[RISCV] Allow vslidedown.vx in isExtractSubvectorCheap for half VT case (#114886)

We have a special case where we allow the extract of the high half of a
vector and consider it cheap. However, we had previously required that
the type have no more than 32 elements for this to work. (Because
64/2=32, and the largest immediate for a vslidedown.vi is 31.)

This has the effect of pessimizing shuffle vector lowering for long
vectors - i.e. at SEW=e8, zvl128b, an m2 or m4 deinterleave can't be
matched because it gets scalarized during DAG construction and can't be
"profitably" rebuilt by DAG combine. Note that for RISCV, scalarization
via insert and extract is extremely expensive (i.e. two vslides per
element), so a slide + two half width shuffles is almost always a net
win. (i.e, this isn't really specific to vnsrl)

Separately, I want to look at the decision to scalarize at all, but it
seems worthwhile adjusting this while we're at it regardless.

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2209,20 +2209,12 @@ bool RISCVTargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
     return true;
 
   // Convervatively only handle extracting half of a vector.
+  // TODO: We can do arbitrary slidedowns, but for now only support extracting
+  // the upper half of a vector until we have more test coverage.
   // TODO: For sizes which aren't multiples of VLEN sizes, this may not be
   // a cheap extract.  However, this case is important in practice for
   // shuffled extracts of longer vectors.  How resolve?
-  if ((ResElts * 2) != SrcElts)
-    return false;
-
-  // Slide can support arbitrary index, but we only treat vslidedown.vi as
-  // cheap.
-  if (Index >= 32)
-    return false;
-
-  // TODO: We can do arbitrary slidedowns, but for now only support extracting
-  // the upper half of a vector until we have more test coverage.
-  return Index == 0 || Index == ResElts;
+  return (ResElts * 2) == SrcElts && (Index == 0 || Index == ResElts);
 }
 
 MVT RISCVTargetLowering::getRegisterTypeForCallingConv(LLVMContext &Context,

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-changes-length.ll

@@ -307,243 +307,14 @@ define <32 x i32> @v32i32_v4i32(<4 x i32>) {
   ret <32 x i32> %2
 }
 
-; TODO: This case should be a simple vnsrl, but gets scalarized instead
 define <32 x i8> @vnsrl_v32i8_v64i8(<64 x i8> %in) {
-; RV32-LABEL: vnsrl_v32i8_v64i8:
-; RV32:       # %bb.0:
-; RV32-NEXT:    addi sp, sp, -128
-; RV32-NEXT:    .cfi_def_cfa_offset 128
-; RV32-NEXT:    sw ra, 124(sp) # 4-byte Folded Spill
-; RV32-NEXT:    sw s0, 120(sp) # 4-byte Folded Spill
-; RV32-NEXT:    .cfi_offset ra, -4
-; RV32-NEXT:    .cfi_offset s0, -8
-; RV32-NEXT:    addi s0, sp, 128
-; RV32-NEXT:    .cfi_def_cfa s0, 0
-; RV32-NEXT:    andi sp, sp, -64
-; RV32-NEXT:    li a0, 64
-; RV32-NEXT:    mv a1, sp
-; RV32-NEXT:    vsetvli zero, a0, e8, m4, ta, ma
-; RV32-NEXT:    vse8.v v8, (a1)
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v10, v8, 1
-; RV32-NEXT:    vmv.x.s a0, v10
-; RV32-NEXT:    li a1, 32
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vmv.v.x v10, a0
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v12, v8, 3
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v12, v8, 5
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v12, v8, 7
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v12, v8, 9
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v12, v8, 11
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v12, v8, 13
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV32-NEXT:    vslidedown.vi v12, v8, 15
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v12, v8, 17
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v12, v8, 19
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v12, v8, 21
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v12, v8, 23
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v12, v8, 25
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v12, v8, 27
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v12, v8, 29
-; RV32-NEXT:    vmv.x.s a0, v12
-; RV32-NEXT:    vslide1down.vx v10, v10, a0
-; RV32-NEXT:    vslidedown.vi v8, v8, 31
-; RV32-NEXT:    vmv.x.s a0, v8
-; RV32-NEXT:    vslide1down.vx v8, v10, a0
-; RV32-NEXT:    lbu a0, 33(sp)
-; RV32-NEXT:    lbu a1, 35(sp)
-; RV32-NEXT:    lbu a2, 37(sp)
-; RV32-NEXT:    lbu a3, 39(sp)
-; RV32-NEXT:    vslide1down.vx v8, v8, a0
-; RV32-NEXT:    vslide1down.vx v8, v8, a1
-; RV32-NEXT:    vslide1down.vx v8, v8, a2
-; RV32-NEXT:    vslide1down.vx v8, v8, a3
-; RV32-NEXT:    lbu a0, 41(sp)
-; RV32-NEXT:    lbu a1, 43(sp)
-; RV32-NEXT:    lbu a2, 45(sp)
-; RV32-NEXT:    lbu a3, 47(sp)
-; RV32-NEXT:    vslide1down.vx v8, v8, a0
-; RV32-NEXT:    vslide1down.vx v8, v8, a1
-; RV32-NEXT:    vslide1down.vx v8, v8, a2
-; RV32-NEXT:    vslide1down.vx v8, v8, a3
-; RV32-NEXT:    lbu a0, 49(sp)
-; RV32-NEXT:    lbu a1, 51(sp)
-; RV32-NEXT:    lbu a2, 53(sp)
-; RV32-NEXT:    lbu a3, 55(sp)
-; RV32-NEXT:    vslide1down.vx v8, v8, a0
-; RV32-NEXT:    vslide1down.vx v8, v8, a1
-; RV32-NEXT:    vslide1down.vx v8, v8, a2
-; RV32-NEXT:    vslide1down.vx v8, v8, a3
-; RV32-NEXT:    lbu a0, 57(sp)
-; RV32-NEXT:    lbu a1, 59(sp)
-; RV32-NEXT:    lbu a2, 61(sp)
-; RV32-NEXT:    lbu a3, 63(sp)
-; RV32-NEXT:    vslide1down.vx v8, v8, a0
-; RV32-NEXT:    vslide1down.vx v8, v8, a1
-; RV32-NEXT:    vslide1down.vx v8, v8, a2
-; RV32-NEXT:    vslide1down.vx v8, v8, a3
-; RV32-NEXT:    addi sp, s0, -128
-; RV32-NEXT:    lw ra, 124(sp) # 4-byte Folded Reload
-; RV32-NEXT:    lw s0, 120(sp) # 4-byte Folded Reload
-; RV32-NEXT:    addi sp, sp, 128
-; RV32-NEXT:    ret
-;
-; RV64-LABEL: vnsrl_v32i8_v64i8:
-; RV64:       # %bb.0:
-; RV64-NEXT:    addi sp, sp, -128
-; RV64-NEXT:    .cfi_def_cfa_offset 128
-; RV64-NEXT:    sd ra, 120(sp) # 8-byte Folded Spill
-; RV64-NEXT:    sd s0, 112(sp) # 8-byte Folded Spill
-; RV64-NEXT:    .cfi_offset ra, -8
-; RV64-NEXT:    .cfi_offset s0, -16
-; RV64-NEXT:    addi s0, sp, 128
-; RV64-NEXT:    .cfi_def_cfa s0, 0
-; RV64-NEXT:    andi sp, sp, -64
-; RV64-NEXT:    li a0, 64
-; RV64-NEXT:    mv a1, sp
-; RV64-NEXT:    vsetvli zero, a0, e8, m4, ta, ma
-; RV64-NEXT:    vse8.v v8, (a1)
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v10, v8, 1
-; RV64-NEXT:    vmv.x.s a0, v10
-; RV64-NEXT:    li a1, 32
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vmv.v.x v10, a0
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v12, v8, 3
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v12, v8, 5
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v12, v8, 7
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v12, v8, 9
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v12, v8, 11
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v12, v8, 13
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
-; RV64-NEXT:    vslidedown.vi v12, v8, 15
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vsetvli zero, a1, e8, m2, ta, ma
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v12, v8, 17
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v12, v8, 19
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v12, v8, 21
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v12, v8, 23
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v12, v8, 25
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v12, v8, 27
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v12, v8, 29
-; RV64-NEXT:    vmv.x.s a0, v12
-; RV64-NEXT:    vslide1down.vx v10, v10, a0
-; RV64-NEXT:    vslidedown.vi v8, v8, 31
-; RV64-NEXT:    vmv.x.s a0, v8
-; RV64-NEXT:    vslide1down.vx v8, v10, a0
-; RV64-NEXT:    lbu a0, 33(sp)
-; RV64-NEXT:    lbu a1, 35(sp)
-; RV64-NEXT:    lbu a2, 37(sp)
-; RV64-NEXT:    lbu a3, 39(sp)
-; RV64-NEXT:    vslide1down.vx v8, v8, a0
-; RV64-NEXT:    vslide1down.vx v8, v8, a1
-; RV64-NEXT:    vslide1down.vx v8, v8, a2
-; RV64-NEXT:    vslide1down.vx v8, v8, a3
-; RV64-NEXT:    lbu a0, 41(sp)
-; RV64-NEXT:    lbu a1, 43(sp)
-; RV64-NEXT:    lbu a2, 45(sp)
-; RV64-NEXT:    lbu a3, 47(sp)
-; RV64-NEXT:    vslide1down.vx v8, v8, a0
-; RV64-NEXT:    vslide1down.vx v8, v8, a1
-; RV64-NEXT:    vslide1down.vx v8, v8, a2
-; RV64-NEXT:    vslide1down.vx v8, v8, a3
-; RV64-NEXT:    lbu a0, 49(sp)
-; RV64-NEXT:    lbu a1, 51(sp)
-; RV64-NEXT:    lbu a2, 53(sp)
-; RV64-NEXT:    lbu a3, 55(sp)
-; RV64-NEXT:    vslide1down.vx v8, v8, a0
-; RV64-NEXT:    vslide1down.vx v8, v8, a1
-; RV64-NEXT:    vslide1down.vx v8, v8, a2
-; RV64-NEXT:    vslide1down.vx v8, v8, a3
-; RV64-NEXT:    lbu a0, 57(sp)
-; RV64-NEXT:    lbu a1, 59(sp)
-; RV64-NEXT:    lbu a2, 61(sp)
-; RV64-NEXT:    lbu a3, 63(sp)
-; RV64-NEXT:    vslide1down.vx v8, v8, a0
-; RV64-NEXT:    vslide1down.vx v8, v8, a1
-; RV64-NEXT:    vslide1down.vx v8, v8, a2
-; RV64-NEXT:    vslide1down.vx v8, v8, a3
-; RV64-NEXT:    addi sp, s0, -128
-; RV64-NEXT:    ld ra, 120(sp) # 8-byte Folded Reload
-; RV64-NEXT:    ld s0, 112(sp) # 8-byte Folded Reload
-; RV64-NEXT:    addi sp, sp, 128
-; RV64-NEXT:    ret
+; CHECK-LABEL: vnsrl_v32i8_v64i8:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a0, 32
+; CHECK-NEXT:    vsetvli zero, a0, e8, m2, ta, ma
+; CHECK-NEXT:    vnsrl.wi v12, v8, 8
+; CHECK-NEXT:    vmv.v.v v8, v12
+; CHECK-NEXT:    ret
   %res = shufflevector <64 x i8> %in, <64 x i8> poison, <32 x i32> <i32 1, i32 3, i32 5, i32 7, i32 9, i32 11, i32 13, i32 15, i32 17, i32 19, i32 21, i32 23, i32 25, i32 27, i32 29, i32 31, i32 33, i32 35, i32 37, i32 39, i32 41, i32 43, i32 45, i32 47, i32 49, i32 51, i32 53, i32 55, i32 57, i32 59, i32 61, i32 63>
   ret <32 x i8> %res
 }