From 10e0360f9e8ad88ae2ddbb6c9ba1913b8c16831d Mon Sep 17 00:00:00 2001
From: Derek Gerstmann <derek.gerstmann@gmail.com>
Date: Thu, 24 Aug 2023 15:22:22 -0700
Subject: [PATCH] [wasm] Enable PIC for WebAssembly on LLVM v18.x (#7803)

* Enable PIC code generation for WebAssembly for LLVM >18.
Enable +mutable-globals to support dynamic linking

* Fix LLVM v18 interface changes for writeArchive()
Add RelLookupTableConverterPass for PIC (in LLVM v18)

* Resolve conflict for writeArchive interface changes.

* Clang format pass

---------

Co-authored-by: Derek Gerstmann <dgerstmann@adobe.com>
---
 src/CodeGen_LLVM.cpp        | 11 +++++++++++
 src/CodeGen_WebAssembly.cpp | 19 +++++++++++++++++++
 src/LLVM_Headers.h          |  3 +++
 src/LLVM_Output.cpp         |  4 ++++
 4 files changed, 37 insertions(+)

diff --git a/src/CodeGen_LLVM.cpp b/src/CodeGen_LLVM.cpp
index b3fc5f176495..b4fd79d88a15 100644
--- a/src/CodeGen_LLVM.cpp
+++ b/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) {
diff --git a/src/CodeGen_WebAssembly.cpp b/src/CodeGen_WebAssembly.cpp
index 3f82f66bc2a2..69d696ce9f8a 100644
--- a/src/CodeGen_WebAssembly.cpp
+++ b/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 {
diff --git a/src/LLVM_Headers.h b/src/LLVM_Headers.h
index 6018c4fedff9..ad3f25365577 100644
--- a/src/LLVM_Headers.h
+++ b/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
 
diff --git a/src/LLVM_Output.cpp b/src/LLVM_Output.cpp
index f0aadfb9a869..04af5aff3609 100644
--- a/src/LLVM_Output.cpp
+++ b/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;