[X86] Reduce i64 to i32 when upper 33 bits are known zeros for add

Author: omkar-mohanty

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -58070,8 +58070,28 @@ static SDValue combineAdd(SDNode *N, SelectionDAG &DAG,
   EVT VT = N->getValueType(0);
   SDValue Op0 = N->getOperand(0);
   SDValue Op1 = N->getOperand(1);
+  unsigned int Opcode = N->getOpcode();
   SDLoc DL(N);
 
+  // Use a 32-bit add+zext if upper 33 bits known zero.
+  if (VT == MVT::i64 && Subtarget.is64Bit()) {
+    APInt HiMask = APInt::getHighBitsSet(64, 33);
+    if (DAG.MaskedValueIsZero(Op0, HiMask) &&
+        DAG.MaskedValueIsZero(Op1, HiMask)) {
+      SDValue LHS = DAG.getNode(ISD::TRUNCATE, DL, MVT::i32, Op0);
+      SDValue RHS = DAG.getNode(ISD::TRUNCATE, DL, MVT::i32, Op1);
+      bool NSW = Op0->getFlags().hasNoSignedWrap();
+      bool NUW = Op0->getFlags().hasNoUnsignedWrap();
+      NSW = NSW & DAG.willNotOverflowAdd(true, LHS, RHS);
+      NUW = NUW & DAG.willNotOverflowAdd(false, LHS, RHS);
+      SDNodeFlags Flags;
+      Flags.setNoUnsignedWrap(NUW);
+      Flags.setNoSignedWrap(NSW);
+      SDValue Sum = DAG.getNode(Opcode, DL, MVT::i32, LHS, RHS, Flags);
+      return DAG.getNode(ISD::ZERO_EXTEND, DL, MVT::i64, Sum);
+    }
+  }
+
   if (SDValue Select = pushAddIntoCmovOfConsts(N, DL, DAG, Subtarget))
     return Select;
 

llvm/test/CodeGen/X86/reduce-i64-add.ll

@@ -0,0 +1,31 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i686-unknown-unknown | FileCheck %s --check-prefix=X86
+; RUN: llc < %s -mtriple=x86_64-linux -verify-machineinstrs | FileCheck %s --check-prefix=X64-LINUX
+; RUN: llc < %s -mtriple=x86_64-win32 | FileCheck %s --check-prefix=X64-WIN32
+
+define i64 @test1(i16 %x) nounwind {
+; X86-LABEL: test1:
+; X86:       # %bb.0: # %entry
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    addl $42, %eax
+; X86-NEXT:    movzwl %ax, %eax
+; X86-NEXT:    xorl %edx, %edx
+; X86-NEXT:    retl
+;
+; X64-LINUX-LABEL: test1:
+; X64-LINUX:       # %bb.0: # %entry
+; X64-LINUX-NEXT:    addl $42, %edi
+; X64-LINUX-NEXT:    movzwl %di, %eax
+; X64-LINUX-NEXT:    retq
+;
+; X64-WIN32-LABEL: test1:
+; X64-WIN32:       # %bb.0: # %entry
+; X64-WIN32-NEXT:    # kill: def $cx killed $cx def $ecx
+; X64-WIN32-NEXT:    addl $42, %ecx
+; X64-WIN32-NEXT:    movzwl %cx, %eax
+; X64-WIN32-NEXT:    retq
+entry:
+  %add = add nuw nsw i16 %x, 42
+  %conv = zext i16 %add to i64
+  ret i64 %conv
+}