[llvm] [RISCV][llvm] Preliminary P extension codegen support (PR #162668)

[Brandon Wu via llvm-commits]

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@@ -4311,6 +4369,34 @@ static SDValue lowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG,
   MVT XLenVT = Subtarget.getXLenVT();
 
   SDLoc DL(Op);
+  // Handle P extension packed vector BUILD_VECTOR with PLI for splat constants
+  if (Subtarget.hasStdExtP() && EnablePExtCodeGen) {
+    bool IsPExtVector =
+        (VT == MVT::v2i16 || VT == MVT::v4i8) ||
+        (Subtarget.is64Bit() &&
+         (VT == MVT::v4i16 || VT == MVT::v8i8 || VT == MVT::v2i32));
+    if (IsPExtVector) {
+      if (SDValue SplatValue = cast<BuildVectorSDNode>(Op)->getSplatValue()) {
+        if (auto *C = dyn_cast<ConstantSDNode>(SplatValue)) {
+          int64_t SplatImm = C->getSExtValue();
+          bool IsValidImm = false;
+
+          // Check immediate range based on vector type
+          if (VT == MVT::v8i8 || VT == MVT::v4i8)
+            // PLI_B uses 8-bit unsigned immediate
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4vtomat wrote:

when I tried to input `add <4 x i8> splat (i8 255), zeroinitializer` to `llc`, it seems that somehow it has been canonicalized to `add <4 x i8> splat (i8 -1), zeroinitializer`, do you think we still need explicit conversion from `uint8` to `int8`?

https://github.com/llvm/llvm-project/pull/162668