[CodegenPrepare] Do not rematerialize gc.relocates across different basic blocks

[igor-laevsky]

Summary

In a case when we have an if condition and base relocate is inserted in a different branch than derived relocate we should check that base dominates derived relocation. Otherwise rematerialized object will be inserted in one branch while all the uses of it will be located in another.

However it seems that introducing dominator tree lookup for such cases is too expensive. I think it is better to simply abort transformation if there is a suspicion that base relocation doesn't dominate derived relocation.

[artagnon]

Pretty silly mistake on my part. Thanks for the clear tests.

lib/CodeGen/CodeGenPrepare.cpp

for (auto &ToReplace : Targets) {
 
     assert(ThisRelocate.getBasePtrIndex() == MasterRelocate.getBasePtrIndex() &&
            "Not relocating a derived object of the original base object");
     if (ThisRelocate.getBasePtrIndex() == ThisRelocate.getDerivedPtrIndex()) {
       // A duplicate relocate call. TODO: coalesce duplicates.
       continue;
     }
 
+    if (RelocatedBase->getParent() != ToReplace->getParent()) {
+      // Base and derived relocates are in different basic blocks.
+      // In this case transform is only valid when base dominates derived
+      // relocate. However it would be too expensive to check dominance
+      // for each such relocate, so we skip the whole transformation.
+      continue;
+    }
+
     Value *Base = ThisRelocate.getBasePtr();
     auto Derived = dyn_cast<GetElementPtrInst>(ThisRelocate.getDerivedPtr());
     if (!Derived || Derived->getPointerOperand() != Base)
       continue;
 
     SmallVector<Value *, 2> OffsetV;
     if (!getGEPSmallConstantIntOffsetV(Derived, OffsetV))
       continue;

test/Transforms/CodeGenPrepare/statepoint-relocate.ll

entry:
        %ptr2 = getelementptr i32, i32* %base, i32 12
        %tok = call i32 (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 0, i32* %base, i32* %ptr, i32* %ptr2)
        %ptr-new = call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok, i32 7, i32 8)
        %ptr2-new = call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok, i32 7, i32 9)
        %ret = load i32, i32* %ptr-new
        ret i32 %ret
 }
 
+define i32 @test_sor_basic_wrong_order(i32* %base) gc "statepoint-example" {
+; CHECK-LABEL: @test_sor_basic_wrong_order
+; Here we have base relocate inserted after derived. Make sure that we don't
+; produce uses of the relocated base pointer before it's definition.
+entry:
+       %ptr = getelementptr i32, i32* %base, i32 15
+       ; CHECK: getelementptr i32, i32* %base, i32 15
+       %tok = call i32 (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 0, i32* %base, i32* %ptr)
+       %ptr-new = call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok, i32 7, i32 8)
+       %base-new = call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok,  i32 7, i32 7)
+       ; CHECK: %base-new = call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok,  i32 7, i32 7)
+       ; CHECK-NEXT: getelementptr i32, i32* %base-new, i32 15
+       %ret = load i32, i32* %ptr-new
+       ret i32 %ret
+}
+
+define i32 @test_sor_noop_cross_bb(i1 %external-cond, i32* %base) gc "statepoint-example" {
+; CHECK-LABEL: @test_sor_noop_cross_bb
+; Here base relocate doesn't dominate derived relocate. Make sure that we don't
+; produce undefined use of the relocated base pointer.
+entry:
+       %ptr = getelementptr i32, i32* %base, i32 15
+       ; CHECK: getelementptr i32, i32* %base, i32 15
+       %tok = call i32 (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 0, i32* %base, i32* %ptr)
+       br i1 %external-cond, label %left, label %right
+
+left:
+       %ptr-new = call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok, i32 7, i32 8)
+       ; CHECK: call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok, i32 7, i32 8)
+       %ret-new = load i32, i32* %ptr-new
+       ret i32 %ret-new
+
+right:
+       %ptr-base = call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok, i32 7, i32 7)
+       ; CHECK: call i32* @llvm.experimental.gc.relocate.p0i32(i32 %tok, i32 7, i32 7)
+       %ret-base = load i32, i32* %ptr-base
+       ret i32 %ret-base
+}
+
 declare i32 @llvm.experimental.gc.statepoint.p0f_i1f(i64, i32, i1 ()*, i32, i32, ...)
 declare i32* @llvm.experimental.gc.relocate.p0i32(i32, i32, i32)
 declare [3 x i32]* @llvm.experimental.gc.relocate.p0a3i32(i32, i32, i32)