Generate correct terminate block under Wasm EH

This fixes failing LLVM assertions during insnsel.

Partially fixes rust-lang#135665.

Author: purplesyringa

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1703,15 +1703,32 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             let mut cs_bx = Bx::build(self.cx, llbb);
             let cs = cs_bx.catch_switch(None, None, &[cp_llbb]);
 
-            // The "null" here is actually a RTTI type descriptor for the
-            // C++ personality function, but `catch (...)` has no type so
-            // it's null. The 64 here is actually a bitfield which
-            // represents that this is a catch-all block.
             bx = Bx::build(self.cx, cp_llbb);
             let null =
                 bx.const_null(bx.type_ptr_ext(bx.cx().data_layout().instruction_address_space));
-            let sixty_four = bx.const_i32(64);
-            funclet = Some(bx.catch_pad(cs, &[null, sixty_four, null]));
+
+            // The `null` in first argument here is actually a RTTI type
+            // descriptor for the C++ personality function, but `catch (...)`
+            // has no type so it's null.
+            let args = if base::wants_msvc_seh(self.cx.sess()) {
+                // This bitmask is a single `HT_IsStdDotDot` flag, which
+                // represents that this is a C++-style `catch (...)` block that
+                // only captures programmatic exceptions, not all SEH
+                // exceptions. The second `null` points to a non-existent
+                // `alloca` instruction, which an LLVM pass would inline into
+                // the initial SEH frame allocation.
+                let adjectives = bx.const_i32(0x40);
+                &[null, adjectives, null] as &[_]
+            } else {
+                // Specifying more arguments than necessary usually doesn't
+                // hurt, but the `WasmEHPrepare` LLVM pass does not recognize
+                // anything other than a single `null` as a `catch (...)` block,
+                // leading to problems down the line during instruction
+                // selection.
+                &[null] as &[_]
+            };
+
+            funclet = Some(bx.catch_pad(cs, args));
         } else {
             llbb = Bx::append_block(self.cx, self.llfn, "terminate");
             bx = Bx::build(self.cx, llbb);

tests/codegen/terminating-catchpad.rs

@@ -0,0 +1,34 @@
+//@ revisions: wasm seh
+//@[wasm] compile-flags: --target wasm32-unknown-emscripten -Z emscripten-wasm-eh
+//@[seh] compile-flags: --target x86_64-pc-windows-msvc
+//@[wasm] needs-llvm-components: webassembly
+//@[seh] needs-llvm-components: x86
+
+// Ensure a catch-all generates:
+// - `catchpad ... [ptr null]` on Wasm (otherwise LLVM gets confused)
+// - `catchpad ... [ptr null, i32 64, ptr null]` on Windows (otherwise we catch SEH exceptions)
+
+#![feature(no_core, lang_items, rustc_attrs)]
+#![crate_type = "lib"]
+#![no_std]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+unsafe extern "C-unwind" {
+    safe fn unwinds();
+}
+
+#[lang = "panic_cannot_unwind"]
+fn panic_cannot_unwind() -> ! {
+    loop {}
+}
+
+#[no_mangle]
+#[rustc_nounwind]
+pub fn doesnt_unwind() {
+    // wasm: %catchpad = catchpad within %catchswitch [ptr null]
+    // seh: %catchpad = catchpad within %catchswitch [ptr null, i32 64, ptr null]
+    unwinds();
+}