diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index b8be612fbe69..b8ba6db9aa80 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -3,7 +3,8 @@
 use std::{assert_matches, iter, ptr};
 
 use rustc_abi::{
-    Align, BackendRepr, Float, HasDataLayout, NumScalableVectors, Primitive, Size, WrappingRange,
+    Align, BackendRepr, Float, HasDataLayout, Integer, NumScalableVectors, Primitive, Size,
+    WrappingRange,
 };
 use rustc_codegen_ssa::base::{compare_simd_types, wants_msvc_seh, wants_wasm_eh};
 use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
@@ -288,10 +289,17 @@ fn codegen_intrinsic_call(
                     bug!("the va_arg intrinsic does not support non-scalar types")
                 };
 
+                // We reject types that would never be passed as varargs in C because
+                // they get promoted to a larger type, specifically integers smaller than
+                // c_int and float type smaller than c_double.
                 match scalar.primitive() {
                     Primitive::Pointer(_) => {
                         // Pointers are always OK.
-                        emit_va_arg(self, args[0], result.layout.ty)
+                    }
+                    Primitive::Int(Integer::I128, _) => {
+                        // FIXME: maybe we should support these? At least on 32-bit powerpc
+                        // the logic in LLVM does not handle i128 correctly though.
+                        bug!("the va_arg intrinsic does not support `i128`/`u128`")
                     }
                     Primitive::Int(..) => {
                         let int_width = self.cx().size_of(result.layout.ty).bits();
@@ -305,27 +313,26 @@ fn codegen_intrinsic_call(
                                 target_c_int_width
                             );
                         }
-                        emit_va_arg(self, args[0], result.layout.ty)
                     }
                     Primitive::Float(Float::F16) => {
                         bug!("the va_arg intrinsic does not support `f16`")
                     }
                     Primitive::Float(Float::F32) => {
-                        if self.cx().sess().target.arch == Arch::Avr {
-                            // c_double is actually f32 on avr.
-                            emit_va_arg(self, args[0], result.layout.ty)
-                        } else {
+                        // c_double is actually f32 on avr.
+                        if self.cx().sess().target.arch != Arch::Avr {
                             bug!("the va_arg intrinsic does not support `f32` on this target")
                         }
                     }
                     Primitive::Float(Float::F64) => {
                         // 64-bit floats are always OK.
-                        emit_va_arg(self, args[0], result.layout.ty)
                     }
                     Primitive::Float(Float::F128) => {
+                        // FIXME(f128) figure out whether we should support this.
                         bug!("the va_arg intrinsic does not support `f128`")
                     }
                 }
+
+                emit_va_arg(self, args[0], result.layout.ty)
             }
 
             sym::volatile_load | sym::unaligned_volatile_load => {