[GlobalISel] Fix buildCopyFromRegs for split vectors (#77448)

Fixes #77055

Author: Pierre-vh

llvm/lib/CodeGen/GlobalISel/CallLowering.cpp

@@ -478,9 +478,43 @@ static void buildCopyFromRegs(MachineIRBuilder &B, ArrayRef<Register> OrigRegs,
   } else {
     // Vector was split, and elements promoted to a wider type.
     // FIXME: Should handle floating point promotions.
-    LLT BVType = LLT::fixed_vector(LLTy.getNumElements(), PartLLT);
-    auto BV = B.buildBuildVector(BVType, Regs);
-    B.buildTrunc(OrigRegs[0], BV);
+    unsigned NumElts = LLTy.getNumElements();
+    LLT BVType = LLT::fixed_vector(NumElts, PartLLT);
+
+    Register BuildVec;
+    if (NumElts == Regs.size())
+      BuildVec = B.buildBuildVector(BVType, Regs).getReg(0);
+    else {
+      // Vector elements are packed in the inputs.
+      // e.g. we have a <4 x s16> but 2 x s32 in regs.
+      assert(NumElts > Regs.size());
+      LLT SrcEltTy = MRI.getType(Regs[0]);
+
+      LLT OriginalEltTy = MRI.getType(OrigRegs[0]).getElementType();
+
+      // Input registers contain packed elements.
+      // Determine how many elements per reg.
+      assert((SrcEltTy.getSizeInBits() % OriginalEltTy.getSizeInBits()) == 0);
+      unsigned EltPerReg =
+          (SrcEltTy.getSizeInBits() / OriginalEltTy.getSizeInBits());
+
+      SmallVector<Register, 0> BVRegs;
+      BVRegs.reserve(Regs.size() * EltPerReg);
+      for (Register R : Regs) {
+        auto Unmerge = B.buildUnmerge(OriginalEltTy, R);
+        for (unsigned K = 0; K < EltPerReg; ++K)
+          BVRegs.push_back(B.buildAnyExt(PartLLT, Unmerge.getReg(K)).getReg(0));
+      }
+
+      // We may have some more elements in BVRegs, e.g. if we have 2 s32 pieces
+      // for a <3 x s16> vector. We should have less than EltPerReg extra items.
+      if (BVRegs.size() > NumElts) {
+        assert((BVRegs.size() - NumElts) < EltPerReg);
+        BVRegs.truncate(NumElts);
+      }
+      BuildVec = B.buildBuildVector(BVType, BVRegs).getReg(0);
+    }
+    B.buildTrunc(OrigRegs[0], BuildVec);
   }
 }