[WebAssembly] Change the default linker for wasm32-wasip2 (#84569)

This commit changes the default linker in the WebAssembly toolchain for
the `wasm32-wasip2` target. This target is being added to the
WebAssembly/wasi-sdk and WebAssembly/wasi-libc projects to target the
Component Model by default, in contrast with the preexisting
`wasm32-wasi` target (in the process of being renamed to
`wasm32-wasip1`) which outputs a core WebAssembly module by default.

The `wasm-component-ld` project currently lives in my GitHub account at
https://github.com/alexcrichton/wasm-component-ld and isn't necessarily
"official" yet, but it's expected to continue to evolve as the
`wasm32-wasip2` target continues to shape up and evolve.

clang/lib/Driver/ToolChains/WebAssembly.cpp

@@ -44,8 +44,15 @@ std::string wasm::Linker::getLinkerPath(const ArgList &Args) const {
           llvm::sys::fs::can_execute(UseLinker))
         return std::string(UseLinker);
 
-      // Accept 'lld', and 'ld' as aliases for the default linker
-      if (UseLinker != "lld" && UseLinker != "ld")
+      // Interpret 'lld' as explicitly requesting `wasm-ld`, so look for that
+      // linker. Note that for `wasm32-wasip2` this overrides the default linker
+      // of `wasm-component-ld`.
+      if (UseLinker == "lld") {
+        return ToolChain.GetProgramPath("wasm-ld");
+      }
+
+      // Allow 'ld' as an alias for the default linker
+      if (UseLinker != "ld")
         ToolChain.getDriver().Diag(diag::err_drv_invalid_linker_name)
             << A->getAsString(Args);
     }
@@ -73,6 +80,16 @@ void wasm::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (Args.hasArg(options::OPT_s))
     CmdArgs.push_back("--strip-all");
 
+  // On `wasip2` the default linker is `wasm-component-ld` which wraps the
+  // execution of `wasm-ld`. Find `wasm-ld` and pass it as an argument of where
+  // to find it to avoid it needing to hunt and rediscover or search `PATH` for
+  // where it is.
+  if (llvm::sys::path::stem(Linker).ends_with_insensitive(
+          "wasm-component-ld")) {
+    CmdArgs.push_back("--wasm-ld-path");
+    CmdArgs.push_back(Args.MakeArgString(ToolChain.GetProgramPath("wasm-ld")));
+  }
+
   Args.addAllArgs(CmdArgs, {options::OPT_L, options::OPT_u});
 
   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
@@ -221,6 +238,12 @@ WebAssembly::WebAssembly(const Driver &D, const llvm::Triple &Triple,
   }
 }
 
+const char *WebAssembly::getDefaultLinker() const {
+  if (getOS() == "wasip2")
+    return "wasm-component-ld";
+  return "wasm-ld";
+}
+
 bool WebAssembly::IsMathErrnoDefault() const { return false; }
 
 bool WebAssembly::IsObjCNonFragileABIDefault() const { return true; }

clang/lib/Driver/ToolChains/WebAssembly.h

@@ -67,7 +67,7 @@ class LLVM_LIBRARY_VISIBILITY WebAssembly final : public ToolChain {
                            llvm::opt::ArgStringList &CmdArgs) const override;
   SanitizerMask getSupportedSanitizers() const override;
 
-  const char *getDefaultLinker() const override { return "wasm-ld"; }
+  const char *getDefaultLinker() const override;
 
   CXXStdlibType GetDefaultCXXStdlibType() const override {
     return ToolChain::CST_Libcxx;

clang/test/Driver/wasm-toolchain.c

@@ -197,3 +197,27 @@
 // RUN: not %clang -### %s --target=wasm32-unknown-unknown --sysroot=%s/no-sysroot-there -fPIC -mno-mutable-globals %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=PIC_NO_MUTABLE_GLOBALS %s
 // PIC_NO_MUTABLE_GLOBALS: error: invalid argument '-fPIC' not allowed with '-mno-mutable-globals'
+
+// Test that `wasm32-wasip2` invokes the `wasm-component-ld` linker by default
+// instead of `wasm-ld`.
+
+// RUN: %clang -### -O2 --target=wasm32-wasip2 %s --sysroot /foo 2>&1 \
+// RUN:   | FileCheck -check-prefix=LINK_WASIP2 %s
+// LINK_WASIP2: "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_WASIP2: wasm-component-ld{{.*}}" "-L/foo/lib/wasm32-wasip2" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
+
+// Test that on `wasm32-wasip2` the `wasm-component-ld` programs is told where
+// to find `wasm-ld` by default.
+
+// RUN: %clang -### -O2 --target=wasm32-wasip2 %s --sysroot /foo 2>&1 \
+// RUN:   | FileCheck -check-prefix=LINK_WASIP2_FIND_WASMLD %s
+// LINK_WASIP2_FIND_WASMLD: "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_WASIP2_FIND_WASMLD: wasm-component-ld{{.*}}" {{.*}} "--wasm-ld-path" "{{.*}}wasm-ld{{.*}}" {{.*}} "[[temp]]" {{.*}}
+
+// If `wasm32-wasip2` is configured with `wasm-ld` as a linker then don't pass
+// the `--wasm-ld-path` flag.
+
+// RUN: %clang -### -O2 --target=wasm32-wasip2 -fuse-ld=lld %s --sysroot /foo 2>&1 \
+// RUN:   | FileCheck -check-prefix=LINK_WASIP2_USE_WASMLD %s
+// LINK_WASIP2_USE_WASMLD: "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_WASIP2_USE_WASMLD: wasm-ld{{.*}}" "-m" "wasm32" {{.*}} "[[temp]]" {{.*}}