Fix the "test" optimization to recognize "dec" as an add of

negative one, as subtracts of immediates are canonicalized
to adds.

llvm-svn: 66180

3 files changed, 31 insertions, 9 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index d56e672e1f8..33eee7a7368 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -5382,13 +5382,19 @@ SDValue X86TargetLowering::EmitTest(SDValue Op, SelectionDAG &DAG) {
            UE = Op.getNode()->use_end(); UI != UE; ++UI)
         if (UI->getOpcode() == ISD::STORE)
           goto default_case;
-      // An add of one will be selected as an INC.
       if (ConstantSDNode *C =
-            dyn_cast<ConstantSDNode>(Op.getNode()->getOperand(1)))
+            dyn_cast<ConstantSDNode>(Op.getNode()->getOperand(1))) {
+        // An add of one will be selected as an INC.
         if (C->getAPIntValue() == 1) {
           Opcode = X86ISD::INC;
           break;
         }
+        // An add of negative one (subtract of one) will be selected as a DEC.
+        if (C->getAPIntValue().isAllOnesValue()) {
+          Opcode = X86ISD::DEC;
+          break;
+        }
+      }
       // Otherwise use a regular EFLAGS-setting add.
       Opcode = X86ISD::ADD;
       break;
@@ -5399,13 +5405,6 @@ SDValue X86TargetLowering::EmitTest(SDValue Op, SelectionDAG &DAG) {
            UE = Op.getNode()->use_end(); UI != UE; ++UI)
         if (UI->getOpcode() == ISD::STORE)
           goto default_case;
-      // A subtract of one will be selected as a DEC.
-      if (ConstantSDNode *C =
-            dyn_cast<ConstantSDNode>(Op.getNode()->getOperand(1)))
-        if (C->getAPIntValue() == 1) {
-          Opcode = X86ISD::DEC;
-          break;
-        }
       // Otherwise use a regular EFLAGS-setting sub.
       Opcode = X86ISD::SUB;
       break;
diff --git a/llvm/test/CodeGen/X86/peep-test.ll b/llvm/test/CodeGen/X86/peep-test-0.ll
index a95b5640443..a95b5640443 100644
--- a/llvm/test/CodeGen/X86/peep-test.ll
+++ b/llvm/test/CodeGen/X86/peep-test-0.ll
diff --git a/llvm/test/CodeGen/X86/peep-test-1.ll b/llvm/test/CodeGen/X86/peep-test-1.ll
new file mode 100644
index 00000000000..b4698e37b42
--- /dev/null
+++ b/llvm/test/CodeGen/X86/peep-test-1.ll
@@ -0,0 +1,23 @@
+; RUN: llvm-as < %s | llc -march=x86 > %t
+; RUN: grep dec %t | count 1
+; RUN: not grep test %t
+; RUN: not grep cmp %t
+
+define void @foo(i32 %n, double* nocapture %p) nounwind {
+	br label %bb
+
+bb:
+	%indvar = phi i32 [ 0, %0 ], [ %indvar.next, %bb ]
+	%i.03 = sub i32 %n, %indvar
+	%1 = getelementptr double* %p, i32 %i.03
+	%2 = load double* %1, align 4
+	%3 = mul double %2, 2.930000e+00
+	store double %3, double* %1, align 4
+	%4 = add i32 %i.03, -1
+	%phitmp = icmp slt i32 %4, 0
+	%indvar.next = add i32 %indvar, 1
+	br i1 %phitmp, label %bb, label %return
+
+return:
+	ret void
+}