[X86] Avoid emitting unnecessary test instructions.

This patch teaches the backend how to check for the 'NoSignedWrap' flag on
binary operations to improve the emission of 'test' instructions.

If the result of a binary operation is known not to overflow we know that
resetting the Overflow flag is unnecessary and so we can avoid emitting
the test instruction.

Patch by Marcello Maggioni.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210468 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index e11522db79e9e149dc79c09e2f2ac32622fe4d1d..df86064d282d50b4399044f7a848ffd2c4499d4d 100644 (file)
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -10074,10 +10074,27 @@ SDValue X86TargetLowering::EmitTest(SDValue Op, unsigned X86CC, SDLoc dl,
     break;
   case X86::COND_G: case X86::COND_GE:
   case X86::COND_L: case X86::COND_LE:
-  case X86::COND_O: case X86::COND_NO:
-    NeedOF = true;
+  case X86::COND_O: case X86::COND_NO: {
+    // Check if we really need to set the
+    // Overflow flag. If NoSignedWrap is present
+    // that is not actually needed.
+    switch (Op->getOpcode()) {
+    case ISD::ADD:
+    case ISD::SUB:
+    case ISD::MUL:
+    case ISD::SHL: {
+      const BinaryWithFlagsSDNode *BinNode =
+          cast<BinaryWithFlagsSDNode>(Op.getNode());
+      if (BinNode->hasNoSignedWrap())
+        break;
+    }
+    default:
+      NeedOF = true;
+      break;
+    }
     break;
   }
+  }
   // See if we can use the EFLAGS value from the operand instead of
   // doing a separate TEST. TEST always sets OF and CF to 0, so unless
   // we prove that the arithmetic won't overflow, we can't use OF or CF.

diff --git a/test/CodeGen/X86/2014-05-29-factorial.ll b/test/CodeGen/X86/2014-05-29-factorial.ll
new file mode 100644 (file)
index 0000000..70255bd
--- /dev/null
+++ b/test/CodeGen/X86/2014-05-29-factorial.ll
@@ -0,0 +1,24 @@
+; RUN: llc < %s -march=x86-64 | FileCheck %s
+; CHECK: decq %rdi
+; CHECK-NOT: testq %rdi, %rdi
+
+define i64 @fact2(i64 %x) {
+entry:
+  br label %while.body
+
+while.body:
+  %result.06 = phi i64 [ %mul, %while.body ], [ 1, %entry ]
+  %x.addr.05 = phi i64 [ %dec, %while.body ], [ %x, %entry ]
+  %mul = mul nsw i64 %result.06, %x.addr.05
+  %dec = add nsw i64 %x.addr.05, -1
+  %cmp = icmp sgt i64 %dec, 0
+  br i1 %cmp, label %while.body, label %while.end.loopexit
+
+while.end.loopexit:
+  %mul.lcssa = phi i64 [ %mul, %while.body ]
+  br label %while.end
+
+while.end:
+  %result.0.lcssa = phi i64 [ %mul.lcssa, %while.end.loopexit ]
+  ret i64 %result.0.lcssa
+}