[llvm] [RISCV][GISEL] Legalize G_EXTRACT_SUBVECTOR (PR #109426)

[Craig Topper via llvm-commits]

================
@@ -0,0 +1,339 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=riscv32 -mattr=+v -run-pass=legalizer %s -o - | FileCheck %s --check-prefixes=CHECK,RV32
+# RUN: llc -mtriple=riscv64 -mattr=+v -run-pass=legalizer %s -o - | FileCheck %s --check-prefixes=CHECK,RV64
+
+# Special handling for i1-element vectors with non-zero index
+---
+name:            extract_subvector_nxv4i1
+legalized:       false
+tracksRegLiveness: true
+body:             |
+  bb.0.entry:
+    ; RV32-LABEL: name: extract_subvector_nxv4i1
+    ; RV32: [[DEF:%[0-9]+]]:_(<vscale x 4 x s1>) = G_IMPLICIT_DEF
+    ; RV32-NEXT: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; RV32-NEXT: [[SPLAT_VECTOR:%[0-9]+]]:_(<vscale x 4 x s8>) = G_SPLAT_VECTOR [[C]](s32)
+    ; RV32-NEXT: [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 1
+    ; RV32-NEXT: [[SPLAT_VECTOR1:%[0-9]+]]:_(<vscale x 4 x s8>) = G_SPLAT_VECTOR [[C1]](s32)
+    ; RV32-NEXT: [[SELECT:%[0-9]+]]:_(<vscale x 4 x s8>) = G_SELECT [[DEF]](<vscale x 4 x s1>), [[SPLAT_VECTOR1]], [[SPLAT_VECTOR]]
+    ; RV32-NEXT: [[READ_VLENB:%[0-9]+]]:_(s32) = G_READ_VLENB
+    ; RV32-NEXT: [[C2:%[0-9]+]]:_(s32) = G_CONSTANT i32 2
+    ; RV32-NEXT: [[LSHR:%[0-9]+]]:_(s32) = G_LSHR [[READ_VLENB]], [[C2]](s32)
+    ; RV32-NEXT: [[VMSET_VL:%[0-9]+]]:_(<vscale x 2 x s1>) = G_VMSET_VL $x0
+    ; RV32-NEXT: [[DEF1:%[0-9]+]]:_(<vscale x 4 x s8>) = G_IMPLICIT_DEF
+    ; RV32-NEXT: [[VSLIDEDOWN_VL:%[0-9]+]]:_(<vscale x 4 x s8>) = G_VSLIDEDOWN_VL [[DEF1]], [[SELECT]], [[LSHR]](s32), [[VMSET_VL]](<vscale x 2 x s1>), $x0, 3
+    ; RV32-NEXT: [[EXTRACT_SUBVECTOR:%[0-9]+]]:_(<vscale x 2 x s8>) = G_EXTRACT_SUBVECTOR [[VSLIDEDOWN_VL]](<vscale x 4 x s8>), 0
----------------
topperc wrote:

> But there are no *legal* G_EXTRACT_SUBVECTORS:
> 
> ```
> 
> getActionDefinitionsBuilder(G_EXTRACT_SUBVECTOR)
> 
>       .customIf(typeIsLegalBoolVec(0, BoolVecTys, ST))
> 
>       .customIf(typeIsLegalIntOrFPVec(0, IntOrFPVecTys, ST));
> 
> ```

That's defect in global isel. There is no way to say that a particular constant is legal.

https://github.com/llvm/llvm-project/pull/109426