[RS4GC] Support vectors of pointers with scalar base pointer

[DaniilSuchkov]

Summary

This patch enables generation of relocations for pointers extracted from vectors when vector of pointers have scalar base pointer.

[reames]

I'm uncomfortable with this patch. The placement in this helper function leaves open the possibility that other callers of findBDVorBase encounter an incorrectly typed result. I'm also concerned that we might end up with a scalar-base, vector-derived pair ending up in the stack map, and I don't think the downstream code knows how to deal with that.

A principled fix would be to integrate getelementptr into the base rewriting logic analogously to extractelement.

In the meantime, I've posted an alternate workaround in https://reviews.llvm.org/D57138. What do you think of that?

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

1023

I think what you actually want here is:
if (Base->getType()->isVectorTy() == Input->getType()->isVectorTy())

i.e. bitcast either scalars, or between vector types.

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

auto getBaseForInput = [&](Value *Input, Instruction *InsertPt) {
     if (isKnownBaseResult(BDV)) {
       Base = BDV;
     } else {
       // Either conflict or base.
       assert(States.count(BDV));
       Base = States[BDV].getBaseValue();
     }
     assert(Base && "Can't be null");
-    // The cast is needed since base traversal may strip away bitcasts
-    if (Base->getType() != Input->getType() && InsertPt)
+    if (Base->getType() != Input->getType() && InsertPt) {
+      if (!Base->getType()->isVectorTy() && !Input->getType()->isVectorTy())
+        // The cast is needed since base traversal may strip away bitcasts.
         Base = new BitCastInst(Base, Input->getType(), "cast", InsertPt);
+      else {
+        // Handle the case when derived pointer is vector and base is scalar.
+        if (!Base->getType()->isVectorTy() && Input->getType()->isVectorTy()) {
+          if (Base->getType() != Input->getType()->getScalarType())
+            Base = new BitCastInst(Base, Input->getType()->getScalarType(),
+                                   "cast", InsertPt);
+          IRBuilder<> Builder(InsertPt);
+          Base = Builder.CreateVectorSplat(
+              cast<VectorType>(Input->getType())->getNumElements(), Base, "base");
+        } else
+          llvm_unreachable("unimplemented case");
+      }
+    }
     return Base;
   };
 
   // Fixup all the inputs of the new PHIs.  Visit order needs to be
   // deterministic and predictable because we're naming newly created
   // instructions.
   for (auto Pair : States) {
     Instruction *BDV = cast<Instruction>(Pair.first);

test/Transforms/RewriteStatepointsForGC/scalar-base-vector.ll

+; RUN: opt < %s -rewrite-statepoints-for-gc -S | FileCheck  %s
+; RUN: opt < %s -passes=rewrite-statepoints-for-gc -S | FileCheck  %s
+
+declare void @do_safepoint()
+declare i8 addrspace(1)* @def_ptr()
+
+define i32 addrspace(1)* @test1(i8 addrspace(1)* %base1, <2 x i64> %offsets) gc "statepoint-example" {
+entry:
+  br i1 undef, label %first, label %second
+
+first:
+  %base2 = call i8 addrspace(1)* @def_ptr() [ "deopt"(i32 0, i32 -1, i32 0, i32 0, i32 0) ]
+  br label %second
+
+second:
+; CHECK-LABEL: @test1(
+; CHECK: gc.statepoint
+; CHECK-DAG: (%ptr.base, %ptr)
+; CHECK-DAG: (%ptr.base, %ptr.base)
+  %phi = phi i8 addrspace(1)* [ %base1, %entry ], [ %base2, %first ]
+  %base.i32 = bitcast i8 addrspace(1)* %phi to i32 addrspace(1)*
+  %vec = getelementptr i32, i32 addrspace(1)* %base.i32, <2 x i64> %offsets
+  %ptr = extractelement <2 x i32 addrspace(1)*> %vec, i32 1
+  call void @do_safepoint() [ "deopt"(i32 0, i32 -1, i32 0, i32 0, i32 0) ]
+  ret i32 addrspace(1)* %ptr
+}