[PATCH] D123796: [RISCV] Don't emit fractional VIDs with negative steps

[Fraser Cormack via Phabricator via llvm-commits]

This revision was landed with ongoing or failed builds.
This revision was automatically updated to reflect the committed changes.
Closed by commit rG3e678cb77264: [RISCV] Don't emit fractional VIDs with negative steps (authored by frasercrmck).

Changed prior to commit:
  https://reviews.llvm.org/D123796?vs=423559&id=424098#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123796/new/

https://reviews.llvm.org/D123796

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-buildvec.ll


Index: llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-buildvec.ll
===================================================================
--- llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-buildvec.ll
+++ llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-buildvec.ll
@@ -746,17 +746,17 @@
   ret <4 x i8> <i8 3, i8 3, i8 1, i8 0>
 }
 
-; FIXME: This is not a valid way to emit this vid sequence: shift-right for
-; division only works for non-negative numbers!
+; We match this as a VID sequence (-3 / 8) + 5 but choose not to introduce
+; division to compute it.
 define <16 x i8> @buildvec_not_vid_v16i8() {
 ; CHECK-LABEL: buildvec_not_vid_v16i8:
 ; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a0, 3
 ; CHECK-NEXT:    vsetivli zero, 16, e8, m1, ta, mu
-; CHECK-NEXT:    vid.v v8
-; CHECK-NEXT:    li a0, -3
-; CHECK-NEXT:    vmul.vx v8, v8, a0
-; CHECK-NEXT:    vsrl.vi v8, v8, 3
-; CHECK-NEXT:    vadd.vi v8, v8, 5
+; CHECK-NEXT:    vmv.s.x v9, a0
+; CHECK-NEXT:    vmv.v.i v8, 0
+; CHECK-NEXT:    vsetivli zero, 7, e8, m1, tu, mu
+; CHECK-NEXT:    vslideup.vi v8, v9, 6
 ; CHECK-NEXT:    ret
   ret <16 x i8> <i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 3, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 0, i8 0>
 }
Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -2186,7 +2186,8 @@
     // a single addi instruction.
     if (((StepOpcode == ISD::MUL && isInt<12>(SplatStepVal)) ||
          (StepOpcode == ISD::SHL && isUInt<5>(SplatStepVal))) &&
-        isPowerOf2_32(StepDenominator) && isInt<5>(Addend)) {
+        isPowerOf2_32(StepDenominator) &&
+        (SplatStepVal >= 0 || StepDenominator == 1) && isInt<5>(Addend)) {
       SDValue VID = DAG.getNode(RISCVISD::VID_VL, DL, ContainerVT, Mask, VL);
       // Convert right out of the scalable type so we can use standard ISD
       // nodes for the rest of the computation. If we used scalable types with


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