[llvm] [BPF] Do not emit names for PTR, CONST, VOLATILE and RESTRICT BTF types (PR #163174)

[Michal R via llvm-commits]

================
@@ -0,0 +1,75 @@
+; RUN: llc -mtriple=bpfel -filetype=obj -o %t1 %s
+; RUN: llvm-objcopy --dump-section='.BTF'=%t2 %t1
+; RUN: %python %p/print_btf.py %t2 | FileCheck -check-prefixes=CHECK-BTF %s
+; RUN: llc -mtriple=bpfeb -filetype=obj -o %t1 %s
+; RUN: llvm-objcopy --dump-section='.BTF'=%t2 %t1
+; RUN: %python %p/print_btf.py %t2 | FileCheck -check-prefixes=CHECK-BTF %s
+;
+; Source:
+;   #![no_std]
+;   #![no_main]
+;
+;   pub struct MyType {
+;       ptr: *const u32,
+;   }
+;
+;   impl MyType {
+;       pub const fn new() -> Self {
+;           let ptr = core::ptr::null();
+;           Self { ptr }
+;       }
+;   }
+;
+;   unsafe impl Sync for MyType {}
+;
+;   #[unsafe(no_mangle)]
+;   pub static X: MyType = MyType::new();
+;
+;   #[cfg(not(test))]
+;   #[panic_handler]
+;   fn panic(_info: &core::panic::PanicInfo) -> ! {
+;       loop {}
+;   }
+; Compilation flag:
+;   cargo +nightly rustc -Zbuild-std=core --target=bpfel-unknown-none -- --emit=llvm-bc
+;   llvm-extract --glob=X $(find target/ -name "*.bc" | head -n 1) -o ptr-named.bc
+;   llvm-dis ptr-named.bc -o ptr-named.ll
+
+; ModuleID = 'ptr-named.bc'
+source_filename = "1m2uqe50qkwxmo53ydydvou91"
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
+target triple = "bpfel"
+
+ at X = constant [8 x i8] zeroinitializer, align 8, !dbg !0
+
+!llvm.module.flags = !{!11, !12, !13, !14}
+!llvm.ident = !{!15}
+!llvm.dbg.cu = !{!16}
+
+; CHECK-BTF:      [1] STRUCT 'MyType' size=8 vlen=1
+; CHECK-BTF-NEXT:         'ptr' type_id=2 bits_offset=0
+; CHECK-BTF-NEXT: [2] PTR '(anon)' type_id=3
+; CHECK-BTF-NEXT: [3] INT 'u32' size=4 bits_offset=0 nr_bits=32 encoding=(none)
+; CHECK-BTF-NEXT: [4] VAR 'X' type_id=1, linkage=global
+; CHECK-BTF-NEXT: [5] DATASEC '.rodata' size=0 vlen=1
+; CHECK-BTF-NEXT:         type_id=4 offset=0 size=8
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "X", scope: !2, file: !3, line: 19, type: !4, isLocal: false, isDefinition: true, align: 64)
+!2 = !DINamespace(name: "ptr_named", scope: null)
+!3 = !DIFile(filename: "ptr-named/src/main.rs", directory: "/tmp/ptr-named", checksumkind: CSK_MD5, checksum: "e37168304600b30cbb5ba168f0384932")
+!4 = !DICompositeType(tag: DW_TAG_structure_type, name: "MyType", scope: !2, file: !5, size: 64, align: 64, flags: DIFlagPublic, elements: !6, templateParams: !10, identifier: "7609fa40332dd486922f074276a171c3")
+!5 = !DIFile(filename: "<unknown>", directory: "")
+!6 = !{!7}
+!7 = !DIDerivedType(tag: DW_TAG_member, name: "ptr", scope: !4, file: !5, baseType: !8, size: 64, align: 64, flags: DIFlagPrivate)
+!8 = !DIDerivedType(tag: DW_TAG_pointer_type, name: "*const u32", baseType: !9, size: 64, align: 64, dwarfAddressSpace: 0)
----------------
vadorovsky wrote:

I was thinking about it, but:

* In C, `const`, `volatile` and `restrict` variables are always unnamed.
* In Rust, these variable types don't exist. Rust frontend, from what I was able to check, never emits `DW_TAG_{const,volatile,restrict}` tags.
  * Both `*const u32` and `*mut u32` used as types simply translate to `DW_TAG_pointer_type`.
  * There are no `volatile` and `reference` keywords.
  * Using volatile operations like [`read_volatile`](https://doc.rust-lang.org/std/ptr/fn.read_volatile.html) doesn't change the DWARF tag, it's still `DW_TAG_pointer_type`.
* Zig has a `volatile` keyword, but it still emits the `DW_TAG_pointer_type` tag.

I'm sanitizing these types anyways for the consistency with the expectation of the kernel and `print_btf.py`, but currently the only reproducible case is Rust frontend emitting named pointers. If you would prefer me to trim down the patch only to removing pointer names, that's fine with me.

https://github.com/llvm/llvm-project/pull/163174