clang "cannot select" when using wasm table builtins and -fPIC

[hoodmane]

Example:

Source file:

static __externref_t table[0];

__externref_t test_builtin_wasm_table_get() {
  return __builtin_wasm_table_get(table, 0);
}

Compiling as follows works:

clang -c -target wasm32 -mreference-types builtins-table.c

but if I add -fPIC, it says:

fatal error: error in backend: Cannot select: intrinsic %llvm.wasm.table.get.externref

Bad/good IR

The IR looks as follows:

@table = internal addrspace(1) global [0 x ptr addrspace(10)] zeroinitializer, align 1

; Function Attrs: noinline nounwind optnone
define hidden ptr addrspace(10) @test_builtin_wasm_table_get() #0 {
entry:
  %0 = call ptr addrspace(10) @llvm.wasm.table.get.externref(ptr addrspace(1) @table, i32 0)
  ret ptr addrspace(10) %0
}

; Function Attrs: nocallback nofree nosync nounwind willreturn memory(read)
declare ptr addrspace(10) @llvm.wasm.table.get.externref(ptr addrspace(1), i32) #1

If I change @table = internal addrspace(1) to @table = local_unnamed_addr addrspace(1) it seems to fix the problem.

@pmatos

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

[hoodmane]

Actually, even with local_unnamed_addr it still doesn't work quite as desired with -fPIC, since it generates relocations of type R_WASM_GLOBAL_INDEX_LEB instead of type R_WASM_TABLE_NUMBER_LEB and so linking does not work. But it moves the problem a bit later.

[pmatos]

Hi. I am currently on holidays until Sept 11. If nobody looks at this until then, I will upon my return.

[pmatos]

I can reproduce the problem and will work on a fix.

[pmatos]

@sbc100 looking at some code you wrote some time ago:

llvm-project/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

Line 1733 in 5fabc8b

if (GV->getValueType()->isFunctionTy()) {

Before Wasm tables being implemented, we used a single table for functions, which explains the condition GV->getValueType()->isFunctionTy(). The issue with this issue is that the symbol for a table is handled in the else condition because it's not a function.

I am not sure this is the correct behaviour. The symbol for a table is compiled to a reference to a table that is then declared with .tabletype. Unsure what this __table_base refers to in the code and it can be used by multiple tables. In any case I think we can solve this issue by adding a code path for Wasm Tables here and appropriately implementing node selection the node. PIC was something I didn't test when implementing tables support.

[sbc100]

Under -fPIC (i.e. dynamic linking) the __table_base symbol is the linker-created global which represents the offset into the __indirect_function_table at which the current DLL's static function addresses are stored. Its similar to __memory_base for the static data section. It should only be used when the address of function is taken and it should only refer to __indirect_function_table.

[sbc100]

Looks like we just need another branch in that if/else to handle table symbols. When that code was written there were only two types of symbols: function symbols and data symbols.

[pmatos]

Looks like we just need another branch in that if/else to handle table symbols. When that code was written there were only two types of symbols: function symbols and data symbols.

Thanks. It seems like the issue is straightforward given tables cannot be export/imported but it will become more complicated when we can have two modules sharing a table.