[llvm] [IR][CodeGen] Specify llvm.gcroot contents are an opaque zero-initialized blob (PR #222333)

[via llvm-commits]

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@@ -86,3 +86,31 @@ entry:
 
   ret void
 }
+
+; The root is an opaque blob, so a root that is neither a pointer nor of
+; pointer size is zero-initialized in full.
+define void @fat_and_array_roots() gc "shadow-stack" {
+; CHECK-LABEL: define void @fat_and_array_roots() gc "shadow-stack" {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    %A = alloca { ptr, i1 }, align 8
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 8 %A, i8 0, i64 16, i1 false)
+; CHECK-NEXT:    %B = alloca [4 x ptr], align 8
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 8 %B, i8 0, i64 32, i1 false)
+; CHECK-NEXT:    %C = alloca ptr, i32 7, align 8
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 8 %C, i8 0, i64 56, i1 false)
+; CHECK-NEXT:    call void @llvm.gcroot(ptr %A, ptr null)
+; CHECK-NEXT:    call void @llvm.gcroot(ptr %B, ptr null)
+; CHECK-NEXT:    call void @llvm.gcroot(ptr %C, ptr null)
+; CHECK-NEXT:    ret void
+; CHECK-NEXT:  }
+entry:
+  %A = alloca { ptr, i1 }
+  %B = alloca [4 x ptr]
+  %C = alloca ptr, i32 7
+
+  call void @llvm.gcroot(ptr %A, ptr null)
+  call void @llvm.gcroot(ptr %B, ptr null)
+  call void @llvm.gcroot(ptr %C, ptr null)
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isuckatcs wrote:

The main issue with this construct is that the GC has no way to figure out how the allocated object looks like. This is probably the main reason a metadata was required for non-pointer roots.

Just by seeing these roots, how would a GC know which addresses it should mark?

https://github.com/llvm/llvm-project/pull/222333