[llvm] eaaa31f - [RISCV][TargetLowering] Special case overflow expansion for (uaddo X, C).

[Hsiangkai Wang via llvm-commits]

Author: Hsiangkai Wang
Date: 2022-05-03T03:51:36Z
New Revision: eaaa31ff2c2bc218802778a19fefa9d4256d9745

URL: https://github.com/llvm/llvm-project/commit/eaaa31ff2c2bc218802778a19fefa9d4256d9745
DIFF: https://github.com/llvm/llvm-project/commit/eaaa31ff2c2bc218802778a19fefa9d4256d9745.diff

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

Follow-up to D122933.

Differential Revision: https://reviews.llvm.org/D124374

Added: 
    

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

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 9732a17a247a0..b209aecfbfa06 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -9044,7 +9044,9 @@ void TargetLowering::expandUADDSUBO(
   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);

diff  --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 4cb3188b5160a..ff63b22766d1b 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -6918,7 +6918,10 @@ void RISCVTargetLowering::ReplaceNodeResults(SDNode *N,
     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 {