLAGS source register operand as
+  // the third source (fourth total) operand.
+  if (OldMI1.getNumOperands() != 4 || OldMI2.getNumOperands() != 4)
+    return;
+
+  assert(NewMI1.getNumOperands() == 4 && NewMI2.getNumOperands() == 4 &&
+         "Unexpected instruction type for reassociation");
+  
+  MachineOperand &OldOp1 = OldMI1.getOperand(3);
+  MachineOperand &OldOp2 = OldMI2.getOperand(3);
+  MachineOperand &NewOp1 = NewMI1.getOperand(3);
+  MachineOperand &NewOp2 = NewMI2.getOperand(3);
+
+  assert(OldOp1.isReg() && OldOp1.getReg() == X86::EFLAGS && OldOp1.isDead() &&
+         "Must have dead EFLAGS operand in reassociable instruction");
+  assert(OldOp2.isReg() && OldOp2.getReg() == X86::EFLAGS && OldOp2.isDead() &&
+         "Must have dead EFLAGS operand in reassociable instruction");
+
+  (void)OldOp1;
+  (void)OldOp2;
+
+  assert(NewOp1.isReg() && NewOp1.getReg() == X86::EFLAGS &&
+         "Unexpected operand in reassociable instruction");
+  assert(NewOp2.isReg() && NewOp2.getReg() == X86::EFLAGS &&
+         "Unexpected operand in reassociable instruction");
+
+  // Mark the new EFLAGS operands as dead to be helpful to subsequent iterations
+  // of this pass or other passes. The EFLAGS operands must be dead in these new
+  // instructions because the EFLAGS operands in the original instructions must
+  // be dead in order for reassociation to occur.
+  NewOp1.setIsDead();
+  NewOp2.setIsDead();
+}
+
 /// Attempt the following reassociation to reduce critical path length:
 ///   B = A op X (Prev)
 ///   C = B op Y (Root)
@@ -6503,15 +6542,16 @@
     BuildMI(*MF, Prev.getDebugLoc(), TII->get(Opcode), NewVR)
       .addReg(RegX, getKillRegState(KillX))
       .addReg(RegY, getKillRegState(KillY));
-  InsInstrs.push_back(MIB1);
-
   MachineInstrBuilder MIB2 =
     BuildMI(*MF, Root.getDebugLoc(), TII->get(Opcode), RegC)
       .addReg(RegA, getKillRegState(KillA))
       .addReg(NewVR, getKillRegState(true));
-  InsInstrs.push_back(MIB2);
 
-  // Record old instructions for deletion.
+  setExtraReassOperands(Root, Prev, *MIB1, *MIB2);
+
+  // Record new instructions for insertion and old instructions for deletion.
+  InsInstrs.push_back(MIB1);
+  InsInstrs.push_back(MIB2);
   DelInstrs.push_back(&Prev);
   DelInstrs.push_back(&Root);
 }


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