Fix cross-compiling i686-pc-windows-gnu from Linux

This is still very rough and serves as a proof-of-concept for fixing
Linux -> 32-bit MinGW cross compilation workflow. Currently, clang and
GCC's MinGW targets both only support DW2 (DWARF) or SJLJ (Set Jump Long
Jump) unwinding on 32-bit Windows.

The default for GCC (and the way it is shipped on every major distro) is
to use SJLJ on Windows, as DWARF cannot traverse non-DWARF frames. This
would work fine, except for the fact that libgcc (our C runtime on the
MinGW platform) exports symbols under a different name when configured
to use SJLJ-style unwinding, and uses a preprocessor macro internally to
alias them.

Because of this, we have to detect this scenario and link to the correct
symbols ourselves. Linking has been tested with a full bootstrap on both
x86_64-unknown-linux-gnu and i686-pc-windows-gnu, as well as
cross-compilation of some of my own projects.

Obviously, the detection is a bit unrefined. Right now we
unconditionally use SJLJ when compiling Linux -> MinGW. I'd like to add
feature detection using compiler build flags or autotools-style
compilation and object analysis. Input on the best way to proceed here
is welcome.

Also, currently there is copy-pasted/duplicated code in libunwind.
Ideally, this could be reduced, but this would likely require a
rethinking of how iOS is special-cased above, to avoid further
duplication. Input on how to best structure this file is requested.

Author: neersighted

Diff for: src/bootstrap/compile.rs

@@ -150,6 +150,11 @@ pub fn std_cargo(builder: &Builder,
         cargo.env("MACOSX_DEPLOYMENT_TARGET", target);
     }
 
+    // FIXME: Temporary detection of SJLJ MinGW compilers.
+    if build.build.build.contains("linux") && target == "i686-pc-windows-gnu" {
+        features.push_str(" sjlj_eh");
+    }
+
     if builder.no_std(target) == Some(true) {
         // for no-std targets we only compile a few no_std crates
         cargo.arg("--features").arg("c mem")

Diff for: src/libstd/Cargo.toml

@@ -58,3 +58,6 @@ wasm_syscall = []
 # the environment for hooking up some thread-related information like the
 # current thread id and accessing/getting the current thread's TCB
 wasm-bindgen-threads = []
+
+# setjmp / longjmp exception handling for i686-pc-windows-gnu
+sjlj_eh = ["unwind/sjlj_eh"]

Diff for: src/libunwind/Cargo.toml

@@ -15,3 +15,6 @@ doc = false
 core = { path = "../libcore" }
 libc = { path = "../rustc/libc_shim" }
 compiler_builtins = { path = "../rustc/compiler_builtins_shim" }
+
+[features]
+sjlj_eh = []

Diff for: src/libunwind/libunwind.rs

@@ -84,7 +84,6 @@ pub type _Unwind_Exception_Cleanup_Fn = extern "C" fn(unwind_code: _Unwind_Reaso
                                                       exception: *mut _Unwind_Exception);
 extern "C" {
     #[unwind(allowed)]
-    pub fn _Unwind_Resume(exception: *mut _Unwind_Exception) -> !;
     pub fn _Unwind_DeleteException(exception: *mut _Unwind_Exception);
     pub fn _Unwind_GetLanguageSpecificData(ctx: *mut _Unwind_Context) -> *mut c_void;
     pub fn _Unwind_GetRegionStart(ctx: *mut _Unwind_Context) -> _Unwind_Ptr;
@@ -216,26 +215,49 @@ if #[cfg(all(any(target_os = "ios", target_os = "netbsd", not(target_arch = "arm
         pc
     }
 }
+} // cfg_if!
 
-if #[cfg(not(all(target_os = "ios", target_arch = "arm")))] {
-    // Not 32-bit iOS
+cfg_if! {
+if #[cfg(all(target_os = "ios", target_arch = "arm"))] {
+    // 32-bit iOS uses SjLj and does not provide _Unwind_Backtrace()
     extern "C" {
         #[unwind(allowed)]
-        pub fn _Unwind_RaiseException(exception: *mut _Unwind_Exception) -> _Unwind_Reason_Code;
-        pub fn _Unwind_Backtrace(trace: _Unwind_Trace_Fn,
-                                 trace_argument: *mut c_void)
-                                 -> _Unwind_Reason_Code;
+        pub fn _Unwind_Resume(exception: *mut _Unwind_Exception) -> !;
+        pub fn _Unwind_SjLj_RaiseException(e: *mut _Unwind_Exception) -> _Unwind_Reason_Code;
     }
-} else {
-    // 32-bit iOS uses SjLj and does not provide _Unwind_Backtrace()
+
+    #[inline]
+    pub unsafe fn _Unwind_RaiseException(exc: *mut _Unwind_Exception) -> _Unwind_Reason_Code {
+        _Unwind_SjLj_RaiseException(exc)
+    }
+
+} else if #[cfg(feature = "sjlj_eh")] {
     extern "C" {
         #[unwind(allowed)]
+        pub fn _Unwind_SjLj_Resume(e: *mut _Unwind_Exception) -> !;
         pub fn _Unwind_SjLj_RaiseException(e: *mut _Unwind_Exception) -> _Unwind_Reason_Code;
+        pub fn _Unwind_Backtrace(trace: _Unwind_Trace_Fn,
+                                 trace_argument: *mut c_void)
+                                 -> _Unwind_Reason_Code;
+    }
+
+    #[inline]
+    pub unsafe fn _Unwind_Resume(exc: *mut _Unwind_Exception) -> ! {
+        _Unwind_SjLj_Resume(exc)
     }
 
     #[inline]
     pub unsafe fn _Unwind_RaiseException(exc: *mut _Unwind_Exception) -> _Unwind_Reason_Code {
         _Unwind_SjLj_RaiseException(exc)
     }
+} else {
+    extern "C" {
+        #[unwind(allowed)]
+        pub fn _Unwind_Resume(exception: *mut _Unwind_Exception) -> !;
+        pub fn _Unwind_RaiseException(exception: *mut _Unwind_Exception) -> _Unwind_Reason_Code;
+        pub fn _Unwind_Backtrace(trace: _Unwind_Trace_Fn,
+                                 trace_argument: *mut c_void)
+                                 -> _Unwind_Reason_Code;
+    }
 }
 } // cfg_if!