[RISCV] Fix v[f]slide1down.vx having VL changed

[lukel97]

v[f]slide1down.vx uses VL to determine where the element is inserted into, so changing the VL changes the result.

This fixes this by setting ActiveElementsAffectsResult, but it's overly conservative. We should relax this later by modelling that it's ok to change the mask, just not VL.

Fixes #106109

[llvmbot]

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

Author: Luke Lau (lukel97)

Changes

v[f]slide1down.vx uses VL to determine where the element is inserted into, so changing the VL changes the result.

This fixes this by setting ActiveElementsAffectsResult, but it's overly conservative. We should relax this later by modelling that it's ok to change the mask, just not VL.

Fixes #106109

---

Full diff: https://github.com/llvm/llvm-project/pull/106110.diff

3 Files Affected:

• (modified) llvm/lib/Target/RISCV/RISCVInstrInfoV.td (+2)
• (added) llvm/test/CodeGen/RISCV/rvv/pr106109.ll (+16)
• (modified) llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll (+5-2)

diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfoV.td b/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
index a84e92b0fda262..878859fc1d0864 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
@@ -1665,6 +1665,7 @@ defm VSLIDEUP_V : VSLD_IV_X_I<"vslideup", 0b001110, /*slidesUp=*/true>;
 defm VSLIDE1UP_V : VSLD1_MV_X<"vslide1up", 0b001110>;
 } // Constraints = "@earlyclobber $vd", RVVConstraint = SlideUp
 defm VSLIDEDOWN_V : VSLD_IV_X_I<"vslidedown", 0b001111, /*slidesUp=*/false>;
+let ActiveElementsAffectResult = 1 in
 defm VSLIDE1DOWN_V : VSLD1_MV_X<"vslide1down", 0b001111>;
 } // Predicates = [HasVInstructions]
 
@@ -1672,6 +1673,7 @@ let Predicates = [HasVInstructionsAnyF] in {
 let Constraints = "@earlyclobber $vd", RVVConstraint = SlideUp in {
 defm VFSLIDE1UP_V : VSLD1_FV_F<"vfslide1up", 0b001110>;
 } // Constraints = "@earlyclobber $vd", RVVConstraint = SlideUp
+let ActiveElementsAffectResult = 1 in
 defm VFSLIDE1DOWN_V : VSLD1_FV_F<"vfslide1down", 0b001111>;
 } // Predicates = [HasVInstructionsAnyF]
 
diff --git a/llvm/test/CodeGen/RISCV/rvv/pr106109.ll b/llvm/test/CodeGen/RISCV/rvv/pr106109.ll
new file mode 100644
index 00000000000000..683753e19f2d5a
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/rvv/pr106109.ll
@@ -0,0 +1,16 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=riscv64 -mattr=+v -verify-machineinstrs | FileCheck %s
+
+define <vscale x 4 x float> @intrinsic_vfslide1down_vf_nxv4f32_nxv4f32_f32(<vscale x 4 x float> %0, <vscale x 4 x float> %false, float %1, <vscale x 4 x i1> %mask) {
+; CHECK-LABEL: intrinsic_vfslide1down_vf_nxv4f32_nxv4f32_f32:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    vsetivli zero, 4, e32, m2, ta, ma
+; CHECK-NEXT:    vfslide1down.vf v8, v8, fa0
+; CHECK-NEXT:    vsetivli zero, 1, e32, m2, ta, ma
+; CHECK-NEXT:    vmerge.vvm v8, v10, v8, v0
+; CHECK-NEXT:    ret
+entry:
+  %a = call <vscale x 4 x float> @llvm.riscv.vfslide1down.nxv4f32.f32(<vscale x 4 x float> undef, <vscale x 4 x float> %0, float %1, i64 4)
+  %b = call <vscale x 4 x float> @llvm.riscv.vmerge.nxv4f32(<vscale x 4 x float> undef, <vscale x 4 x float> %false, <vscale x 4 x float> %a, <vscale x 4 x i1> %mask, i64 1)
+  ret <vscale x 4 x float> %b
+}
diff --git a/llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll b/llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll
index 6700920cebff0a..2ff775d6d14f43 100644
--- a/llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-vops.ll
@@ -776,12 +776,15 @@ define <vscale x 2 x i32> @vpselect_vslide1up(<vscale x 2 x i32> %passthru, <vsc
   ret <vscale x 2 x i32> %b
 }
 
+; FIXME: We can still fold this given that the vmerge and the vslide1down have
+; the same vl.
 declare <vscale x 2 x i32> @llvm.riscv.vslide1down.nxv2i32.i32(<vscale x 2 x i32>, <vscale x 2 x i32>, i32, i64)
 define <vscale x 2 x i32> @vpselect_vslide1down(<vscale x 2 x i32> %passthru, <vscale x 2 x i32> %v, i32 %x, <vscale x 2 x i1> %m, i32 zeroext %vl) {
 ; CHECK-LABEL: vpselect_vslide1down:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    vsetvli zero, a1, e32, m1, ta, mu
-; CHECK-NEXT:    vslide1down.vx v8, v9, a0, v0.t
+; CHECK-NEXT:    vsetvli zero, a1, e32, m1, ta, ma
+; CHECK-NEXT:    vslide1down.vx v9, v9, a0
+; CHECK-NEXT:    vmerge.vvm v8, v8, v9, v0
 ; CHECK-NEXT:    ret
   %1 = zext i32 %vl to i64
   %a = call <vscale x 2 x i32> @llvm.riscv.vslide1down.nxv2i32.i32(<vscale x 2 x i32> undef, <vscale x 2 x i32> %v, i32 %x, i64 %1)

[topperc]

LGTM