[AArch64] Improve the codegen for sdiv 2

Similar to X86, if X's size is BitWidth, then X sdiv 2 can be expressived as
```
 X += X >> (BitWidth - 1)
 X = X >> 1
```

Author: vfdff

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -17747,6 +17747,12 @@ AArch64TargetLowering::BuildSDIVPow2(SDNode *N, const APInt &Divisor,
       !(Divisor.isPowerOf2() || Divisor.isNegatedPowerOf2()))
     return SDValue();
 
+  // If the divisor is 2 or -2, the default expansion is better. It will add
+  // (N->getValueType(0) >> (BitWidth - 1)) to it before shifting right.
+  if (Divisor == 2 ||
+      Divisor == APInt(Divisor.getBitWidth(), -2, /*isSigned*/ true))
+    return SDValue();
+
   return TargetLowering::buildSDIVPow2WithCMov(N, Divisor, DAG, Created);
 }
 

llvm/test/CodeGen/AArch64/aarch64-bit-gen.ll

@@ -202,9 +202,8 @@ define <4 x i32> @test_bit_sink_operand(<4 x i32> %src, <4 x i32> %dst, <4 x i32
 ; CHECK-SD:       // %bb.0: // %entry
 ; CHECK-SD-NEXT:    sub sp, sp, #32
 ; CHECK-SD-NEXT:    .cfi_def_cfa_offset 32
-; CHECK-SD-NEXT:    cmp w0, #0
+; CHECK-SD-NEXT:    add w8, w0, w0, lsr #31
 ; CHECK-SD-NEXT:    mov w9, wzr
-; CHECK-SD-NEXT:    cinc w8, w0, lt
 ; CHECK-SD-NEXT:    asr w8, w8, #1
 ; CHECK-SD-NEXT:  .LBB11_1: // %do.body
 ; CHECK-SD-NEXT:    // =>This Inner Loop Header: Depth=1

llvm/test/CodeGen/AArch64/sdivpow2.ll

@@ -90,8 +90,7 @@ define i64 @test7(i64 %x) {
 define i64 @test8(i64 %x) {
 ; ISEL-LABEL: test8:
 ; ISEL:       // %bb.0:
-; ISEL-NEXT:    cmp x0, #0
-; ISEL-NEXT:    cinc x8, x0, lt
+; ISEL-NEXT:    add x8, x0, x0, lsr #63
 ; ISEL-NEXT:    asr x0, x8, #1
 ; ISEL-NEXT:    ret
 ;
@@ -110,10 +109,8 @@ define i32 @sdiv_int(i32 %begin, i32 %first) #0 {
 ; ISEL-LABEL: sdiv_int:
 ; ISEL:       // %bb.0:
 ; ISEL-NEXT:    sub w8, w0, w1
-; ISEL-NEXT:    add w9, w8, #1
-; ISEL-NEXT:    add w10, w8, #2
-; ISEL-NEXT:    cmp w9, #0
-; ISEL-NEXT:    csinc w8, w10, w8, lt
+; ISEL-NEXT:    add w8, w8, #1
+; ISEL-NEXT:    add w8, w8, w8, lsr #31
 ; ISEL-NEXT:    sub w0, w0, w8, asr #1
 ; ISEL-NEXT:    ret
 ;