[RISCV] Add DAG combine for (vmv_s_x_vl (undef) (vmv_x_s X).

[topperc]

We can use the original vector as long as the type of X matches the result type of the vmv_s_x_vl.

[llvmbot]

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

Author: Craig Topper (topperc)

Changes

We can use the original vector as long as the type of X matches the result type of the vmv_s_x_vl.

---

Full diff: https://github.com/llvm/llvm-project/pull/90524.diff

2 Files Affected:

• (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+5)
• (modified) llvm/test/CodeGen/RISCV/rvv/fixed-vector-i8-index-cornercase.ll (+5-12)

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 68f4ec5ef49f31..65f2736d9e47c5 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -16791,6 +16791,11 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
     SDValue Scalar = N->getOperand(1);
     SDValue VL = N->getOperand(2);
 
+    if (Scalar.getOpcode() == RISCVISD::VMV_X_S &&
+        Scalar.getOperand(0).getValueType() == N->getValueType(0) &&
+        isa<ConstantSDNode>(N->getOperand(2)) && N->getOperand(0).isUndef())
+      return Scalar.getOperand(0);
+
     // Use M1 or smaller to avoid over constraining register allocation
     const MVT M1VT = getLMUL1VT(VT);
     if (M1VT.bitsLT(VT)) {
diff --git a/llvm/test/CodeGen/RISCV/rvv/fixed-vector-i8-index-cornercase.ll b/llvm/test/CodeGen/RISCV/rvv/fixed-vector-i8-index-cornercase.ll
index 2874db6debd740..875f4f239028b4 100644
--- a/llvm/test/CodeGen/RISCV/rvv/fixed-vector-i8-index-cornercase.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/fixed-vector-i8-index-cornercase.ll
@@ -26,17 +26,13 @@ define <512 x i8> @single_source(<512 x i8> %a) {
 ; CHECK-NEXT:    vmv.v.x v8, a1
 ; CHECK-NEXT:    vslide1down.vx v8, v8, a0
 ; CHECK-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; CHECK-NEXT:    vslidedown.vi v17, v16, 5
-; CHECK-NEXT:    vmv.x.s a0, v17
-; CHECK-NEXT:    vmv.s.x v24, a0
+; CHECK-NEXT:    vslidedown.vi v24, v16, 5
 ; CHECK-NEXT:    li a0, 432
 ; CHECK-NEXT:    li a1, 431
 ; CHECK-NEXT:    vsetvli zero, a0, e8, m8, tu, ma
 ; CHECK-NEXT:    vslideup.vx v8, v24, a1
 ; CHECK-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
 ; CHECK-NEXT:    vslidedown.vi v16, v16, 4
-; CHECK-NEXT:    vmv.x.s a0, v16
-; CHECK-NEXT:    vmv.s.x v16, a0
 ; CHECK-NEXT:    li a0, 466
 ; CHECK-NEXT:    li a1, 465
 ; CHECK-NEXT:    vsetvli zero, a0, e8, m8, tu, ma
@@ -109,20 +105,17 @@ define <512 x i8> @two_source(<512 x i8> %a, <512 x i8> %b) {
 ; CHECK-NEXT:    addi a1, sp, 512
 ; CHECK-NEXT:    vsetvli zero, a0, e8, m8, ta, ma
 ; CHECK-NEXT:    vse8.v v8, (a1)
+; CHECK-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
+; CHECK-NEXT:    vslidedown.vi v0, v24, 5
 ; CHECK-NEXT:    vmv.x.s a1, v24
+; CHECK-NEXT:    vsetvli zero, a0, e8, m8, ta, ma
 ; CHECK-NEXT:    vmv.v.x v8, a1
-; CHECK-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; CHECK-NEXT:    vslidedown.vi v25, v24, 5
-; CHECK-NEXT:    vmv.x.s a1, v25
-; CHECK-NEXT:    vmv.s.x v0, a1
 ; CHECK-NEXT:    li a1, 432
 ; CHECK-NEXT:    li a2, 431
 ; CHECK-NEXT:    vsetvli zero, a1, e8, m8, tu, ma
 ; CHECK-NEXT:    vslideup.vx v8, v0, a2
 ; CHECK-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
 ; CHECK-NEXT:    vslidedown.vi v24, v24, 4
-; CHECK-NEXT:    vmv.x.s a1, v24
-; CHECK-NEXT:    vmv.s.x v24, a1
 ; CHECK-NEXT:    li a1, 466
 ; CHECK-NEXT:    li a2, 465
 ; CHECK-NEXT:    vsetvli zero, a1, e8, m8, tu, ma
@@ -130,9 +123,9 @@ define <512 x i8> @two_source(<512 x i8> %a, <512 x i8> %b) {
 ; CHECK-NEXT:    vslideup.vx v8, v24, a2
 ; CHECK-NEXT:    vmv.s.x v24, a1
 ; CHECK-NEXT:    li a1, 478
+; CHECK-NEXT:    li a2, 477
 ; CHECK-NEXT:    vsetvli zero, a1, e8, m8, tu, ma
 ; CHECK-NEXT:    lbu a1, 1012(sp)
-; CHECK-NEXT:    li a2, 477
 ; CHECK-NEXT:    vslideup.vx v8, v24, a2
 ; CHECK-NEXT:    vmv.s.x v24, a1
 ; CHECK-NEXT:    li a1, 501

[lukel97]

Why do we need to check VL is constant?

[topperc]

We don't. I found this code in my downstream and didn't scrutinize it.

[lukel97]

Suggested change

	N->getOperand(0).isUndef())
	Passthru.isUndef())

[wangpc-pp]

LGTM.