[clang] [CIR] Add CIRGen for cir.unreachable and cir.trap (PR #151363)

[Sirui Mu via cfe-commits]

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@@ -269,6 +270,22 @@ RValue CIRGenFunction::emitBuiltinExpr(const GlobalDecl &gd, unsigned builtinID,
   case Builtin::BI__builtin_rotateright32:
   case Builtin::BI__builtin_rotateright64:
     return emitRotate(e, /*isRotateLeft=*/false);
+
+  case Builtin::BI__builtin_trap: {
+    builder.create<cir::TrapOp>(loc);
+    // Note that cir.trap is a terminator so we need to start a new dummy block
+    // to preserve the builder's insertion point.
+    builder.createBlock(builder.getBlock()->getParent());
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Lancern wrote:

> Also is this really needed? Classic codegen adds additional block only for unreachable.

In the classic CodeGen, calls to `__builtin_unreachable` gets lowered to the `unreachable` instruction, and calls to `__builtin_trap` gets lowered to an intrinsic call to `@llvm.trap()`. The former is a terminator instruction, which prehibits any further instructions from following it in the same block. The latter is not a terminator and it could be followed by more instructions in the same block (these instructions would be DCE-ed however). So in the classic CodeGen we don't have to start a new block after calling `__builtin_trap`.

On the contrary, in CIR world, both of `cir.unreachable` and `cir.trap` are terminators. Thus we have to start a new block after them to hold those instructions that follow them.

> Also it is unclear to me why this one is inlined while unreachable uses emitUnreachable as in classic codegen. We should probably add emitTrap too.

`emitUnreachable` is there because we have a sanitifer for unreachable code. We don't have sanitizers for traps, so we just inlined the creation of `cir.trap`.

Maybe we could put `builder.createBlock` in `emitUnreachable` and `emitTrap` though.

https://github.com/llvm/llvm-project/pull/151363