[AArch64] Use ccmn to compare negative immediates between -1 and -31 (#95825)

Because ccmn is like a + b, it works when using cmp but with values that
when negated are in the range of 1 through 31

Author: AZero13

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -3519,6 +3519,12 @@ static SDValue emitConditionalComparison(SDValue LHS, SDValue RHS,
       RHS = DAG.getNode(ISD::FP_EXTEND, DL, MVT::f32, RHS);
     }
     Opcode = AArch64ISD::FCCMP;
+  } else if (ConstantSDNode *Const = dyn_cast<ConstantSDNode>(RHS)) {
+    APInt Imm = Const->getAPIntValue();
+    if (Imm.isNegative() && Imm.sgt(-32)) {
+      Opcode = AArch64ISD::CCMN;
+      RHS = DAG.getConstant(Imm.abs(), DL, Const->getValueType(0));
+    }
   } else if (RHS.getOpcode() == ISD::SUB) {
     SDValue SubOp0 = RHS.getOperand(0);
     if (isNullConstant(SubOp0) && (CC == ISD::SETEQ || CC == ISD::SETNE)) {

llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp

@@ -4667,18 +4667,19 @@ MachineInstr *AArch64InstructionSelector::emitConditionalComparison(
     Register LHS, Register RHS, CmpInst::Predicate CC,
     AArch64CC::CondCode Predicate, AArch64CC::CondCode OutCC,
     MachineIRBuilder &MIB) const {
-  // TODO: emit CMN as an optimization.
   auto &MRI = *MIB.getMRI();
   LLT OpTy = MRI.getType(LHS);
   unsigned CCmpOpc;
   std::optional<ValueAndVReg> C;
   if (CmpInst::isIntPredicate(CC)) {
     assert(OpTy.getSizeInBits() == 32 || OpTy.getSizeInBits() == 64);
     C = getIConstantVRegValWithLookThrough(RHS, MRI);
-    if (C && C->Value.ult(32))
+    if (!C || C->Value.sgt(31) || C->Value.slt(-31))
+      CCmpOpc = OpTy.getSizeInBits() == 32 ? AArch64::CCMPWr : AArch64::CCMPXr;
+    else if (C->Value.ule(31))
       CCmpOpc = OpTy.getSizeInBits() == 32 ? AArch64::CCMPWi : AArch64::CCMPXi;
     else
-      CCmpOpc = OpTy.getSizeInBits() == 32 ? AArch64::CCMPWr : AArch64::CCMPXr;
+      CCmpOpc = OpTy.getSizeInBits() == 32 ? AArch64::CCMNWi : AArch64::CCMNXi;
   } else {
     assert(OpTy.getSizeInBits() == 16 || OpTy.getSizeInBits() == 32 ||
            OpTy.getSizeInBits() == 64);
@@ -4703,6 +4704,8 @@ MachineInstr *AArch64InstructionSelector::emitConditionalComparison(
       MIB.buildInstr(CCmpOpc, {}, {LHS});
   if (CCmpOpc == AArch64::CCMPWi || CCmpOpc == AArch64::CCMPXi)
     CCmp.addImm(C->Value.getZExtValue());
+  else if (CCmpOpc == AArch64::CCMNWi || CCmpOpc == AArch64::CCMNXi)
+    CCmp.addImm(C->Value.abs().getZExtValue());
   else
     CCmp.addReg(RHS);
   CCmp.addImm(NZCV).addImm(Predicate);

llvm/test/CodeGen/AArch64/arm64-ccmp.ll

@@ -706,10 +706,9 @@ define i32 @select_noccmp3(i32 %v0, i32 %v1, i32 %v2) {
 ; GISEL:       ; %bb.0:
 ; GISEL-NEXT:    mov w8, #99 ; =0x63
 ; GISEL-NEXT:    sub w9, w0, #45
-; GISEL-NEXT:    mov w10, #-23 ; =0xffffffe9
 ; GISEL-NEXT:    cmp w0, #77
 ; GISEL-NEXT:    ccmp w0, w8, #4, ne
-; GISEL-NEXT:    ccmp w9, w10, #2, eq
+; GISEL-NEXT:    ccmn w9, #23, #2, eq
 ; GISEL-NEXT:    ccmp w0, #14, #0, lo
 ; GISEL-NEXT:    csel w0, w1, w2, hs
 ; GISEL-NEXT:    ret

llvm/test/CodeGen/AArch64/cmp-chains.ll

@@ -242,5 +242,19 @@ define i32 @true_or3(i32 %0, i32 %1, i32 %2) {
   %9 = zext i1 %8 to i32
   ret i32 %9
 }
-;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
-; CHECK: {{.*}}
+
+; (b > -3 && a < c)
+define i32 @neg_range_int(i32 %a, i32 %b, i32 %c) {
+; CHECK-LABEL: neg_range_int:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    cmp w0, w2
+; CHECK-NEXT:    ccmn w1, #3, #4, lt
+; CHECK-NEXT:    csel w0, w1, w0, gt
+; CHECK-NEXT:    ret
+  %cmp = icmp sgt i32 %b, -3
+  %cmp1 = icmp slt i32 %a, %c
+  %or.cond = and i1 %cmp, %cmp1
+  %retval.0 = select i1 %or.cond, i32 %b, i32 %a
+  ret i32 %retval.0
+}
+