[AArch64] canCreateUndefOrPoisonForTargetNode - AArch64ISD::VASHR\VLSHR\VSHL can't create undef/poison (#156445)

We can always fold freeze(VSHIFT(x,c)) -> VSHIFT(freeze(x),c) as VASHR\VLSHR\VSHL should always have an in-range constant shift amount.

Test coverage can be tricky, so I've hijacked some computeKnownBits/ComputeNumSignBits tests to show that value tracking can still analyze the shift node as the FREEZE will have been discarded by the canCreateUndefOrPoison/isGuaranteedNotToBeUndefOrPoison logic in getFreeze().

Author: RKSimon

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -31004,6 +31004,9 @@ bool AArch64TargetLowering::canCreateUndefOrPoisonForTargetNode(
   case AArch64ISD::MOVIshift:
   case AArch64ISD::MVNImsl:
   case AArch64ISD::MVNIshift:
+  case AArch64ISD::VASHR:
+  case AArch64ISD::VLSHR:
+  case AArch64ISD::VSHL:
     return false;
   }
   return TargetLowering::canCreateUndefOrPoisonForTargetNode(

llvm/test/CodeGen/AArch64/vector-compress.ll

@@ -12,16 +12,15 @@ define <4 x i32> @test_compress_v4i32(<4 x i32> %vec, <4 x i1> %mask) {
 ; CHECK-NEXT:    shl.4s v1, v1, #31
 ; CHECK-NEXT:    cmlt.4s v1, v1, #0
 ; CHECK-NEXT:    mov.s w9, v1[1]
-; CHECK-NEXT:    mov.s w10, v1[2]
 ; CHECK-NEXT:    fmov w11, s1
+; CHECK-NEXT:    mov.s w10, v1[2]
+; CHECK-NEXT:    and x12, x11, #0x1
 ; CHECK-NEXT:    bfi x8, x11, #2, #1
-; CHECK-NEXT:    and x11, x11, #0x1
-; CHECK-NEXT:    and x9, x9, #0x1
-; CHECK-NEXT:    and w10, w10, #0x1
-; CHECK-NEXT:    add x9, x11, x9
 ; CHECK-NEXT:    mov x11, sp
+; CHECK-NEXT:    and x9, x9, #0x1
+; CHECK-NEXT:    add x9, x12, x9
 ; CHECK-NEXT:    st1.s { v0 }[1], [x8]
-; CHECK-NEXT:    add w10, w9, w10
+; CHECK-NEXT:    sub w10, w9, w10
 ; CHECK-NEXT:    orr x9, x11, x9, lsl #2
 ; CHECK-NEXT:    bfi x11, x10, #2, #2
 ; CHECK-NEXT:    st1.s { v0 }[2], [x9]
@@ -93,7 +92,8 @@ define <2 x double> @test_compress_v2f64(<2 x double> %vec, <2 x i1> %mask) {
 ; CHECK-NEXT:    shl.2d v1, v1, #63
 ; CHECK-NEXT:    cmlt.2d v1, v1, #0
 ; CHECK-NEXT:    fmov x9, d1
-; CHECK-NEXT:    bfi x8, x9, #3, #1
+; CHECK-NEXT:    and x9, x9, #0x8
+; CHECK-NEXT:    orr x8, x8, x9
 ; CHECK-NEXT:    st1.d { v0 }[1], [x8]
 ; CHECK-NEXT:    ldr q0, [sp], #16
 ; CHECK-NEXT:    ret
@@ -420,16 +420,15 @@ define <3 x i32> @test_compress_narrow(<3 x i32> %vec, <3 x i1> %mask) {
 ; CHECK-NEXT:    shl.4s v1, v1, #31
 ; CHECK-NEXT:    cmlt.4s v1, v1, #0
 ; CHECK-NEXT:    mov.s w8, v1[1]
-; CHECK-NEXT:    mov.s w9, v1[2]
 ; CHECK-NEXT:    fmov w10, s1
+; CHECK-NEXT:    mov.s w9, v1[2]
+; CHECK-NEXT:    and x12, x10, #0x1
 ; CHECK-NEXT:    bfi x11, x10, #2, #1
-; CHECK-NEXT:    and x10, x10, #0x1
-; CHECK-NEXT:    and x8, x8, #0x1
-; CHECK-NEXT:    and w9, w9, #0x1
-; CHECK-NEXT:    add x8, x10, x8
 ; CHECK-NEXT:    mov x10, sp
+; CHECK-NEXT:    and x8, x8, #0x1
+; CHECK-NEXT:    add x8, x12, x8
 ; CHECK-NEXT:    st1.s { v0 }[1], [x11]
-; CHECK-NEXT:    add w9, w8, w9
+; CHECK-NEXT:    sub w9, w8, w9
 ; CHECK-NEXT:    orr x8, x10, x8, lsl #2
 ; CHECK-NEXT:    bfi x10, x9, #2, #2
 ; CHECK-NEXT:    st1.s { v0 }[2], [x8]

llvm/unittests/Target/AArch64/AArch64SelectionDAGTest.cpp

@@ -172,6 +172,9 @@ TEST_F(AArch64SelectionDAGTest, ComputeNumSignBits_VASHR) {
   auto VecA = DAG->getConstant(0xaa, Loc, VecVT);
   auto Op2 = DAG->getNode(AArch64ISD::VASHR, Loc, VecVT, VecA, Shift);
   EXPECT_EQ(DAG->ComputeNumSignBits(Op2), 5u);
+  // VASHR can't create undef/poison - FREEZE(VASHR(C1,C2)) -> VASHR(C1,C2).
+  auto Fr2 = DAG->getFreeze(Op2);
+  EXPECT_EQ(DAG->ComputeNumSignBits(Fr2), 5u);
 }
 
 TEST_F(AArch64SelectionDAGTest, SimplifyDemandedVectorElts_EXTRACT_SUBVECTOR) {
@@ -564,6 +567,11 @@ TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_VASHR) {
   Known = DAG->computeKnownBits(Op1);
   EXPECT_EQ(Known.Zero, APInt(8, 0x00));
   EXPECT_EQ(Known.One, APInt(8, 0xFF));
+
+  auto Fr1 = DAG->getFreeze(Op1);
+  Known = DAG->computeKnownBits(Fr1);
+  EXPECT_EQ(Known.Zero, APInt(8, 0x00));
+  EXPECT_EQ(Known.One, APInt(8, 0xFF));
 }
 
 // Piggy-backing on the AArch64 tests to verify SelectionDAG::computeKnownBits.
@@ -584,6 +592,11 @@ TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_VLSHR) {
   Known = DAG->computeKnownBits(Op1);
   EXPECT_EQ(Known.Zero, APInt(8, 0xFE));
   EXPECT_EQ(Known.One, APInt(8, 0x1));
+
+  auto Fr1 = DAG->getFreeze(Op1);
+  Known = DAG->computeKnownBits(Fr1);
+  EXPECT_EQ(Known.Zero, APInt(8, 0xFE));
+  EXPECT_EQ(Known.One, APInt(8, 0x1));
 }
 
 // Piggy-backing on the AArch64 tests to verify SelectionDAG::computeKnownBits.
@@ -604,6 +617,11 @@ TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_VSHL) {
   Known = DAG->computeKnownBits(Op1);
   EXPECT_EQ(Known.Zero, APInt(8, 0x7F));
   EXPECT_EQ(Known.One, APInt(8, 0x80));
+
+  auto Fr1 = DAG->getFreeze(Op1);
+  Known = DAG->computeKnownBits(Fr1);
+  EXPECT_EQ(Known.Zero, APInt(8, 0x7F));
+  EXPECT_EQ(Known.One, APInt(8, 0x80));
 }
 
 TEST_F(AArch64SelectionDAGTest, isSplatValue_Fixed_BUILD_VECTOR) {