[wasm] Enable PIC for WebAssembly on LLVM v18.x

src/CodeGen_LLVM.cpp

@@ -1151,6 +1151,17 @@ void CodeGen_LLVM::optimize_module() {
     using OptimizationLevel = llvm::OptimizationLevel;
     OptimizationLevel level = OptimizationLevel::O3;
 
+#if LLVM_VERSION >= 180
+    if (tm->isPositionIndependent()) {
+        // Add a pass that converts lookup tables to relative lookup tables to make them PIC-friendly.
+        // See https://bugs.llvm.org/show_bug.cgi?id=45244
+        pb.registerOptimizerLastEPCallback(
+            [&](ModulePassManager &mpm, OptimizationLevel level) {
+                mpm.addPass(RelLookupTableConverterPass());
+            });
+    }
+#endif
+
     if (get_target().has_feature(Target::SanitizerCoverage)) {
         pb.registerOptimizerLastEPCallback(
             [&](ModulePassManager &mpm, OptimizationLevel level) {

src/CodeGen_WebAssembly.cpp

@@ -344,6 +344,14 @@ string CodeGen_WebAssembly::mattrs() const {
         sep = ",";
     }
 
+    // PIC implies +mutable-globals because the PIC ABI used by the linker
+    // depends on importing and exporting mutable globals. Also -pthread implies
+    // mutable-globals too, so quitely enable it if either of these are specified.
+    if (use_pic() || target.has_feature(Target::WasmThreads)) {
+        s << sep << "+mutable-globals";
+        sep = ",";
+    }
+
     // Recent Emscripten builds assume that specifying `-pthread` implies bulk-memory too,
     // so quietly enable it if either of these are specified.
     if (target.has_feature(Target::WasmBulkMemory) || target.has_feature(Target::WasmThreads)) {
@@ -362,7 +370,18 @@ bool CodeGen_WebAssembly::use_soft_float_abi() const {
 }
 
 bool CodeGen_WebAssembly::use_pic() const {
+#if LLVM_VERSION >= 180
+    // Issues with WASM PIC and dynamic linking only got fixed in LLVM v18.x (June 26th 2023)
+    // See https://reviews.llvm.org/D153293
+
+    // Always emitting PIC "does add a little bloat to the object files, due to the extra
+    // indirection, but when linked into a static binary 100% of this can be removed by
+    // wasm-opt in release builds."
+    // See https://github.com/halide/Halide/issues/7796
+    return true;
+#else
     return false;
+#endif
 }
 
 int CodeGen_WebAssembly::native_vector_bits() const {

src/LLVM_Headers.h

@@ -102,6 +102,9 @@
 #include <llvm/Transforms/Scalar/GVN.h>
 #include <llvm/Transforms/Utils/ModuleUtils.h>
 #include <llvm/Transforms/Utils/SymbolRewriter.h>
+#if LLVM_VERSION >= 180
+#include <llvm/Transforms/Utils/RelLookupTableConverter.h>
+#endif
 
 // IWYU pragma: end_exports
 

src/LLVM_Output.cpp

@@ -397,6 +397,10 @@ void emit_file(const llvm::Module &module_in, Internal::LLVMOStream &out,
     pass_manager.add(llvm::createRewriteSymbolsPass());
 #endif
 
+    if (target_machine->isPositionIndependent()) {
+        Internal::debug(1) << "Target machine is Position Independent!\n";
+    }
+
     // Override default to generate verbose assembly.
     target_machine->Options.MCOptions.AsmVerbose = true;