[llvm] r210468 - [X86] Avoid emitting unnecessary test instructions.

[Andrea Di Biagio]

Author: adibiagio
Date: Mon Jun  9 07:34:50 2014
New Revision: 210468

URL: http://llvm.org/viewvc/llvm-project?rev=210468&view=rev
Log:
[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.


Added:
    llvm/trunk/test/CodeGen/X86/2014-05-29-factorial.ll
Modified:
    llvm/trunk/lib/Target/X86/X86ISelLowering.cpp

Modified: llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86ISelLowering.cpp?rev=210468&r1=210467&r2=210468&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86ISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86ISelLowering.cpp Mon Jun  9 07:34:50 2014
@@ -10074,10 +10074,27 @@ SDValue X86TargetLowering::EmitTest(SDVa
     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.

Added: llvm/trunk/test/CodeGen/X86/2014-05-29-factorial.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/2014-05-29-factorial.ll?rev=210468&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/X86/2014-05-29-factorial.ll (added)
+++ llvm/trunk/test/CodeGen/X86/2014-05-29-factorial.ll Mon Jun  9 07:34:50 2014
@@ -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
+}