[llvm] [AArch64][GlobalISel] Prefer to use Vector Truncate (PR #105692)

[Matt Arsenault via llvm-commits]

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@@ -3313,6 +3313,120 @@ static bool isConstValidTrue(const TargetLowering &TLI, unsigned ScalarSizeBits,
          isConstTrueVal(TLI, Cst, IsVector, IsFP);
 }
 
+// This combine tries to reduce the number of scalarised G_TRUNC instructions by
+// using vector truncates instead
+//
+// EXAMPLE:
+// %a(i32), %b(i32) = G_UNMERGE_VALUES %src(<2 x i32>)
+// %T_a(i16) = G_TRUNC %a(i32)
+// %T_b(i16) = G_TRUNC %b(i32)
+// %Undef(i16) = G_IMPLICIT_DEF(i16)
+// %dst(v4i16) = G_BUILD_VECTORS %T_a(i16), %T_b(i16), %Undef(i16), %Undef(i16)
+//
+// ===>
+// %Undef(<2 x i32>) = G_IMPLICIT_DEF(<2 x i32>)
+// %Mid(<4 x s32>) = G_CONCAT_VECTORS %src(<2 x i32>), %Undef(<2 x i32>)
+// %dst(<4 x s16>) = G_TRUNC %Mid(<4 x s32>)
+//
+// This will ALSO match:
+// %a(i32), %b(i32) = G_UNMERGE_VALUES %src(<2 x i32>)
+// %Undef(i32) = G_IMPLICIT_DEF(i32)
+// %dst(v4i32) = G_BUILD_VECTORS %T_a(i32), %T_b(i32), %Undef(i32), %Undef(i32)
+//
+// ===>
+// %Undef(<2 x i32>) = G_IMPLICIT_DEF(<2 x i32>)
+// %Mid(<4 x s32>) = G_CONCAT_VECTORS %src(<2 x i32>), %Undef(<2 x i32>)
+//
+bool CombinerHelper::matchUseVectorTruncate(MachineInstr &MI,
+                                            Register &MatchInfo) {
+  assert(MI.getOpcode() == TargetOpcode::G_BUILD_VECTOR &&
+         "Expected G_BUILD_VECTOR instruction\n");
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arsenm wrote:

better to `cast<GBuildVector>(MI)` than use a custom assertion 

https://github.com/llvm/llvm-project/pull/105692