[llvm] [RISCV] Widen i1 AnyOf reductions (PR #134898)

[Craig Topper via llvm-commits]

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@@ -103,6 +105,82 @@ bool RISCVCodeGenPrepare::visitAnd(BinaryOperator &BO) {
   return true;
 }
 
+// With EVL tail folding, an AnyOf reduction will generate an i1 vp.merge like
+// follows:
+//
+// loop:
+//   %phi = phi <vscale x 4 x i1> [ zeroinitializer, %entry ], [ %rec, %loop ]
+//   %cmp = icmp ...
+//   %or = or <vscale x 4 x i1> %phi, %cmp
+//   %rec = call <vscale x 4 x i1> @llvm.vp.merge(%mask, %or, %phi, %evl)
+//   ...
+// middle:
+//   %res = call i1 @llvm.vector.reduce.or(<vscale x 4 x i1> %rec)
+//
+// However RVV doesn't have any tail undisturbed mask instructions and so we
+// need a convoluted sequence of mask instructions to lower the i1 vp.merge: see
+// llvm/test/CodeGen/RISCV/rvv/vpmerge-sdnode.ll.
+//
+// To avoid that this widens the i1 vp.merge to an i8 vp.merge, which will
+// usually be folded into a masked vor.vv.
+//
+// loop:
+//   %phi = phi <vscale x 4 x i8> [ zeroinitializer, %entry ], [ %rec, %loop ]
+//   %cmp = icmp ...
+//   %zext = zext <vscale x 4 x i1> %cmp to <vscale x 4 x i8>
+//   %or = or <vscale x 4 x i8> %phi, %cmp
+//   %rec = call <vscale x 4 x i8> @llvm.vp.merge(%mask, %or, %phi, %evl)
+//   %trunc = trunc <vscale x 4 x i8> %rec to <vscale x 4 x i1>
+//   ...
+// middle:
+//   %res = call i1 @llvm.vector.reduce.or(<vscale x 4 x i1> %rec)
+//
+// The trunc will normally be sunk outside of the loop, but even if there are
+// users inside the loop it is still profitable.
+bool RISCVCodeGenPrepare::widenVPMerge(IntrinsicInst &II) {
+  if (!II.getType()->getScalarType()->isIntegerTy(1))
+    return false;
+
+  Value *Mask, *PhiV, *Cond, *EVL;
+
+  using namespace PatternMatch;
+  if (!match(&II,
+             m_Intrinsic<Intrinsic::vp_merge>(
+                 m_Value(Mask), m_OneUse(m_c_Or(m_Value(PhiV), m_Value(Cond))),
+                 m_Deferred(PhiV), m_Value(EVL))))
+    return false;
+
+  auto *Phi = dyn_cast<PHINode>(PhiV);
+  auto *Start = dyn_cast<Constant>(Phi->getIncomingValue(0));
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topperc wrote:

I think it is possible to have a phi with no operands so I'm not sure you can assume value 0 exists.

https://github.com/llvm/llvm-project/pull/134898