[PATCH] D124374: [RISCV][TargetLowering] Special case overflow expansion for (uaddo X, C).

[Hsiangkai Wang via Phabricator via llvm-commits]

HsiangKai updated this revision to Diff 425941.
HsiangKai added a comment.

Address comments.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D124374/new/

https://reviews.llvm.org/D124374

Files:
  llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp


Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -6971,7 +6971,10 @@
     SDValue Overflow;
     if (IsAdd && isOneConstant(RHS)) {
       // Special case uaddo X, 1 overflowed if the addition result is 0.
-      // FIXME: We can do this for any constant RHS by using (X + C) < C.
+      // The general case (X + C) < C is not necessarily beneficial. Although we
+      // reduce the live range of X, we may introduce the materialization of
+      // constant C, especially when the setcc result is used by branch. We have
+      // no compare with constant and branch instructions.
       Overflow = DAG.getSetCC(DL, N->getValueType(1), Res,
                               DAG.getConstant(0, DL, MVT::i64), ISD::SETEQ);
     } else {
Index: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -8823,7 +8823,9 @@
   if (IsAdd && isOneConstant(RHS)) {
     // Special case: uaddo X, 1 overflowed if X+1 is 0. This potential reduces
     // the live range of X. We assume comparing with 0 is cheap.
-    // TODO: This generalizes to (X + C) < C.
+    // The general case (X + C) < C is not necessarily beneficial. Although we
+    // reduce the live range of X, we may introduce the materialization of
+    // constant C.
     SetCC =
         DAG.getSetCC(dl, SetCCType, Result,
                      DAG.getConstant(0, dl, Node->getValueType(0)), ISD::SETEQ);


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