diff --git a/cranelift/codegen/src/verifier/mod.rs b/cranelift/codegen/src/verifier/mod.rs
index 1048248a28..30d14bb62d 100644
--- a/cranelift/codegen/src/verifier/mod.rs
+++ b/cranelift/codegen/src/verifier/mod.rs
@@ -61,9 +61,9 @@ use crate::dbg::DisplayList;
 use crate::dominator_tree::DominatorTree;
 use crate::entity::SparseSet;
 use crate::flowgraph::{BlockPredecessor, ControlFlowGraph};
-use crate::ir;
 use crate::ir::entities::AnyEntity;
 use crate::ir::instructions::{BranchInfo, CallInfo, InstructionFormat, ResolvedConstraint};
+use crate::ir::{self, ArgumentExtension};
 use crate::ir::{
     types, ArgumentPurpose, Block, Constant, DynamicStackSlot, FuncRef, Function, GlobalValue,
     Inst, JumpTable, MemFlags, Opcode, SigRef, StackSlot, Type, Value, ValueDef, ValueList,
@@ -1648,45 +1648,57 @@ impl<'a> Verifier<'a> {
     }
 
     fn typecheck_function_signature(&self, errors: &mut VerifierErrors) -> VerifierStepResult<()> {
-        self.func
+        let params = self
+            .func
             .signature
             .params
             .iter()
             .enumerate()
-            .filter(|(_, &param)| param.value_type == types::INVALID)
-            .for_each(|(i, _)| {
+            .map(|p| (true, p));
+        let returns = self
+            .func
+            .signature
+            .returns
+            .iter()
+            .enumerate()
+            .map(|p| (false, p));
+
+        for (is_argument, (i, param)) in params.chain(returns) {
+            let is_return = !is_argument;
+            let item = if is_argument {
+                "Parameter"
+            } else {
+                "Return value"
+            };
+
+            if param.value_type == types::INVALID {
                 errors.report((
                     AnyEntity::Function,
-                    format!("Parameter at position {} has an invalid type", i),
+                    format!("{item} at position {i} has an invalid type"),
                 ));
-            });
+            }
 
-        self.func
-            .signature
-            .returns
-            .iter()
-            .enumerate()
-            .filter(|(_, &ret)| ret.value_type == types::INVALID)
-            .for_each(|(i, _)| {
-                errors.report((
-                    AnyEntity::Function,
-                    format!("Return value at position {} has an invalid type", i),
-                ))
-            });
-
-        self.func
-            .signature
-            .returns
-            .iter()
-            .enumerate()
-            .for_each(|(i, ret)| {
-                if let ArgumentPurpose::StructArgument(_) = ret.purpose {
+            if let ArgumentPurpose::StructArgument(_) = param.purpose {
+                if is_return {
                     errors.report((
                         AnyEntity::Function,
-                        format!("Return value at position {} can't be an struct argument", i),
+                        format!("{item} at position {i} can't be an struct argument"),
                     ))
                 }
-            });
+            }
+
+            let ty_allows_extension = param.value_type.is_int();
+            let has_extension = param.extension != ArgumentExtension::None;
+            if !ty_allows_extension && has_extension {
+                errors.report((
+                    AnyEntity::Function,
+                    format!(
+                        "{} at position {} has invalid extension {:?}",
+                        item, i, param.extension
+                    ),
+                ));
+            }
+        }
 
         if errors.has_error() {
             Err(())
diff --git a/cranelift/filetests/filetests/verifier/argument-extension.clif b/cranelift/filetests/filetests/verifier/argument-extension.clif
new file mode 100644
index 0000000000..198a758f82
--- /dev/null
+++ b/cranelift/filetests/filetests/verifier/argument-extension.clif
@@ -0,0 +1,26 @@
+test verifier
+
+function %float_with_sext(f32 sext) -> f32 { ; error: Parameter at position 0 has invalid extension Sext
+block0(v0: f32):
+    return v0
+}
+
+function %float_with_uext(f32 uext) -> f32 { ; error: Parameter at position 0 has invalid extension Uext
+block0(v0: f32):
+    return v0
+}
+
+function %float_ret_with_sext(f32) -> f32 sext { ; error: Return value at position 0 has invalid extension Sext
+block0(v0: f32):
+    return v0
+}
+
+function %float_ret_with_uext(f32) -> f32 uext { ; error: Return value at position 0 has invalid extension Uext
+block0(v0: f32):
+    return v0
+}
+
+function %simd_ext(i32x4 sext) -> i32x4 { ; error: Parameter at position 0 has invalid extension Sext
+block0(v0: i32x4):
+    return v0
+}
diff --git a/cranelift/fuzzgen/src/function_generator.rs b/cranelift/fuzzgen/src/function_generator.rs
index 2888cbca97..d5b51c5c59 100644
--- a/cranelift/fuzzgen/src/function_generator.rs
+++ b/cranelift/fuzzgen/src/function_generator.rs
@@ -1230,10 +1230,14 @@ where
         let value_type = self.generate_type()?;
         // TODO: There are more argument purposes to be explored...
         let purpose = ArgumentPurpose::Normal;
-        let extension = match self.u.int_in_range(0..=2)? {
-            2 => ArgumentExtension::Sext,
-            1 => ArgumentExtension::Uext,
-            _ => ArgumentExtension::None,
+        let extension = if value_type.is_int() {
+            *self.u.choose(&[
+                ArgumentExtension::Sext,
+                ArgumentExtension::Uext,
+                ArgumentExtension::None,
+            ])?
+        } else {
+            ArgumentExtension::None
         };
 
         Ok(AbiParam {