[GISEL] Fix bugs and clarify spec of G_EXTRACT_SUBVECTOR (#108848)

The implementation was missing the fact that `G_EXTRACT_SUBVECTOR`
destination and source vector can be different types.

Also fix a bug in the MIR builder for `G_EXTRACT_SUBVECTOR` to generate
the correct opcode.

Clarify the G_EXTRACT_SUBVECTOR specification.

1 files changed, 19 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/MachineVerifier.cpp b/llvm/lib/CodeGen/MachineVerifier.cpp
index 759201e..6eed73c 100644
--- a/llvm/lib/CodeGen/MachineVerifier.cpp
+++ b/llvm/lib/CodeGen/MachineVerifier.cpp
@@ -1778,10 +1778,25 @@ void MachineVerifier::verifyPreISelGenericInstruction(const MachineInstr *MI) {
       break;
     }
 
-    if (IndexOp.getImm() != 0 &&
-        SrcTy.getElementCount().getKnownMinValue() % IndexOp.getImm() != 0) {
-      report("Index must be a multiple of the source vector's minimum vector "
-             "length",
+    if (SrcTy.isScalable() != DstTy.isScalable()) {
+      report("Vector types must both be fixed or both be scalable", MI);
+      break;
+    }
+
+    uint64_t Idx = IndexOp.getImm();
+    uint64_t DstMinLen = DstTy.getElementCount().getKnownMinValue();
+    if (Idx % DstMinLen != 0) {
+      report("Index must be a multiple of the destination vector's minimum "
+             "vector length",
+             MI);
+      break;
+    }
+
+    uint64_t SrcMinLen = SrcTy.getElementCount().getKnownMinValue();
+    if (SrcTy.isScalable() == DstTy.isScalable() &&
+        (Idx >= SrcMinLen || Idx + DstMinLen > SrcMinLen)) {
+      report("Source type and index must not cause extract to overrun to the "
+             "destination type",
              MI);
       break;
     }