[PATCH] D96659: [WebAssembly] Add new relocation for location relative data

[Yuta Saito via Phabricator via llvm-commits]

kateinoigakukun added inline comments.


================
Comment at: lld/test/wasm/reloc-relative.s:3
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+# RUN: wasm-ld --no-entry --no-gc-sections --allow-undefined -fatal-warnings -o %t.wasm %t.o %t.hello32.o
+# RUN: obj2yaml %t.wasm | FileCheck %s
----------------
sbc100 wrote:
> Do you need `--allow-undefined` here?
Yes, hello.s requires `puts` symbol but this test doesn't use printing function, so I put `--allow-undefined` to ignore such unrelated symbols.


```
## hello.s

  .globl  hello
hello:
  .functype hello () -> ()
  i32.const hello_str
  call  puts
  end_function

  .section  .rodata.hello_str,"",@
  .globl  hello_str
hello_str:
  .asciz  "hello\n"
  .size hello_str, 7

  .functype puts (i32) -> ()
```


================
Comment at: lld/test/wasm/reloc-relative.s:29
+bar:
+  .int32 hello_str - bar
+  .size bar, 4
----------------
sbc100 wrote:
> How does this not trigger the `can not be placed in a different section` error in the object writer?
```
S + A - P
```

`R_WASM_MEMORY_ADDR_LOCREL_I32` allows `S` to be a symbol in a different section, but doesn't allow `P` to be in a different section. This limitation is because a relocation entry can have only one symbol, so the base address (`P`) needs to be relative to the writing location.

In this case,  `hello_str` is an external section symbol, but `S` can be a symbol in a different section and `bar` is placed in the same section with writing location, so does not trigger `can not be placed in a different section`. 
This expression emits the below relocation entry.

```
Relocation {
  Type: R_WASM_MEMORY_ADDR_LOCREL_I32 (23)
  Offset: 0x0
  Symbol: hello_str
  Addend: 0
}
```



Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D96659/new/

https://reviews.llvm.org/D96659