[llvm] cce64e7 - [DAG] visitTRUNCATE - move GetDemandedBits AFTER SimplifyDemandedBits.

[Simon Pilgrim via llvm-commits]

Author: Simon Pilgrim
Date: 2022-07-04T11:25:40+01:00
New Revision: cce64e7a9cba91b31f072a906d4c85479e9eae9f

URL: https://github.com/llvm/llvm-project/commit/cce64e7a9cba91b31f072a906d4c85479e9eae9f
DIFF: https://github.com/llvm/llvm-project/commit/cce64e7a9cba91b31f072a906d4c85479e9eae9f.diff

LOG: [DAG] visitTRUNCATE - move GetDemandedBits AFTER SimplifyDemandedBits.

Another cleanup step before removing GetDemandedBits entirely.

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index aa688d9dda3c..ce570abef92e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13243,18 +13243,6 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
     }
   }
 
-  // See if we can simplify the input to this truncate through knowledge that
-  // only the low bits are being used.
-  // For example "trunc (or (shl x, 8), y)" // -> trunc y
-  // Currently we only perform this optimization on scalars because vectors
-  // may have 
diff erent active low bits.
-  if (!VT.isVector()) {
-    APInt Mask =
-        APInt::getLowBitsSet(N0.getValueSizeInBits(), VT.getSizeInBits());
-    if (SDValue Shorter = DAG.GetDemandedBits(N0, Mask))
-      return DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, Shorter);
-  }
-
   // fold (truncate (load x)) -> (smaller load x)
   // fold (truncate (srl (load x), c)) -> (smaller load (x+c/evtbits))
   if (!LegalTypes || TLI.isTypeDesirableForOp(N0.getOpcode(), VT)) {
@@ -13341,6 +13329,18 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
   if (SimplifyDemandedBits(SDValue(N, 0)))
     return SDValue(N, 0);
 
+  // See if we can simplify the input to this truncate through knowledge that
+  // only the low bits are being used.
+  // For example "trunc (or (shl x, 8), y)" // -> trunc y
+  // Currently we only perform this optimization on scalars because vectors
+  // may have 
diff erent active low bits.
+  if (!VT.isVector()) {
+    APInt Mask =
+        APInt::getLowBitsSet(N0.getValueSizeInBits(), VT.getSizeInBits());
+    if (SDValue Shorter = DAG.GetDemandedBits(N0, Mask))
+      return DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, Shorter);
+  }
+
   // fold (truncate (extract_subvector(ext x))) ->
   //      (extract_subvector x)
   // TODO: This can be generalized to cover cases where the truncate and extract