[Wasm] Specify the least valid pointer for 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.

Author: kateinoigakukun

lib/Driver/WebAssemblyToolChains.cpp

@@ -1,5 +1,4 @@
-//===---- WebAssemblyToolChains.cpp - Job invocations (WebAssembly-specific)
-//------===//
+//===---- WebAssemblyToolChains.cpp - Job invocations (WebAssembly-specific) ------===//
 //
 // This source file is part of the Swift.org open source project
 //
@@ -189,6 +188,9 @@ toolchains::WebAssembly::constructInvocation(const DynamicLinkJobAction &job,
   // 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);

lib/IRGen/SwiftTargetInfo.cpp

@@ -169,6 +169,13 @@ 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,
@@ -240,6 +247,9 @@ 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.

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__)
+
+#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

@@ -207,4 +207,12 @@
 #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