Don't polyfill i128 for compiler-builtins

Author: LegNeato

Cargo.lock

@@ -22,6 +22,7 @@ use rustc_middle::ty::layout::{
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_session::config::OptLevel;
 use rustc_span::Span;
+use rustc_span::def_id::LOCAL_CRATE;
 use rustc_target::callconv::FnAbi;
 use rustc_target::spec::{HasTargetSpec, Target};
 use tracing::{debug, trace};
@@ -136,7 +137,14 @@ macro_rules! imath_builder_methods {
     ($($self_:ident.$name:ident($($arg:ident),*) => $llvm_capi:ident => $op:block)+) => {
         $(fn $name(&mut $self_, $($arg: &'ll Value),*) -> &'ll Value {
             // Dispatch to i128 emulation when any operand is 128 bits wide.
-            if $($self_.is_i128($arg))||*
+            // When compiling compiler_builtins itself we must avoid emulation that recursively calls
+            // the same builtins, so fall back to LLVM's native operation.
+            if $self_.cx.tcx.is_compiler_builtins(LOCAL_CRATE) {
+                unsafe {
+                    trace!("binary expr (builtin crate): {:?} with args {:?}", stringify!($name), [$($arg),*]);
+                    llvm::$llvm_capi($self_.llbuilder, $($arg,)* UNNAMED)
+                }
+            } else if $($self_.is_i128($arg))||*
                 $op
             else {
                 unsafe {