[WebAssembly] call_indirect causes indirect function table import

For wasm-ld table linking work to proceed, object files should indicate
if they use an indirect function table.  In the future this will be done
by the usual symbols and relocations mechanism, but until that support
lands in the linker, the presence of an `__indirect_function_table` in
the object file's import section shows that the object file needs an
indirect function table.

Prior to https://reviews.llvm.org/D91637, this condition was met by all
object files residualizing an `__indirect_function_table` import.

Since https://reviews.llvm.org/D91637, the intention has been that only
those object files needing an indirect function table would have the
`__indirect_function_table` import.  However, we missed the case of
object files which use the table via `call_indirect` but which
themselves do not declare any indirect functions.

This changeset makes it so that when we lower a call to `call_indirect`,
that we ensure that a `__indirect_function_table` symbol is present and
that it will be propagated to the linker.

A followup patch will revise this mechanism to make an explicit link
between `call_indirect` and its associated indirect function table; see
https://reviews.llvm.org/D90948.

Differential Revision: https://reviews.llvm.org/D92840

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index c8d2e5db..683678b 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -405,6 +405,13 @@ void WasmObjectWriter::writeHeader(const MCAssembler &Asm) {
 
 void WasmObjectWriter::executePostLayoutBinding(MCAssembler &Asm,
                                                 const MCAsmLayout &Layout) {
+  // As a stopgap measure until call_indirect instructions start explicitly
+  // referencing the indirect function table via TABLE_NUMBER relocs, ensure
+  // that the indirect function table import makes it to the output if anything
+  // in the compilation unit has caused it to be present.
+  if (auto *Sym = Asm.getContext().lookupSymbol("__indirect_function_table"))
+    Asm.registerSymbol(*Sym);
+
   // Build a map of sections to the function that defines them, for use
   // in recordRelocation.
   for (const MCSymbol &S : Asm.symbols()) {