wasm exception handling

Author: mirkootter

compiler/rustc_codegen_llvm/src/context.rs

@@ -8,7 +8,7 @@ use crate::llvm_util;
 use crate::type_::Type;
 use crate::value::Value;
 
-use rustc_codegen_ssa::base::wants_msvc_seh;
+use rustc_codegen_ssa::base::{wants_msvc_seh, wants_wasm_eh};
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::base_n;
 use rustc_data_structures::fx::FxHashMap;
@@ -532,19 +532,24 @@ impl<'ll, 'tcx> MiscMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         if let Some(llpersonality) = self.eh_personality.get() {
             return llpersonality;
         }
+
+        let name = if wants_msvc_seh(self.sess()) {
+            Some("__CxxFrameHandler3")
+        } else if wants_wasm_eh(self.sess()) {
+            Some("__gxx_wasm_personality_v0")
+        } else {
+            None
+        };
+
         let tcx = self.tcx;
         let llfn = match tcx.lang_items().eh_personality() {
-            Some(def_id) if !wants_msvc_seh(self.sess()) => self.get_fn_addr(
+            Some(def_id) if name.is_none() => self.get_fn_addr(
                 ty::Instance::resolve(tcx, ty::ParamEnv::reveal_all(), def_id, ty::List::empty())
                     .unwrap()
                     .unwrap(),
             ),
             _ => {
-                let name = if wants_msvc_seh(self.sess()) {
-                    "__CxxFrameHandler3"
-                } else {
-                    "rust_eh_personality"
-                };
+                let name = name.unwrap_or("rust_eh_personality");
                 if let Some(llfn) = self.get_declared_value(name) {
                     llfn
                 } else {

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -7,7 +7,7 @@ use crate::type_of::LayoutLlvmExt;
 use crate::va_arg::emit_va_arg;
 use crate::value::Value;
 
-use rustc_codegen_ssa::base::{compare_simd_types, wants_msvc_seh};
+use rustc_codegen_ssa::base::{compare_simd_types, wants_msvc_seh, wants_wasm_eh};
 use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
 use rustc_codegen_ssa::errors::{ExpectedPointerMutability, InvalidMonomorphization};
 use rustc_codegen_ssa::mir::operand::OperandRef;
@@ -452,6 +452,8 @@ fn try_intrinsic<'ll>(
         bx.store(bx.const_i32(0), dest, ret_align);
     } else if wants_msvc_seh(bx.sess()) {
         codegen_msvc_try(bx, try_func, data, catch_func, dest);
+    } else if wants_wasm_eh(bx.sess()) {
+        codegen_wasm_try(bx, try_func, data, catch_func, dest);
     } else if bx.sess().target.os == "emscripten" {
         codegen_emcc_try(bx, try_func, data, catch_func, dest);
     } else {
@@ -610,6 +612,80 @@ fn codegen_msvc_try<'ll>(
     bx.store(ret, dest, i32_align);
 }
 
+// WASM's definition of the `rust_try` function.
+fn codegen_wasm_try<'ll>(
+    bx: &mut Builder<'_, 'll, '_>,
+    try_func: &'ll Value,
+    data: &'ll Value,
+    catch_func: &'ll Value,
+    dest: &'ll Value,
+) {
+    let (llty, llfn) = get_rust_try_fn(bx, &mut |mut bx| {
+        bx.set_personality_fn(bx.eh_personality());
+
+        let normal = bx.append_sibling_block("normal");
+        let catchswitch = bx.append_sibling_block("catchswitch");
+        let catchpad = bx.append_sibling_block("catchpad");
+        let caught = bx.append_sibling_block("caught");
+
+        let try_func = llvm::get_param(bx.llfn(), 0);
+        let data = llvm::get_param(bx.llfn(), 1);
+        let catch_func = llvm::get_param(bx.llfn(), 2);
+
+        // We're generating an IR snippet that looks like:
+        //
+        //   declare i32 @rust_try(%try_func, %data, %catch_func) {
+        //      %slot = alloca i8*
+        //      invoke %try_func(%data) to label %normal unwind label %catchswitch
+        //
+        //   normal:
+        //      ret i32 0
+        //
+        //   catchswitch:
+        //      %cs = catchswitch within none [%catchpad_rust] unwind to caller
+        //
+        //   catchpad:
+        //      %tok = catchpad within %cs [null]
+        //      %ptr = call @llvm.wasm.get.exception(token %tok)
+        //      %sel = call @llvm.wasm.get.ehselector(token %tok)
+        //      call %catch_func(%data, %ptr)
+        //      catchret from %tok to label %caught
+        //
+        //   caught:
+        //      ret i32 1
+        //   }
+        //
+        let try_func_ty = bx.type_func(&[bx.type_i8p()], bx.type_void());
+        bx.invoke(try_func_ty, None, None, try_func, &[data], normal, catchswitch, None);
+
+        bx.switch_to_block(normal);
+        bx.ret(bx.const_i32(0));
+
+        bx.switch_to_block(catchswitch);
+        let cs = bx.catch_switch(None, None, &[catchpad]);
+
+        bx.switch_to_block(catchpad);
+        let null = bx.const_null(bx.type_i8p());
+        let funclet = bx.catch_pad(cs, &[null]);
+
+        let ptr = bx.call_intrinsic("llvm.wasm.get.exception", &[funclet.cleanuppad()]);
+        let _sel = bx.call_intrinsic("llvm.wasm.get.ehselector", &[funclet.cleanuppad()]);
+
+        let catch_ty = bx.type_func(&[bx.type_i8p(), bx.type_i8p()], bx.type_void());
+        bx.call(catch_ty, None, None, catch_func, &[data, ptr], Some(&funclet));
+        bx.catch_ret(&funclet, caught);
+
+        bx.switch_to_block(caught);
+        bx.ret(bx.const_i32(1));
+    });
+
+    // Note that no invoke is used here because by definition this function
+    // can't panic (that's what it's catching).
+    let ret = bx.call(llty, None, None, llfn, &[try_func, data, catch_func], None);
+    let i32_align = bx.tcx().data_layout.i32_align.abi;
+    bx.store(ret, dest, i32_align);
+}
+
 // Definition of the standard `try` function for Rust using the GNU-like model
 // of exceptions (e.g., the normal semantics of LLVM's `landingpad` and `invoke`
 // instructions).

compiler/rustc_codegen_ssa/src/base.rs

@@ -357,6 +357,13 @@ pub fn cast_shift_expr_rhs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 }
 
+// Returns `true` if this session's target will use native wasm
+// exceptions. This means that the VM does the unwinding for
+// us
+pub fn wants_wasm_eh(sess: &Session) -> bool {
+    sess.target.is_like_wasm && sess.target.os != "emscripten"
+}
+
 /// Returns `true` if this session's target will use SEH-based unwinding.
 ///
 /// This is only true for MSVC targets, and even then the 64-bit MSVC target
@@ -366,6 +373,13 @@ pub fn wants_msvc_seh(sess: &Session) -> bool {
     sess.target.is_like_msvc
 }
 
+/// Returns `true` if this session's target requires the new exception
+/// handling LLVM IR instructions (catchpad / cleanuppad / ... instead
+/// of landingpad)
+pub fn wants_new_eh_instructions(sess: &Session) -> bool {
+    wants_wasm_eh(sess) || wants_msvc_seh(sess)
+}
+
 pub fn memcpy_ty<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     bx: &mut Bx,
     dst: Bx::Value,

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -79,8 +79,8 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
             lltarget = fx.landing_pad_for(target);
         }
         if is_cleanupret {
-            // MSVC cross-funclet jump - need a trampoline
-            debug_assert!(base::wants_msvc_seh(fx.cx.tcx().sess));
+            // Cross-funclet jump - need a trampoline
+            debug_assert!(base::wants_new_eh_instructions(fx.cx.tcx().sess));
             debug!("llbb_with_cleanup: creating cleanup trampoline for {:?}", target);
             let name = &format!("{:?}_cleanup_trampoline_{:?}", self.bb, target);
             let trampoline_llbb = Bx::append_block(fx.cx, fx.llfn, name);
@@ -177,9 +177,16 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
             mir::UnwindAction::Continue => None,
             mir::UnwindAction::Unreachable => None,
             mir::UnwindAction::Terminate => {
-                if fx.mir[self.bb].is_cleanup && base::wants_msvc_seh(fx.cx.tcx().sess) {
-                    // SEH will abort automatically if an exception tries to
+                if fx.mir[self.bb].is_cleanup && base::wants_new_eh_instructions(fx.cx.tcx().sess) {
+                    // MSVC SEH will abort automatically if an exception tries to
                     // propagate out from cleanup.
+
+                    // FIXME(@mirkootter): For wasm, we currently do not support terminate during
+                    // cleanup, because this requires a few more changes: The current code
+                    // caches the `terminate_block` for each function; funclet based code - however -
+                    // requires a different terminate_block for each funclet
+                    // Until this is implemented, we just do not unwind inside cleanup blocks
+
                     None
                 } else {
                     Some(fx.terminate_block())
@@ -1528,7 +1535,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     // FIXME(eddyb) rename this to `eh_pad_for_uncached`.
     fn landing_pad_for_uncached(&mut self, bb: mir::BasicBlock) -> Bx::BasicBlock {
         let llbb = self.llbb(bb);
-        if base::wants_msvc_seh(self.cx.sess()) {
+        if base::wants_new_eh_instructions(self.cx.sess()) {
             let cleanup_bb = Bx::append_block(self.cx, self.llfn, &format!("funclet_{:?}", bb));
             let mut cleanup_bx = Bx::build(self.cx, cleanup_bb);
             let funclet = cleanup_bx.cleanup_pad(None, &[]);

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -169,7 +169,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         start_bx.set_personality_fn(cx.eh_personality());
     }
 
-    let cleanup_kinds = base::wants_msvc_seh(cx.tcx().sess).then(|| analyze::cleanup_kinds(&mir));
+    let cleanup_kinds =
+        base::wants_new_eh_instructions(cx.tcx().sess).then(|| analyze::cleanup_kinds(&mir));
 
     let cached_llbbs: IndexVec<mir::BasicBlock, CachedLlbb<Bx::BasicBlock>> =
         mir.basic_blocks