Add a white list of known non-safepoint functions

These are functions we emit in codegen directly that are known to not contain any safepoint

Author: yuyichao

src/llvm-late-gc-lowering.cpp

@@ -729,6 +729,12 @@ State LateLowerGCFrame::LocalScan(Function &F) {
                 if (CI->canReturnTwice()) {
                     S.ReturnsTwice.push_back(CI);
                 }
+                if (auto callee = CI->getCalledFunction()) {
+                    // Known functions emitted in codegen that are not safepoints
+                    if (callee == pointer_from_objref_func || callee->getName() == "memcmp") {
+                        continue;
+                    }
+                }
                 int SafepointNumber = NoteSafepoint(S, BBS, CI);
                 BBS.HasSafepoint = true;
                 BBS.TopmostSafepoint = SafepointNumber;

test/codegen.jl

@@ -61,4 +61,35 @@ if opt_level > 0
     test_loads_no_call(get_llvm(core_sizeof, Tuple{Vector}), [Iptr, "i16"])
 end
 
-@test !contains(get_llvm(isequal, Tuple{Nullable{BigFloat}, Nullable{BigFloat}}), "%gcframe")
+# Make sure we will not elide the allocation
+@noinline create_ref1() = Ref(1)
+function pointer_not_safepoint()
+    a = create_ref1()
+    unsafe_store!(Ptr{Int}(pointer_from_objref(a)), 3)
+    return a[]
+end
+@test pointer_not_safepoint() == 3
+
+struct LargeStruct
+    x::NTuple{100,Int}
+    LargeStruct() = new()
+end
+
+const large_struct = LargeStruct()
+@noinline create_ref_struct() = Ref(large_struct)
+function compare_large_struct(a)
+    b = create_ref_struct()
+    if a[] === b[]
+        b[].x[1]
+    else
+        a[].x[2]
+    end
+end
+
+if opt_level > 0
+    @test !contains(get_llvm(isequal, Tuple{Nullable{BigFloat}, Nullable{BigFloat}}), "%gcframe")
+    @test !contains(get_llvm(pointer_not_safepoint, Tuple{}), "%gcframe")
+    compare_large_struct_ir = get_llvm(compare_large_struct, Tuple{typeof(create_ref_struct())})
+    @test contains(compare_large_struct_ir, "call i32 @memcmp")
+    @test !contains(compare_large_struct_ir, "%gcframe")
+end