[llvm] [RISCV] Expand constant multiplication for targets without M extension (PR #137195)

[Philip Reames via llvm-commits]

================
@@ -15531,20 +15631,23 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
   if (DAG.getMachineFunction().getFunction().hasMinSize())
     return SDValue();
 
-  if (DCI.isBeforeLegalize() || DCI.isCalledByLegalizer())
-    return SDValue();
-
   if (VT != Subtarget.getXLenVT())
     return SDValue();
 
-  const bool HasShlAdd =
-      Subtarget.hasStdExtZba() || Subtarget.hasVendorXTHeadBa();
-
   ConstantSDNode *CNode = dyn_cast<ConstantSDNode>(N->getOperand(1));
   if (!CNode)
     return SDValue();
   uint64_t MulAmt = CNode->getZExtValue();
 
+  if (!Subtarget.hasStdExtM() && !Subtarget.hasStdExtZmmul())
----------------
preames wrote:

Code structure wise, you want this to fall into the generic expansion code for the no-M case, and then only resort to expensive expansion if none of the existing rules work.

Consider something like the following:
bool ShouldExpandMul = (!DCI.isBeforeLegalize() && !DCI.isCalledByLegalizer()) || !Subtarget.hasStdExtZmmul().

Then put the call to expandMulToBasicOps under the Subtarget.hasStdExtZmmul() guard at the bottom.


https://github.com/llvm/llvm-project/pull/137195