[PATCH] D49966: [X86] Performing DAG pruning before selection of LEA instructions.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


================
Comment at: lib/Target/X86/X86ISelDAGToDAG.cpp:712
+  unsigned Opcode = N->getOpcode();
+  if (Opcode == ISD::SHL) {
+    SDNode *Shift = N->getOperand(1).getNode();
----------------
jbhateja wrote:
> lebedev.ri wrote:
> > craig.topper wrote:
> > > Why are we matching the pattern in reverse? Normally we would look for a sub followed by a shift. Why are we starting with a shift and looking backwards?
> > Early return?
> 
> Following DAG transformation is being done in this revision 
> 
>   %1 = SHL X , 2
>   %2 = SUB Y , %1
>          |
>          V
>   %1 = SUB 0 , X
>   %2 = SHL %1 , 2  -> A 
>   %3 = ADD Y , %2  -> B
> 
> We are trying to remap SHL + SUB pattern so that it can be consumed easily, transformation is currently not kicking in for any SUB patterns but only for case where SUB is a USER of SHL.
> 
But why can't you look for subtracts that have shl as an operand? That's how nearly every DAG pattern match is written. I'm trying to understand why this one is different. A call to use_begin is pretty unusual the DAG.


Repository:
  rL LLVM

https://reviews.llvm.org/D49966