[DAGCombine] Invert vselect to make TrueValue is binop

[ChunyuLiao]

Before:
binop N0, (vselect Cond, IDC, FVal) --> vselect Cond, N0, (binop N0, FVal)

After for special case:
binop N0, (vselect Cond, IDC, FVal) --> vselect InvertCond, N0, (binop FVal, N0)
It is possible to better eliminate the vmerge instruction for RISCV.

I haven't seen any regression in other targets.

[llvmbot]

@llvm/pr-subscribers-llvm-selectiondag

@llvm/pr-subscribers-backend-risc-v

Author: Liao Chunyu (ChunyuLiao)

Changes

Before:
binop N0, (vselect Cond, IDC, FVal) --> vselect Cond, N0, (binop N0, FVal)

After for special case:
binop N0, (vselect Cond, IDC, FVal) --> vselect InvertCond, N0, (binop FVal, N0)
It is possible to better eliminate the vmerge instruction for RISCV.

I haven't seen any regression in other targets.

---

Full diff: https://github.com/llvm/llvm-project/pull/167499.diff

3 Files Affected:

• (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+9)
• (modified) llvm/test/CodeGen/RISCV/intrinsic-cttz-elts-vscale.ll (+14-17)
• (modified) llvm/test/CodeGen/RISCV/rvv/vadd-sdnode.ll (+15)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index df353c4d91b1a..12e5e2ba1f811 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2508,6 +2508,15 @@ static SDValue foldSelectWithIdentityConstant(SDNode *N, SelectionDAG &DAG,
                                                FVal)) {
     SDValue F0 = DAG.getFreeze(N0);
     SDValue NewBO = DAG.getNode(Opcode, SDLoc(N), VT, F0, FVal, N->getFlags());
+    // For RISCV prefer to N0 == FVal
+    if (Cond.getOpcode() == ISD::SETCC) {
+      EVT CVT = Cond->getValueType(0);
+      ISD::CondCode NotCC = ISD::getSetCCInverse(
+          cast<CondCodeSDNode>(Cond.getOperand(2))->get(), CVT);
+      SDValue NCond = DAG.getSetCC(SDLoc(N), CVT, Cond.getOperand(0),
+                                   Cond.getOperand(1), NotCC);
+      return DAG.getSelect(SDLoc(N), VT, NCond, NewBO, F0);
+    }
     return DAG.getSelect(SDLoc(N), VT, Cond, F0, NewBO);
   }
   // binop N0, (vselect Cond, TVal, IDC) --> vselect Cond, (binop N0, TVal), N0
diff --git a/llvm/test/CodeGen/RISCV/intrinsic-cttz-elts-vscale.ll b/llvm/test/CodeGen/RISCV/intrinsic-cttz-elts-vscale.ll
index a06c7505d543d..b258ad0f0bca0 100644
--- a/llvm/test/CodeGen/RISCV/intrinsic-cttz-elts-vscale.ll
+++ b/llvm/test/CodeGen/RISCV/intrinsic-cttz-elts-vscale.ll
@@ -11,14 +11,13 @@ define i32 @ctz_nxv4i32(<vscale x 4 x i32> %a) #0 {
 ; RV32-NEXT:    vsetvli a1, zero, e16, m1, ta, ma
 ; RV32-NEXT:    vid.v v10
 ; RV32-NEXT:    li a1, -1
-; RV32-NEXT:    vsetvli zero, zero, e32, m2, ta, ma
-; RV32-NEXT:    vmsne.vi v0, v8, 0
 ; RV32-NEXT:    srli a0, a0, 1
+; RV32-NEXT:    vmv.v.x v11, a0
+; RV32-NEXT:    vsetvli zero, zero, e32, m2, ta, ma
+; RV32-NEXT:    vmseq.vi v0, v8, 0
 ; RV32-NEXT:    vsetvli zero, zero, e16, m1, ta, ma
-; RV32-NEXT:    vmv.v.x v8, a0
-; RV32-NEXT:    vmadd.vx v10, a1, v8
-; RV32-NEXT:    vmv.v.i v8, 0
-; RV32-NEXT:    vmerge.vvm v8, v8, v10, v0
+; RV32-NEXT:    vmadd.vx v10, a1, v11
+; RV32-NEXT:    vmerge.vim v8, v10, 0, v0
 ; RV32-NEXT:    vredmaxu.vs v8, v8, v8
 ; RV32-NEXT:    vmv.x.s a1, v8
 ; RV32-NEXT:    sub a0, a0, a1
@@ -32,14 +31,13 @@ define i32 @ctz_nxv4i32(<vscale x 4 x i32> %a) #0 {
 ; RV64-NEXT:    vsetvli a1, zero, e16, m1, ta, ma
 ; RV64-NEXT:    vid.v v10
 ; RV64-NEXT:    li a1, -1
-; RV64-NEXT:    vsetvli zero, zero, e32, m2, ta, ma
-; RV64-NEXT:    vmsne.vi v0, v8, 0
 ; RV64-NEXT:    srli a0, a0, 1
+; RV64-NEXT:    vmv.v.x v11, a0
+; RV64-NEXT:    vsetvli zero, zero, e32, m2, ta, ma
+; RV64-NEXT:    vmseq.vi v0, v8, 0
 ; RV64-NEXT:    vsetvli zero, zero, e16, m1, ta, ma
-; RV64-NEXT:    vmv.v.x v8, a0
-; RV64-NEXT:    vmadd.vx v10, a1, v8
-; RV64-NEXT:    vmv.v.i v8, 0
-; RV64-NEXT:    vmerge.vvm v8, v8, v10, v0
+; RV64-NEXT:    vmadd.vx v10, a1, v11
+; RV64-NEXT:    vmerge.vim v8, v10, 0, v0
 ; RV64-NEXT:    vredmaxu.vs v8, v8, v8
 ; RV64-NEXT:    vmv.x.s a1, v8
 ; RV64-NEXT:    sub a0, a0, a1
@@ -113,13 +111,12 @@ define i64 @ctz_nxv8i1_no_range(<vscale x 8 x i16> %a) {
 ; RV64-NEXT:    vsetvli a1, zero, e64, m8, ta, ma
 ; RV64-NEXT:    vid.v v16
 ; RV64-NEXT:    li a1, -1
+; RV64-NEXT:    vmv.v.x v24, a0
 ; RV64-NEXT:    vsetvli zero, zero, e16, m2, ta, ma
-; RV64-NEXT:    vmsne.vi v0, v8, 0
+; RV64-NEXT:    vmseq.vi v0, v8, 0
 ; RV64-NEXT:    vsetvli zero, zero, e64, m8, ta, ma
-; RV64-NEXT:    vmv.v.x v8, a0
-; RV64-NEXT:    vmadd.vx v16, a1, v8
-; RV64-NEXT:    vmv.v.i v8, 0
-; RV64-NEXT:    vmerge.vvm v8, v8, v16, v0
+; RV64-NEXT:    vmadd.vx v16, a1, v24
+; RV64-NEXT:    vmerge.vim v8, v16, 0, v0
 ; RV64-NEXT:    vredmaxu.vs v8, v8, v8
 ; RV64-NEXT:    vmv.x.s a1, v8
 ; RV64-NEXT:    sub a0, a0, a1
diff --git a/llvm/test/CodeGen/RISCV/rvv/vadd-sdnode.ll b/llvm/test/CodeGen/RISCV/rvv/vadd-sdnode.ll
index a9a13147f5c9b..3292c064bb4e9 100644
--- a/llvm/test/CodeGen/RISCV/rvv/vadd-sdnode.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/vadd-sdnode.ll
@@ -814,6 +814,21 @@ define <vscale x 8 x i32> @vadd_vv_mask_nxv8i32(<vscale x 8 x i32> %va, <vscale
   ret <vscale x 8 x i32> %vc
 }
 
+define <vscale x 8 x i32> @vadd_vv_mask_nxv8i32_novmerge(<vscale x 8 x i32> %va, <vscale x 8 x i32> %vb,
+; CHECK-LABEL: vadd_vv_mask_nxv8i32_novmerge:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli a0, zero, e32, m4, ta, mu
+; CHECK-NEXT:    vmsne.vi v0, v8, 1
+; CHECK-NEXT:    vadd.vv v16, v16, v12, v0.t
+; CHECK-NEXT:    vmv.v.v v8, v16
+; CHECK-NEXT:    ret
+                                                         <vscale x 8 x i32> %vc) {
+  %mask = icmp eq <vscale x 8 x i32> %va, splat (i32 1)
+  %vs = select <vscale x 8 x i1> %mask, <vscale x 8 x i32> zeroinitializer , <vscale x 8 x i32> %vb
+  %vr = add nsw <vscale x 8 x i32> %vc, %vs
+  ret <vscale x 8 x i32> %vr
+}
+
 define <vscale x 8 x i32> @vadd_vx_mask_nxv8i32(<vscale x 8 x i32> %va, i32 signext %b, <vscale x 8 x i1> %mask) {
 ; CHECK-LABEL: vadd_vx_mask_nxv8i32:
 ; CHECK:       # %bb.0:

[RKSimon]

I suspect X86 AVX512 predicates might benefit from this as well - I can't recall if we have existing code to canonicalize this