[AArch64] All bits of an exact right shift are demanded (#97448)

When building a vector which contains zero elements, the AArch64 ISel
replaces those elements with `undef`, if they are right shifted out.

However, these elements need to stay zero if the right shift is exact,
or otherwise we will be introducing undefined behavior.

Should allow #92528 to be
recommitted.

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -22142,6 +22142,10 @@ static SDValue performVectorShiftCombine(SDNode *N,
     if (DCI.DAG.ComputeNumSignBits(Op.getOperand(0)) > ShiftImm)
       return Op.getOperand(0);
 
+  // If the shift is exact, the shifted out bits matter.
+  if (N->getFlags().hasExact())
+    return SDValue();
+
   APInt ShiftedOutBits = APInt::getLowBitsSet(OpScalarSize, ShiftImm);
   APInt DemandedMask = ~ShiftedOutBits;
 

llvm/test/CodeGen/AArch64/shr-exact-demanded-bits.ll

@@ -0,0 +1,35 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s | FileCheck %s
+target triple = "aarch64-linux"
+
+define <2 x i32> @f(i8  %0, i8  %1) {
+; CHECK-LABEL: f:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    movi v0.2d, #0000000000000000
+; CHECK-NEXT:    mov v0.b[3], w0
+; CHECK-NEXT:    mov v0.b[7], w1
+; CHECK-NEXT:    sshr v0.2s, v0.2s, #24
+; CHECK-NEXT:    ret
+  %3 = insertelement <2 x i8> poison, i8 %0, i64 0
+  %4 = insertelement <2 x i8> %3, i8 %1, i64 1
+  %5 = shufflevector <2 x i8> %4, <2 x i8> <i8 0, i8 poison>, <8 x i32> <i32 2, i32 2, i32 2, i32 0, i32 2, i32 2, i32 2, i32 1>
+  %6 = bitcast <8 x i8> %5 to <2 x i32>
+  %7 = ashr exact <2 x i32> %6, <i32 24, i32 24>
+  ret <2 x i32> %7
+}
+
+define <2 x i32> @g(i8  %0, i8  %1) {
+; CHECK-LABEL: g:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    movi v0.2d, #0000000000000000
+; CHECK-NEXT:    mov v0.b[3], w0
+; CHECK-NEXT:    mov v0.b[7], w1
+; CHECK-NEXT:    ushr v0.2s, v0.2s, #24
+; CHECK-NEXT:    ret
+  %3 = insertelement <2 x i8> poison, i8 %0, i64 0
+  %4 = insertelement <2 x i8> %3, i8 %1, i64 1
+  %5 = shufflevector <2 x i8> %4, <2 x i8> <i8 0, i8 poison>, <8 x i32> <i32 2, i32 2, i32 2, i32 0, i32 2, i32 2, i32 2, i32 1>
+  %6 = bitcast <8 x i8> %5 to <2 x i32>
+  %7 = lshr exact <2 x i32> %6, <i32 24, i32 24>
+  ret <2 x i32> %7
+}