rustbuild: allow dynamically linking LLVM

The makefiles and `mklldeps.py` called `llvm-config --shared-mode` to
find out if LLVM defaulted to shared or static libraries, and just went
with that.  But under rustbuild, `librustc_llvm/build.rs` was assuming
that LLVM should be static, and even forcing `--link-static` for 3.9+.

Now that build script also uses `--shared-mode` to learn the default,
which should work better for pre-3.9 configured for dynamic linking, as
it wasn't possible back then to choose differently via `llvm-config`.

Further, the configure script now has a new `--enable-llvm-link-shared`
option, which allows one to manually override `--link-shared` on 3.9+
instead of forcing static.

configure

@@ -624,6 +624,7 @@ opt ccache 0 "invoke gcc/clang via ccache to reuse object files between builds"
 opt local-rust 0 "use an installed rustc rather than downloading a snapshot"
 opt local-rebuild 0 "assume local-rust matches the current version, for rebuilds; implies local-rust, and is implied if local-rust already matches the current version"
 opt llvm-static-stdcpp 0 "statically link to libstdc++ for LLVM"
+opt llvm-link-shared 0 "prefer shared linking to LLVM (llvm-config --link-shared)"
 opt rpath 1 "build rpaths into rustc itself"
 opt stage0-landing-pads 1 "enable landing pads during bootstrap with stage0"
 # This is used by the automation to produce single-target nightlies

src/bootstrap/compile.rs

@@ -212,6 +212,9 @@ pub fn rustc<'a>(build: &'a Build, target: &str, compiler: &Compiler<'a>) {
         cargo.env("LLVM_STATIC_STDCPP",
                   compiler_file(build.cxx(target), "libstdc++.a"));
     }
+    if build.config.llvm_link_shared {
+        cargo.env("LLVM_LINK_SHARED", "1");
+    }
     if let Some(ref s) = build.config.rustc_default_linker {
         cargo.env("CFG_DEFAULT_LINKER", s);
     }

src/bootstrap/config.rs

@@ -53,6 +53,7 @@ pub struct Config {
     pub llvm_release_debuginfo: bool,
     pub llvm_version_check: bool,
     pub llvm_static_stdcpp: bool,
+    pub llvm_link_shared: bool,
 
     // rust codegen options
     pub rust_optimize: bool,
@@ -343,6 +344,7 @@ impl Config {
                 ("OPTIMIZE_LLVM", self.llvm_optimize),
                 ("LLVM_VERSION_CHECK", self.llvm_version_check),
                 ("LLVM_STATIC_STDCPP", self.llvm_static_stdcpp),
+                ("LLVM_LINK_SHARED", self.llvm_link_shared),
                 ("OPTIMIZE", self.rust_optimize),
                 ("DEBUG_ASSERTIONS", self.rust_debug_assertions),
                 ("DEBUGINFO", self.rust_debuginfo),

src/librustc_llvm/build.rs

@@ -123,13 +123,24 @@ fn main() {
        .cpp_link_stdlib(None) // we handle this below
        .compile("librustllvm.a");
 
+    // Find out LLVM's default linking mode.
+    let mut cmd = Command::new(&llvm_config);
+    cmd.arg("--shared-mode");
+    let mut llvm_kind = if output(&mut cmd).trim() == "shared" {
+        "dylib"
+    } else {
+        "static"
+    };
+
     // Link in all LLVM libraries, if we're uwring the "wrong" llvm-config then
     // we don't pick up system libs because unfortunately they're for the host
     // of llvm-config, not the target that we're attempting to link.
     let mut cmd = Command::new(&llvm_config);
     cmd.arg("--libs");
 
-    // Force static linking with "--link-static" if available.
+    // Force static linking with "--link-static" if available, or
+    // force "--link-shared" if the configuration requested it.
+    let llvm_link_shared = env::var_os("LLVM_LINK_SHARED").is_some();
     let mut version_cmd = Command::new(&llvm_config);
     version_cmd.arg("--version");
     let version_output = output(&mut version_cmd);
@@ -137,7 +148,13 @@ fn main() {
     if let (Some(major), Some(minor)) = (parts.next().and_then(|s| s.parse::<u32>().ok()),
                                          parts.next().and_then(|s| s.parse::<u32>().ok())) {
         if major > 3 || (major == 3 && minor >= 9) {
-            cmd.arg("--link-static");
+            if llvm_link_shared {
+                cmd.arg("--link-shared");
+                llvm_kind = "dylib";
+            } else {
+                cmd.arg("--link-static");
+                llvm_kind = "static";
+            }
         }
     }
 
@@ -174,7 +191,7 @@ fn main() {
         }
 
         let kind = if name.starts_with("LLVM") {
-            "static"
+            llvm_kind
         } else {
             "dylib"
         };