Add codegen test for BTF relocations

Test the `btf_field_exists`, `btf_field_byte_offset` and
`btf_field_byte_size` builtins and make sure they emit correct LLVM
intrinsic calls.

Author: vadorovsky

tests/codegen-llvm/btf-field-info-minicore.rs

@@ -0,0 +1,103 @@
+//@ add-minicore
+//@ needs-llvm-components: bpf
+//@ compile-flags: --target bpfel-unknown-none -Cdebuginfo=2
+
+#![feature(allow_internal_unstable, btf_relocations, decl_macro, no_core)]
+#![no_core]
+#![no_std]
+#![no_main]
+
+extern crate minicore;
+use minicore::*;
+
+#[allow_internal_unstable(builtin_syntax)]
+pub macro field_byte_offset($Container:ty, $($fields:expr)+ $(,)?) {{
+    if builtin # btf_field_exists($Container, $($fields)+) {
+        ::minicore::Option::Some(builtin # btf_field_byte_offset($Container, $($fields)+))
+    } else {
+        ::minicore::Option::None
+    }
+}}
+
+#[allow_internal_unstable(builtin_syntax)]
+pub macro field_byte_size($Container:ty, $($fields:expr)+ $(,)?) {{
+    if builtin # btf_field_exists($Container, $($fields)+) {
+        ::minicore::Option::Some(builtin # btf_field_byte_size($Container, $($fields)+))
+    } else {
+        ::minicore::Option::None
+    }
+}}
+
+#[btf_relocatable]
+#[repr(C)]
+pub struct Inner {
+    pub x: u32,
+    pub y: u64,
+}
+
+#[btf_relocatable]
+#[repr(C)]
+pub union Payload {
+    pub word: u64,
+    pub half: u32,
+}
+
+#[btf_relocatable]
+#[repr(C)]
+pub struct Outer {
+    pub pad: u32,
+    pub inner: Inner,
+    pub payload: Payload,
+}
+
+// Each `Option` query emits a `BPF_CORE_FIELD_EXISTS` relocation (kind 2), followed by either
+// `BPF_CORE_FIELD_BYTE_OFFSET` (kind 0) or `BPF_CORE_FIELD_BYTE_SIZE` (kind 1). The value between
+// the second and third colons is the compile-time fallback.
+//
+// CHECK-DAG: @"llvm.Outer:2:1$0:1" = external global i32, !llvm.preserve.access.index
+// CHECK-DAG: @"llvm.Outer:0:8$0:1" = external global i32, !llvm.preserve.access.index
+// CHECK-DAG: @"llvm.Outer:1:16$0:1" = external global i32, !llvm.preserve.access.index
+//
+// CHECK-DAG: @"llvm.Outer:2:1$0:1:1" = external global i32, !llvm.preserve.access.index
+// CHECK-DAG: @"llvm.Outer:0:16$0:1:1" = external global i32, !llvm.preserve.access.index
+// CHECK-DAG: @"llvm.Outer:1:8$0:1:1" = external global i32, !llvm.preserve.access.index
+//
+// CHECK-DAG: @"llvm.Outer:2:1$0:2:1" = external global i32, !llvm.preserve.access.index
+// CHECK-DAG: @"llvm.Outer:0:24$0:2:1" = external global i32, !llvm.preserve.access.index
+// CHECK-DAG: @"llvm.Outer:1:4$0:2:1" = external global i32, !llvm.preserve.access.index
+
+// CHECK-LABEL: define{{.*}} @field_offset(
+#[unsafe(no_mangle)]
+pub fn field_offset() -> Option<usize> {
+    field_byte_offset!(Outer, inner)
+}
+
+// CHECK-LABEL: define{{.*}} @field_size(
+#[unsafe(no_mangle)]
+pub fn field_size() -> Option<usize> {
+    field_byte_size!(Outer, inner)
+}
+
+// CHECK-LABEL: define{{.*}} @nested_field_offset(
+#[unsafe(no_mangle)]
+pub fn nested_field_offset() -> Option<usize> {
+    field_byte_offset!(Outer, inner.y)
+}
+
+// CHECK-LABEL: define{{.*}} @nested_field_size(
+#[unsafe(no_mangle)]
+pub fn nested_field_size() -> Option<usize> {
+    field_byte_size!(Outer, inner.y)
+}
+
+// CHECK-LABEL: define{{.*}} @union_field_offset(
+#[unsafe(no_mangle)]
+pub fn union_field_offset() -> Option<usize> {
+    field_byte_offset!(Outer, payload.half)
+}
+
+// CHECK-LABEL: define{{.*}} @union_field_size(
+#[unsafe(no_mangle)]
+pub fn union_field_size() -> Option<usize> {
+    field_byte_size!(Outer, payload.half)
+}