[BPF] Add preserve_access_index attribute for record definition

This patch introduced a new bpf specific attribute which can
be added to struct or union definition. For example,
  struct s { ... } __attribute__((preserve_access_index));
  union u { ... } __attribute__((preserve_access_index));
The goal is to simplify user codes for cases
where preserve access index happens for certain struct/union,
so user does not need to use clang __builtin_preserve_access_index
for every members.

The attribute has no effect if -g is not specified.

When the attribute is specified and -g is specified, any member
access defined by that structure or union, including array subscript
access and inner records, will be preserved through
  __builtin_preserve_{array,struct,union}_access_index()
IR intrinsics, which will enable relocation generation
in bpf backend.

The following is an example to illustrate the usage:
  -bash-4.4$ cat t.c
  #define __reloc__ __attribute__((preserve_access_index))
  struct s1 {
    int c;
  } __reloc__;

  struct s2 {
    union {
      struct s1 b[3];
    };
  } __reloc__;

  struct s3 {
    struct s2 a;
  } __reloc__;

  int test(struct s3 *arg) {
    return arg->a.b[2].c;
  }
  -bash-4.4$ clang -target bpf -g -S -O2 t.c

A relocation with access string "0:0:0:0:2:0" will be generated
representing access offset of arg->a.b[2].c.

forward declaration with attribute is also handled properly such
that the attribute is copied and populated in real record definition.

Differential Revision: https://reviews.llvm.org/D69759

1 files changed, 32 insertions, 0 deletions

diff --git a/clang/test/CodeGen/bpf-attr-preserve-access-index-5.c b/clang/test/CodeGen/bpf-attr-preserve-access-index-5.c
new file mode 100644
index 00000000000..281bcdd782c
--- /dev/null
+++ b/clang/test/CodeGen/bpf-attr-preserve-access-index-5.c
@@ -0,0 +1,32 @@
+// REQUIRES: bpf-registered-target
+// RUN: %clang -target bpf -emit-llvm -S -g %s -o - | FileCheck %s
+
+#define __reloc__ __attribute__((preserve_access_index))
+
+// chain of records, attribute may be in inner record.
+struct s1 {
+  int c;
+} __reloc__;
+typedef struct s1 __s1;
+
+struct s2 {
+  union {
+    __s1 b[3];
+  } __reloc__;
+};
+typedef struct s2 __s2;
+
+struct s3 {
+  __s2 a;
+} __reloc__;
+typedef struct s3 __s3;
+
+int test(__s3 *arg) {
+  return arg->a.b[2].c;
+}
+
+// CHECK: call %struct.s2* @llvm.preserve.struct.access.index.p0s_struct.s2s.p0s_struct.s3s(%struct.s3* %{{[0-9a-z]+}}, i32 0, i32 0)
+// CHECK-NOT: call %union.anon* @llvm.preserve.struct.access.index.p0s_union.anons.p0s_struct.s2s
+// CHECK: call %union.anon* @llvm.preserve.union.access.index.p0s_union.anons.p0s_union.anons(%union.anon* %{{[0-9a-z]+}}, i32 0)
+// CHECK: call %struct.s1* @llvm.preserve.array.access.index.p0s_struct.s1s.p0a3s_struct.s1s([3 x %struct.s1]* %{{[0-9a-z]+}}, i32 1, i32 2)
+// CHECK: call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.s1s(%struct.s1* %{{[0-9a-z]+}}, i32 0, i32 0)