[WASM] Fix least valid pointer value

WebAssembly doesn't reserve low addresses But without "extra inhabitants" of
the pointer representation, runtime performance and memory footprint are
worse. So assume that compiler driver uses wasm-ld and --global-base=1024 to
reserve low 1KB.

Author: kateinoigakukun

lib/Driver/ToolChains.h

@@ -118,6 +118,9 @@ class LLVM_LIBRARY_VISIBILITY WebAssembly : public ToolChain {
                                      const JobContext &context) const override;
   InvocationInfo constructInvocation(const StaticLinkJobAction &job,
                                      const JobContext &context) const override;
+  void validateArguments(DiagnosticEngine &diags,
+                         const llvm::opt::ArgList &args,
+                         StringRef defaultTarget) const override;
 
 public:
   WebAssembly(const Driver &D, const llvm::Triple &Triple) : ToolChain(D, Triple) {}

lib/Driver/WebAssemblyToolChains.cpp

@@ -12,6 +12,8 @@
 
 #include "ToolChains.h"
 
+#include "swift/ABI/System.h"
+#include "swift/AST/DiagnosticsDriver.h"
 #include "swift/Basic/Dwarf.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/Platform.h"
@@ -192,6 +194,15 @@ toolchains::WebAssembly::constructInvocation(const DynamicLinkJobAction &job,
     Arguments.push_back("-v");
   }
 
+  // WebAssembly doesn't reserve low addresses But without "extra inhabitants"
+  // of the pointer representation, runtime performance and memory footprint are
+  // worse. So assume that compiler driver uses wasm-ld and --global-base=1024
+  // to reserve low 1KB.
+  Arguments.push_back("-Xlinker");
+  Arguments.push_back(context.Args.MakeArgString(
+      Twine("--global-base=") +
+      std::to_string(SWIFT_ABI_WASM32_LEAST_VALID_POINTER)));
+
   // These custom arguments should be right before the object file at the end.
   context.Args.AddAllArgs(Arguments, options::OPT_linker_option_Group);
   context.Args.AddAllArgs(Arguments, options::OPT_Xlinker);
@@ -208,6 +219,23 @@ toolchains::WebAssembly::constructInvocation(const DynamicLinkJobAction &job,
   return II;
 }
 
+void validateLinkerArguments(DiagnosticEngine &diags,
+                             ArgStringList linkerArgs) {
+  for (auto arg : linkerArgs) {
+    if (StringRef(arg).startswith("--global-base=")) {
+      diags.diagnose(SourceLoc(), diag::error_option_not_supported, arg,
+                     "wasm32");
+    }
+  }
+}
+void toolchains::WebAssembly::validateArguments(DiagnosticEngine &diags,
+                                                const llvm::opt::ArgList &args,
+                                                StringRef defaultTarget) const {
+  ArgStringList linkerArgs;
+  args.AddAllArgValues(linkerArgs, options::OPT_Xlinker);
+  validateLinkerArguments(diags, linkerArgs);
+}
+
 ToolChain::InvocationInfo
 toolchains::WebAssembly::constructInvocation(const StaticLinkJobAction &job,
                                          const JobContext &context) const {

lib/IRGen/SwiftTargetInfo.cpp

@@ -140,6 +140,13 @@ static void configureSystemZ(IRGenModule &IGM, const llvm::Triple &triple,
   target.SwiftRetainIgnoresNegativeValues = true;
 }
 
+/// Configures target-specific information for wasm32 platforms.
+static void configureWasm32(IRGenModule &IGM, const llvm::Triple &triple,
+                            SwiftTargetInfo &target) {
+  target.LeastValidPointerValue =
+    SWIFT_ABI_WASM32_LEAST_VALID_POINTER;
+}
+
 /// Configure a default target.
 SwiftTargetInfo::SwiftTargetInfo(
   llvm::Triple::ObjectFormatType outputObjectFormat,
@@ -196,7 +203,9 @@ SwiftTargetInfo SwiftTargetInfo::get(IRGenModule &IGM) {
   case llvm::Triple::systemz:
     configureSystemZ(IGM, triple, target);
     break;
-
+  case llvm::Triple::wasm32:
+    configureWasm32(IGM, triple, target);
+    break;
   default:
     // FIXME: Complain here? Default target info is unlikely to be correct.
     break;

stdlib/public/SwiftShims/HeapObject.h

@@ -190,6 +190,22 @@ static_assert(alignof(HeapObject) == alignof(void*),
 #define _swift_BridgeObject_TaggedPointerBits                                  \
   (__swift_uintptr_t) SWIFT_ABI_DEFAULT_BRIDGEOBJECT_TAG_64
 
+#elif defined(__wasm32__)
+extern unsigned char __global_base;
+#define _swift_abi_LeastValidPointerValue                                      \
+  (__swift_uintptr_t) SWIFT_ABI_WASM32_LEAST_VALID_POINTER
+
+#define _swift_abi_SwiftSpareBitsMask                                          \
+  (__swift_uintptr_t) SWIFT_ABI_DEFAULT_SWIFT_SPARE_BITS_MASK
+
+#define _swift_abi_ObjCReservedBitsMask                                        \
+  (__swift_uintptr_t) SWIFT_ABI_DEFAULT_OBJC_RESERVED_BITS_MASK
+#define _swift_abi_ObjCReservedLowBits                                         \
+  (unsigned) SWIFT_ABI_DEFAULT_OBJC_NUM_RESERVED_LOW_BITS
+
+#define _swift_BridgeObject_TaggedPointerBits                                  \
+  (__swift_uintptr_t) SWIFT_ABI_DEFAULT_BRIDGEOBJECT_TAG_32
+
 #else
 
 #define _swift_abi_LeastValidPointerValue                                      \

stdlib/public/SwiftShims/System.h

@@ -192,4 +192,12 @@
 #define SWIFT_ABI_S390X_OBJC_WEAK_REFERENCE_MARKER_VALUE \
   (1<<SWIFT_ABI_S390X_OBJC_NUM_RESERVED_LOW_BITS)
 
+/*********************************** wasm32 ************************************/
+
+// WebAssembly doesn't reserve low addresses But without "extra inhabitants" of
+// the pointer representation, runtime performance and memory footprint are
+// worse. So assume that compiler driver uses wasm-ld and --global-base=1024 to
+// reserve low 1KB.
+#define SWIFT_ABI_WASM32_LEAST_VALID_POINTER 1024
+
 #endif // SWIFT_STDLIB_SHIMS_ABI_SYSTEM_H