[llvm] [RISCV] Adjust LMUL if not used to avoid VL toggle (PR #69259)

[Philip Reames via llvm-commits]

================
@@ -1463,6 +1463,57 @@ static bool canMutatePriorConfig(const MachineInstr &PrevMI,
   return areCompatibleVTYPEs(PriorVType, VType, Used);
 }
 
+// If LMUL or the SEW/LMUL ratio aren't demanded and MI and NextMI have the same
+// AVL, then we can try and change MI's LMUL so that we can avoid setting VL in
+// NextMI, e.g:
+//
+// vsetivli  zero, 4, e32, m1, ta, ma
+// vsetivli  zero, 4, e16, mf4, ta, ma
+//
+// vsetivli  zero, 4, e32, mf2, ta, ma
+// vsetvli   zero, zero, e16, mf4, ta, ma
+//
+// If possible, returns the new VTYPE that should be used for MI.
+static std::optional<unsigned>
+canAdjustSEWLMULRatio(const MachineInstr &MI, const MachineInstr &NextMI,
+                      const DemandedFields &Used) {
+  if (Used.LMUL || Used.SEWLMULRatio)
+    return std::nullopt;
+  if (!NextMI.getOperand(0).isDead())
+    return std::nullopt;
+  // If we end up increasing the SEW/LMUL ratio, then we will decrease VLMAX,
+  // which means we might end up changing VL in the case that AVL > VLMAX. So
+  // bail if the exact VL value is needed.
+  //
+  // TODO: We could potentially relax this when we know we're increasing VLMAX.
+  if (Used.VLAny)
+    return std::nullopt;
+
+  // If NextMI is already zero, zero then bail. If MI is zero, zero then we
+  // won't be able to tell if it has the same AVL as NextMI, so also bail.
+  if (isVLPreservingConfig(MI) || isVLPreservingConfig(NextMI))
+    return std::nullopt;
+
+  VSETVLIInfo NextMIInfo = getInfoForVSETVLI(NextMI);
----------------
preames wrote:

I don't remember if it's safe to call getInfoForVSETVLI here on the post-rewrite MI.  However, the existence of isNonZeroAVL seems to indicate I thought it wasn't previously?

We should either write this in terms of the raw MI, or insert assertions sufficient to prove this is safe during insertion, and rewrite the existing post pass logic in terms of the VSETVLIInfo object.

https://github.com/llvm/llvm-project/pull/69259