Add LLVM backend support for funnel shifts

Author: sayantn

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -383,7 +383,9 @@ impl<'ll, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
             | sym::rotate_left
             | sym::rotate_right
             | sym::saturating_add
-            | sym::saturating_sub => {
+            | sym::saturating_sub
+            | sym::unchecked_funnel_shl
+            | sym::unchecked_funnel_shr => {
                 let ty = args[0].layout.ty;
                 if !ty.is_integral() {
                     tcx.dcx().emit_err(InvalidMonomorphization::BasicIntegerType {
@@ -437,6 +439,19 @@ impl<'ll, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
 
                         self.call_intrinsic(llvm_name, &[llty], &[val, val, raw_shift])
                     }
+                    sym::unchecked_funnel_shl | sym::unchecked_funnel_shr => {
+                        let is_left = name == sym::unchecked_funnel_shl;
+                        let lhs = args[0].immediate();
+                        let rhs = args[1].immediate();
+                        let raw_shift = args[2].immediate();
+                        let llvm_name = format!("llvm.fsh{}", if is_left { 'l' } else { 'r' });
+
+                        // llvm expects shift to be the same type as the values, but rust
+                        // always uses `u32`.
+                        let raw_shift = self.intcast(raw_shift, self.val_ty(lhs), false);
+
+                        self.call_intrinsic(llvm_name, &[llty], &[lhs, rhs, raw_shift])
+                    }
                     sym::saturating_add | sym::saturating_sub => {
                         let is_add = name == sym::saturating_add;
                         let lhs = args[0].immediate();

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -449,6 +449,9 @@ pub(crate) fn check_intrinsic_type(
         }
         sym::unchecked_shl | sym::unchecked_shr => (2, 0, vec![param(0), param(1)], param(0)),
         sym::rotate_left | sym::rotate_right => (1, 0, vec![param(0), tcx.types.u32], param(0)),
+        sym::unchecked_funnel_shl | sym::unchecked_funnel_shr => {
+            (1, 0, vec![param(0), param(0), tcx.types.u32], param(0))
+        }
         sym::unchecked_add | sym::unchecked_sub | sym::unchecked_mul => {
             (1, 0, vec![param(0), param(0)], param(0))
         }

compiler/rustc_span/src/symbol.rs

@@ -2269,6 +2269,8 @@ symbols! {
         unboxed_closures,
         unchecked_add,
         unchecked_div,
+        unchecked_funnel_shl,
+        unchecked_funnel_shr,
         unchecked_mul,
         unchecked_rem,
         unchecked_shl,