[llvm] [RISCV] Reorganize select lowering to pull binop expansion early (PR #156974)
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 4 15:08:19 PDT 2025
https://github.com/preames created https://github.com/llvm/llvm-project/pull/156974
This is purely stylistic, but I think makes the code easier to follow.
It isn't quite NFC because it undoes the arithmetic lowering for the select c, simm12, 0 cases for a processor with both conditional move forwarding and zicond.
>From d757cc5a3b19732791123963ab4bdb8fcf03aba5 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Thu, 4 Sep 2025 14:59:47 -0700
Subject: [PATCH 1/2] [RISCV] Reorganize select lowering to pull binop
expansion early
This is purely stylistic, but I think makes the code easier to follow.
It isn't quite NFC because it undoes the airthmetic lowering for the
select c, simm12, 0 cases for a processor with both conditional
move forwarding and zicond.
---
llvm/lib/Target/RISCV/RISCVISelLowering.cpp | 43 +++++++--------------
llvm/test/CodeGen/RISCV/cmov-branch-opt.ll | 10 ++---
2 files changed, 19 insertions(+), 34 deletions(-)
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index f8ec1be1fd8d6..5012966592a98 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -9106,8 +9106,12 @@ static std::optional<bool> matchSetCC(SDValue LHS, SDValue RHS,
return std::nullopt;
}
-static SDValue combineSelectToBinOp(SDNode *N, SelectionDAG &DAG,
- const RISCVSubtarget &Subtarget) {
+static bool isSimm12Constant(SDValue V) {
+ return isa<ConstantSDNode>(V) && V->getAsAPIntVal().isSignedIntN(12);
+}
+
+static SDValue lowerSelectToBinOp(SDNode *N, SelectionDAG &DAG,
+ const RISCVSubtarget &Subtarget) {
SDValue CondV = N->getOperand(0);
SDValue TrueV = N->getOperand(1);
SDValue FalseV = N->getOperand(2);
@@ -9127,14 +9131,17 @@ static SDValue combineSelectToBinOp(SDNode *N, SelectionDAG &DAG,
return DAG.getNode(ISD::OR, DL, VT, Neg, DAG.getFreeze(TrueV));
}
+ const bool HasCZero = VT.isScalarInteger() &&
+ (Subtarget.hasStdExtZicond() || Subtarget.hasVendorXVentanaCondOps());
+
// (select c, 0, y) -> (c-1) & y
- if (isNullConstant(TrueV)) {
+ if (isNullConstant(TrueV) && (!HasCZero || isSimm12Constant(FalseV))) {
SDValue Neg = DAG.getNode(ISD::ADD, DL, VT, CondV,
DAG.getAllOnesConstant(DL, VT));
return DAG.getNode(ISD::AND, DL, VT, Neg, DAG.getFreeze(FalseV));
}
// (select c, y, 0) -> -c & y
- if (isNullConstant(FalseV)) {
+ if (isNullConstant(FalseV) && (!HasCZero || isSimm12Constant(TrueV))) {
SDValue Neg = DAG.getNegative(CondV, DL, VT);
return DAG.getNode(ISD::AND, DL, VT, Neg, DAG.getFreeze(TrueV));
}
@@ -9240,10 +9247,6 @@ foldBinOpIntoSelectIfProfitable(SDNode *BO, SelectionDAG &DAG,
return DAG.getSelect(DL, VT, Sel.getOperand(0), NewT, NewF);
}
-static bool isSimm12Constant(SDValue V) {
- return isa<ConstantSDNode>(V) && V->getAsAPIntVal().isSignedIntN(12);
-}
-
SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
SDValue CondV = Op.getOperand(0);
SDValue TrueV = Op.getOperand(1);
@@ -9259,6 +9262,10 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
return DAG.getNode(ISD::VSELECT, DL, VT, CondSplat, TrueV, FalseV);
}
+ // Try some other optimizations before falling back to generic lowering.
+ if (SDValue V = lowerSelectToBinOp(Op.getNode(), DAG, Subtarget))
+ return V;
+
// When Zicond or XVentanaCondOps is present, emit CZERO_EQZ and CZERO_NEZ
// nodes to implement the SELECT. Performing the lowering here allows for
// greater control over when CZERO_{EQZ/NEZ} are used vs another branchless
@@ -9266,19 +9273,6 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
if ((Subtarget.hasStdExtZicond() || Subtarget.hasVendorXVentanaCondOps()) &&
VT.isScalarInteger()) {
- // select c, simm12, 0 -> andi (sub x0, c), simm12
- if (isSimm12Constant(TrueV) && isNullConstant(FalseV)) {
- SDValue Mask = DAG.getNegative(CondV, DL, VT);
- return DAG.getNode(ISD::AND, DL, VT, TrueV, Mask);
- }
-
- // select c, 0, simm12 -> andi (addi c, -1), simm12
- if (isNullConstant(TrueV) && isSimm12Constant(FalseV)) {
- SDValue Mask = DAG.getNode(ISD::ADD, DL, VT, CondV,
- DAG.getSignedConstant(-1, DL, XLenVT));
- return DAG.getNode(ISD::AND, DL, VT, FalseV, Mask);
- }
-
// (select c, t, 0) -> (czero_eqz t, c)
if (isNullConstant(FalseV))
return DAG.getNode(RISCVISD::CZERO_EQZ, DL, VT, TrueV, CondV);
@@ -9332,10 +9326,6 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
DAG.getNode(RISCVISD::CZERO_EQZ, DL, VT, TrueV, CondV));
}
- // Try some other optimizations before falling back to generic lowering.
- if (SDValue V = combineSelectToBinOp(Op.getNode(), DAG, Subtarget))
- return V;
-
// (select c, c1, c2) -> (add (czero_nez c2 - c1, c), c1)
// (select c, c1, c2) -> (add (czero_eqz c1 - c2, c), c2)
if (isa<ConstantSDNode>(TrueV) && isa<ConstantSDNode>(FalseV)) {
@@ -9438,9 +9428,6 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
SDNodeFlags::Disjoint);
}
- if (SDValue V = combineSelectToBinOp(Op.getNode(), DAG, Subtarget))
- return V;
-
if (Op.hasOneUse()) {
unsigned UseOpc = Op->user_begin()->getOpcode();
if (isBinOp(UseOpc) && DAG.isSafeToSpeculativelyExecute(UseOpc)) {
diff --git a/llvm/test/CodeGen/RISCV/cmov-branch-opt.ll b/llvm/test/CodeGen/RISCV/cmov-branch-opt.ll
index 351b02494ae85..6608874286e34 100644
--- a/llvm/test/CodeGen/RISCV/cmov-branch-opt.ll
+++ b/llvm/test/CodeGen/RISCV/cmov-branch-opt.ll
@@ -149,9 +149,8 @@ define signext i32 @test4(i32 signext %x, i32 signext %y, i32 signext %z) {
;
; CMOV-ZICOND-LABEL: test4:
; CMOV-ZICOND: # %bb.0:
-; CMOV-ZICOND-NEXT: snez a0, a2
-; CMOV-ZICOND-NEXT: addi a0, a0, -1
-; CMOV-ZICOND-NEXT: andi a0, a0, 3
+; CMOV-ZICOND-NEXT: li a0, 3
+; CMOV-ZICOND-NEXT: czero.nez a0, a0, a2
; CMOV-ZICOND-NEXT: ret
;
; SFB-NOZICOND-LABEL: test4:
@@ -165,9 +164,8 @@ define signext i32 @test4(i32 signext %x, i32 signext %y, i32 signext %z) {
;
; SFB-ZICOND-LABEL: test4:
; SFB-ZICOND: # %bb.0:
-; SFB-ZICOND-NEXT: snez a0, a2
-; SFB-ZICOND-NEXT: addi a0, a0, -1
-; SFB-ZICOND-NEXT: andi a0, a0, 3
+; SFB-ZICOND-NEXT: li a0, 3
+; SFB-ZICOND-NEXT: czero.nez a0, a0, a2
; SFB-ZICOND-NEXT: ret
%c = icmp eq i32 %z, 0
%a = select i1 %c, i32 3, i32 0
>From b00a6ac10bcaa61849469130c9acd17f9c983d5c Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Thu, 4 Sep 2025 15:02:23 -0700
Subject: [PATCH 2/2] clang-format
---
llvm/lib/Target/RISCV/RISCVISelLowering.cpp | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 5012966592a98..d26f5ab8f3e88 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -9131,13 +9131,14 @@ static SDValue lowerSelectToBinOp(SDNode *N, SelectionDAG &DAG,
return DAG.getNode(ISD::OR, DL, VT, Neg, DAG.getFreeze(TrueV));
}
- const bool HasCZero = VT.isScalarInteger() &&
- (Subtarget.hasStdExtZicond() || Subtarget.hasVendorXVentanaCondOps());
+ const bool HasCZero =
+ VT.isScalarInteger() &&
+ (Subtarget.hasStdExtZicond() || Subtarget.hasVendorXVentanaCondOps());
// (select c, 0, y) -> (c-1) & y
if (isNullConstant(TrueV) && (!HasCZero || isSimm12Constant(FalseV))) {
- SDValue Neg = DAG.getNode(ISD::ADD, DL, VT, CondV,
- DAG.getAllOnesConstant(DL, VT));
+ SDValue Neg =
+ DAG.getNode(ISD::ADD, DL, VT, CondV, DAG.getAllOnesConstant(DL, VT));
return DAG.getNode(ISD::AND, DL, VT, Neg, DAG.getFreeze(FalseV));
}
// (select c, y, 0) -> -c & y
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