[wasm-ld] --unresolved-symbols=ignore-all does not actually resolve undefined functions to null

[SingleAccretion]

Reproduction steps:

1. test.c:

extern void undefined_func(void);

int main(void) {
    void* a = (void*)&undefined_func;
    asm (
        ""
        : "=r"(a)
        : "0"(a)
    );

    return (a == 0) ? 1 : 2;
}

2. $env:EMCC_DEBUG=1 / export EMCC_DEBUG=1.
3. emcc test.c -nostdlib -O2
4. emcc will fail with a few wasm-ld errors like below:

wasm-ld: error: symbol exported via --export not found: stackSave
wasm-ld: error: symbol exported via --export not found: stackRestore
wasm-ld: error: symbol exported via --export not found: stackAlloc
wasm-ld: error: symbol exported via --export not found: __errno_location
wasm-ld: error: symbol exported via --export not found: __get_temp_ret
wasm-ld: error: symbol exported via --export not found: __set_temp_ret
emcc: error: 'C:/Users/Accretion/source/dotnet/emsdk/upstream/bin\wasm-ld.exe -o a.out.wasm C:\Users\ACCRET~1\AppData\Local\Temp\emscripten_temp\test_0.o -LC:\Users\Accretion\source\dotnet\emsdk\upstream\emscripten\cache\sysroot\lib\wasm32-emscripten -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --import-undefined --strip-debug --export-if-defined=main --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export=stackSave --export=stackRestore --export=stackAlloc --export=__wasm_call_ctors --export=__errno_location --export=__get_temp_ret --export=__set_temp_ret --export-table -z stack-size=5242880 --initial-memory=16777216 --no-entry --max-memory=16777216 --global-base=1024' failed (returned 1)

5. Copy the linker command line, delete the problematic export=<...> arguments, delete --import-undefined, add --unresolved-symbols=ignore-all. Run it again with the modified command line.
6. wasm-objdump -j Elem -x a.out.wasm:

Elem[1]:
 - segment[0] flags=0 table=0 count=1 - init i32=1
  - elem[1] = func[1] <undefined_stub>

7. wasm-objdump -d a.out.wasm:

Code Disassembly:

000089 func[0] <__wasm_call_ctors>:
 00008a: 0b                         | end
00008c func[1] <undefined_stub>:
 00008d: 00                         | unreachable
 00008e: 0b                         | end
000090 func[2] <__original_main>:
 000091: 01 7f                      | local[2] type=i32
 000093: 41 81 80 80 80 00          | i32.const 1
 000099: 21 00                      | local.set 0
 00009b: 41 02                      | i32.const 2
 00009d: 41 01                      | i32.const 1
 00009f: 20 00                      | local.get 0
 0000a1: 1b                         | select
 0000a2: 0b                         | end
0000a4 func[3] <main>:
 0000a5: 10 82 80 80 80 00          | call 2 <__original_main>
 0000ab: 0b                         | end

Expected behavior (documentation): undefined_stub is not in the indirect function table, &undefined_func is resolved to zero.
Actual behavior: &undefined_func is resolved to &undefined_stub / 1.

Analysis: scanRelocations will first process the function index relocation, invoking out.elemSec->addEntry(cast<FunctionSymbol>(sym));, adding it to the indirect function table, and only then replace the function with a stub (set isStub = true), hence, the check in ElemSection::addEntry is not effective.

[llvmbot]

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