Auto merge of #26569 - alexcrichton:msvc-llvm-update, r=brson

Now that LLVM has been updated, the only remaining roadblock to implementing
unwinding for MSVC is to fill out the runtime support in `std::rt::unwind::seh`.
This commit does precisely that, fixing up some other bits and pieces along the
way:

* The `seh` unwinding module now uses `RaiseException` to initiate a panic.
* The `rust_try.ll` file was rewritten for MSVC (as it's quite different) and is
  located at `rust_try_msvc_64.ll`, only included on MSVC builds for now.
* The personality function for all landing pads generated by LLVM is hard-wired
  to `__C_specific_handler` instead of the standard `rust_eh_personality` lang
  item. This is required to get LLVM to emit SEH unwinding information instead
  of DWARF unwinding information. This also means that on MSVC the
  `rust_eh_personality` function is entirely unused (but is defined as it's a
  lang item).

More details about how panicking works on SEH can be found in the
`rust_try_msvc_64.ll` or `seh.rs` files, but I'm always open to adding more
comments!

A key aspect of this PR is missing, however, which is that **unwinding is still
turned off by default for MSVC**. There is a [bug in llvm][llvm-bug] which
causes optimizations to inline enough landing pads that LLVM chokes. If the
compiler is optimized at `-O1` (where inlining isn't enabled) then it can
bootstrap with unwinding enabled, but when optimized at `-O2` (inlining is
enabled) then it hits a fatal LLVM error.

[llvm-bug]: https://llvm.org/bugs/show_bug.cgi?id=23884

Author: bors

mk/cfg/x86_64-pc-windows-msvc.mk

@@ -23,64 +23,6 @@ CFG_RUN_x86_64-pc-windows-msvc=$(2)
 CFG_RUN_TARG_x86_64-pc-windows-msvc=$(call CFG_RUN_x86_64-pc-windows-msvc,,$(2))
 CFG_GNU_TRIPLE_x86_64-pc-windows-msvc := x86_64-pc-win32
 
-# These two environment variables are scraped by the `./configure` script and
-# are necessary for `cl.exe` to find standard headers (the INCLUDE variable) and
-# for `link.exe` to find standard libraries (the LIB variable).
-ifdef CFG_MSVC_INCLUDE_PATH
-export INCLUDE := $(CFG_MSVC_INCLUDE_PATH)
-endif
-ifdef CFG_MSVC_LIB_PATH
-export LIB := $(CFG_MSVC_LIB_PATH)
-endif
-
-# Unfortunately `link.exe` is also a program in `/usr/bin` on MinGW installs,
-# but it's not the one that we want. As a result we make sure that our detected
-# `link.exe` shows up in PATH first.
-ifdef CFG_MSVC_LINK
-export PATH := $(CFG_MSVC_ROOT)/VC/bin/amd64:$(PATH)
-endif
-
-# There are more comments about this available in the target specification for
-# Windows MSVC in the compiler, but the gist of it is that we use `llvm-ar.exe`
-# instead of `lib.exe` for assembling archives, so we need to inject this custom
-# dependency here.
-NATIVE_TOOL_DEPS_core_T_x86_64-pc-windows-msvc += llvm-ar.exe
-INSTALLED_BINS_x86_64-pc-windows-msvc += llvm-ar.exe
-
-# When working with MSVC on windows, each DLL needs to explicitly declare its
-# interface to the outside world through some means. The options for doing so
-# include:
-#
-# 1. A custom attribute on each function itself
-# 2. A linker argument saying what to export
-# 3. A file which lists all symbols that need to be exported
-#
-# The Rust compiler takes care (1) for us for all Rust code by annotating all
-# public-facing functions with dllexport, but we have a few native dependencies
-# which need to cross the DLL boundary. The most important of these dependencies
-# is LLVM which is linked into `rustc_llvm.dll` but primarily used from
-# `rustc_trans.dll`. This means that many of LLVM's C API functions need to be
-# exposed from `rustc_llvm.dll` to be forwarded over the boundary.
-#
-# Unfortunately, at this time, LLVM does not handle this sort of exportation on
-# Windows for us, so we're forced to do it ourselves if we want it (which seems
-# like the path of least resistance right now). To do this we generate a `.DEF`
-# file [1] which we then custom-pass to the linker when building the rustc_llvm
-# crate. This DEF file list all symbols that are exported from
-# `src/librustc_llvm/lib.rs` and is generated by a small python script.
-#
-# Fun times!
-#
-# [1]: https://msdn.microsoft.com/en-us/library/28d6s79h.aspx
-RUSTFLAGS_rustc_llvm_T_x86_64-pc-windows-msvc += \
-	-C link-args="-DEF:x86_64-pc-windows-msvc/rt/rustc_llvm.def"
-CUSTOM_DEPS_rustc_llvm_T_x86_64-pc-windows-msvc += \
-	x86_64-pc-windows-msvc/rt/rustc_llvm.def
-
-x86_64-pc-windows-msvc/rt/rustc_llvm.def: $(S)src/etc/mklldef.py \
-			$(S)src/librustc_llvm/lib.rs
-	$(CFG_PYTHON) $^ $@ rustc_llvm-$(CFG_FILENAME_EXTRA)
-
 # All windows nightiles are currently a GNU triple, so this MSVC triple is not
 # bootstrapping from itself. This is relevant during stage0, and other parts of
 # the build system take this into account.

mk/platform.mk

@@ -238,3 +238,72 @@ endef
 
 $(foreach target,$(CFG_TARGET), \
   $(eval $(call CFG_MAKE_TOOLCHAIN,$(target))))
+
+# These two environment variables are scraped by the `./configure` script and
+# are necessary for `cl.exe` to find standard headers (the INCLUDE variable) and
+# for `link.exe` to find standard libraries (the LIB variable).
+ifdef CFG_MSVC_INCLUDE_PATH
+export INCLUDE := $(CFG_MSVC_INCLUDE_PATH)
+endif
+ifdef CFG_MSVC_LIB_PATH
+export LIB := $(CFG_MSVC_LIB_PATH)
+endif
+
+# Unfortunately `link.exe` is also a program in `/usr/bin` on MinGW installs,
+# but it's not the one that we want. As a result we make sure that our detected
+# `link.exe` shows up in PATH first.
+ifdef CFG_MSVC_LINK
+export PATH := $(CFG_MSVC_ROOT)/VC/bin/amd64:$(PATH)
+endif
+
+# There are more comments about this available in the target specification for
+# Windows MSVC in the compiler, but the gist of it is that we use `llvm-ar.exe`
+# instead of `lib.exe` for assembling archives, so we need to inject this custom
+# dependency here.
+define ADD_LLVM_AR_TO_MSVC_DEPS
+ifeq ($$(findstring msvc,$(1)),msvc)
+NATIVE_TOOL_DEPS_core_T_$(1) += llvm-ar.exe
+INSTALLED_BINS_$(1) += llvm-ar.exe
+endif
+endef
+
+$(foreach target,$(CFG_TARGET), \
+  $(eval $(call ADD_LLVM_AR_TO_MSVC_DEPS,$(target))))
+
+# When working with MSVC on windows, each DLL needs to explicitly declare its
+# interface to the outside world through some means. The options for doing so
+# include:
+#
+# 1. A custom attribute on each function itself
+# 2. A linker argument saying what to export
+# 3. A file which lists all symbols that need to be exported
+#
+# The Rust compiler takes care (1) for us for all Rust code by annotating all
+# public-facing functions with dllexport, but we have a few native dependencies
+# which need to cross the DLL boundary. The most important of these dependencies
+# is LLVM which is linked into `rustc_llvm.dll` but primarily used from
+# `rustc_trans.dll`. This means that many of LLVM's C API functions need to be
+# exposed from `rustc_llvm.dll` to be forwarded over the boundary.
+#
+# Unfortunately, at this time, LLVM does not handle this sort of exportation on
+# Windows for us, so we're forced to do it ourselves if we want it (which seems
+# like the path of least resistance right now). To do this we generate a `.DEF`
+# file [1] which we then custom-pass to the linker when building the rustc_llvm
+# crate. This DEF file list all symbols that are exported from
+# `src/librustc_llvm/lib.rs` and is generated by a small python script.
+#
+# Fun times!
+#
+# [1]: https://msdn.microsoft.com/en-us/library/28d6s79h.aspx
+define ADD_RUSTC_LLVM_DEF_TO_MSVC
+ifeq ($$(findstring msvc,$(1)),msvc)
+RUSTFLAGS_rustc_llvm_T_$(1) += -C link-args="-DEF:$(1)/rt/rustc_llvm.def"
+CUSTOM_DEPS_rustc_llvm_T_$(1) += $(1)/rt/rustc_llvm.def
+
+$(1)/rt/rustc_llvm.def: $$(S)src/etc/mklldef.py $$(S)src/librustc_llvm/lib.rs
+	$$(CFG_PYTHON) $$^ $$@ rustc_llvm-$$(CFG_FILENAME_EXTRA)
+endif
+endef
+
+$(foreach target,$(CFG_TARGET), \
+  $(eval $(call ADD_RUSTC_LLVM_DEF_TO_MSVC,$(target))))

mk/rt.mk

@@ -53,9 +53,12 @@ NATIVE_DEPS_hoedown_$(1) := hoedown/src/autolink.c \
 NATIVE_DEPS_miniz_$(1) = miniz.c
 NATIVE_DEPS_rust_builtin_$(1) := rust_builtin.c \
 			rust_android_dummy.c
-NATIVE_DEPS_rustrt_native_$(1) := \
-			rust_try.ll \
-			arch/$$(HOST_$(1))/record_sp.S
+NATIVE_DEPS_rustrt_native_$(1) := arch/$$(HOST_$(1))/record_sp.S
+ifeq ($$(findstring msvc,$(1)),msvc)
+NATIVE_DEPS_rustrt_native_$(1) += rust_try_msvc_64.ll
+else
+NATIVE_DEPS_rustrt_native_$(1) += rust_try.ll
+endif
 NATIVE_DEPS_rust_test_helpers_$(1) := rust_test_helpers.c
 NATIVE_DEPS_morestack_$(1) := arch/$$(HOST_$(1))/morestack.S
 

src/librustc_trans/back/link.rs

@@ -1214,11 +1214,13 @@ fn add_upstream_rust_crates(cmd: &mut Linker, sess: &Session,
 
         // Just need to tell the linker about where the library lives and
         // what its name is
-        if let Some(dir) = cratepath.parent() {
+        let parent = cratepath.parent();
+        if let Some(dir) = parent {
             cmd.include_path(&fix_windows_verbatim_for_gcc(dir));
         }
         let filestem = cratepath.file_stem().unwrap().to_str().unwrap();
-        cmd.link_dylib(&unlib(&sess.target, filestem));
+        cmd.link_rust_dylib(&unlib(&sess.target, filestem),
+                            parent.unwrap_or(Path::new("")));
     }
 }
 

src/librustc_trans/back/linker.rs

@@ -11,6 +11,7 @@
 use std::ffi::OsString;
 use std::path::{Path, PathBuf};
 use std::process::Command;
+use std::fs;
 
 use rustc_back::archive;
 use session::Session;
@@ -25,6 +26,7 @@ use session::config;
 /// MSVC linker (e.g. `link.exe`) is being used.
 pub trait Linker {
     fn link_dylib(&mut self, lib: &str);
+    fn link_rust_dylib(&mut self, lib: &str, path: &Path);
     fn link_framework(&mut self, framework: &str);
     fn link_staticlib(&mut self, lib: &str);
     fn link_rlib(&mut self, lib: &Path);
@@ -67,6 +69,10 @@ impl<'a> Linker for GnuLinker<'a> {
     fn position_independent_executable(&mut self) { self.cmd.arg("-pie"); }
     fn args(&mut self, args: &[String]) { self.cmd.args(args); }
 
+    fn link_rust_dylib(&mut self, lib: &str, _path: &Path) {
+        self.cmd.arg("-l").arg(lib);
+    }
+
     fn link_framework(&mut self, framework: &str) {
         self.cmd.arg("-framework").arg(framework);
     }
@@ -189,6 +195,18 @@ impl<'a> Linker for MsvcLinker<'a> {
     fn link_dylib(&mut self, lib: &str) {
         self.cmd.arg(&format!("{}.lib", lib));
     }
+
+    fn link_rust_dylib(&mut self, lib: &str, path: &Path) {
+        // When producing a dll, the MSVC linker may not actually emit a
+        // `foo.lib` file if the dll doesn't actually export any symbols, so we
+        // check to see if the file is there and just omit linking to it if it's
+        // not present.
+        let name = format!("{}.lib", lib);
+        if fs::metadata(&path.join(&name)).is_ok() {
+            self.cmd.arg(name);
+        }
+    }
+
     fn link_staticlib(&mut self, lib: &str) {
         self.cmd.arg(&format!("{}.lib", lib));
     }

src/librustc_trans/trans/cleanup.rs

@@ -856,18 +856,36 @@ impl<'blk, 'tcx> CleanupHelperMethods<'blk, 'tcx> for FunctionContext<'blk, 'tcx
         // this function, so we just codegen a generic reference to it. We don't
         // specify any of the types for the function, we just make it a symbol
         // that LLVM can later use.
+        //
+        // Note that MSVC is a little special here in that we don't use the
+        // `eh_personality` lang item at all. Currently LLVM has support for
+        // both Dwarf and SEH unwind mechanisms for MSVC targets and uses the
+        // *name of the personality function* to decide what kind of unwind side
+        // tables/landing pads to emit. It looks like Dwarf is used by default,
+        // injecting a dependency on the `_Unwind_Resume` symbol for resuming
+        // an "exception", but for MSVC we want to force SEH. This means that we
+        // can't actually have the personality function be our standard
+        // `rust_eh_personality` function, but rather we wired it up to the
+        // CRT's custom `__C_specific_handler` personality funciton, which
+        // forces LLVM to consider landing pads as "landing pads for SEH".
+        let target = &self.ccx.sess().target.target;
         let llpersonality = match pad_bcx.tcx().lang_items.eh_personality() {
-            Some(def_id) => {
+            Some(def_id) if !target.options.is_like_msvc => {
                 callee::trans_fn_ref(pad_bcx.ccx(), def_id, ExprId(0),
                                      pad_bcx.fcx.param_substs).val
             }
-            None => {
+            _ => {
                 let mut personality = self.ccx.eh_personality().borrow_mut();
                 match *personality {
                     Some(llpersonality) => llpersonality,
                     None => {
+                        let name = if target.options.is_like_msvc {
+                            "__C_specific_handler"
+                        } else {
+                            "rust_eh_personality"
+                        };
                         let fty = Type::variadic_func(&[], &Type::i32(self.ccx));
-                        let f = declare::declare_cfn(self.ccx, "rust_eh_personality", fty,
+                        let f = declare::declare_cfn(self.ccx, name, fty,
                                                      self.ccx.tcx().types.i32);
                         *personality = Some(f);
                         f