diff --git a/cranelift/filetests/isa/x86/legalize-libcall.cton b/cranelift/filetests/isa/x86/legalize-libcall.cton
index d633bd4e46..fdad2a3cf2 100644
--- a/cranelift/filetests/isa/x86/legalize-libcall.cton
+++ b/cranelift/filetests/isa/x86/legalize-libcall.cton
@@ -3,6 +3,7 @@ test legalizer
 ; Pre-SSE 4.1, we need to use runtime library calls for floating point rounding operations.
 set is_64bit
 set is_pic
+set call_conv=system_v
 isa x86
 
 function %floor(f32) -> f32 {
@@ -11,6 +12,6 @@ ebb0(v0: f32):
     return v1
 }
 ; check: function %floor(f32 [%xmm0]) -> f32 [%xmm0] fast {
-; check: sig0 = (f32) -> f32 fast
+; check: sig0 = (f32 [%xmm0]) -> f32 [%xmm0] system_v
 ; check: fn0 = %FloorF32 sig0
 ; check: v1 = call fn0(v0)
diff --git a/cranelift/filetests/wasm/f32-arith.cton b/cranelift/filetests/wasm/f32-arith.cton
index d21316d293..7ce8eb4142 100644
--- a/cranelift/filetests/wasm/f32-arith.cton
+++ b/cranelift/filetests/wasm/f32-arith.cton
@@ -4,9 +4,15 @@ test compile
 set is_64bit=0
 isa x86 haswell
 
+set is_64bit=0
+isa x86 baseline
+
 set is_64bit=1
 isa x86 haswell
 
+set is_64bit=1
+isa x86 baseline
+
 ; Constants.
 
 function %f32_const() -> f32 {
diff --git a/cranelift/filetests/wasm/f64-arith.cton b/cranelift/filetests/wasm/f64-arith.cton
index 7b870ab2cb..150d9a0d8e 100644
--- a/cranelift/filetests/wasm/f64-arith.cton
+++ b/cranelift/filetests/wasm/f64-arith.cton
@@ -4,6 +4,9 @@ test compile
 set is_64bit=1
 isa x86 haswell
 
+set is_64bit=1
+isa x86 baseline
+
 ; Constants.
 
 function %f64_const() -> f64 {
diff --git a/cranelift/filetests/wasm/i32-arith.cton b/cranelift/filetests/wasm/i32-arith.cton
index 7fb388054f..f6ee971c69 100644
--- a/cranelift/filetests/wasm/i32-arith.cton
+++ b/cranelift/filetests/wasm/i32-arith.cton
@@ -4,9 +4,15 @@ test compile
 set is_64bit=0
 isa x86 haswell
 
+set is_64bit=0
+isa x86 baseline
+
 set is_64bit=1
 isa x86 haswell
 
+set is_64bit=1
+isa x86 baseline
+
 ; Constants.
 
 function %i32_const() -> i32 {
diff --git a/cranelift/filetests/wasm/i64-arith.cton b/cranelift/filetests/wasm/i64-arith.cton
index 6cf6f2e4a2..52016039e0 100644
--- a/cranelift/filetests/wasm/i64-arith.cton
+++ b/cranelift/filetests/wasm/i64-arith.cton
@@ -4,6 +4,9 @@ test compile
 set is_64bit=1
 isa x86 haswell
 
+set is_64bit=1
+isa x86 baseline
+
 ; Constants.
 
 function %i64_const() -> i64 {
diff --git a/lib/codegen/src/legalizer/boundary.rs b/lib/codegen/src/legalizer/boundary.rs
index 3c0a198f7f..4a865f2965 100644
--- a/lib/codegen/src/legalizer/boundary.rs
+++ b/lib/codegen/src/legalizer/boundary.rs
@@ -33,11 +33,9 @@ use std::vec::Vec;
 /// change the entry block arguments, calls, or return instructions, so this can leave the function
 /// in a state with type discrepancies.
 pub fn legalize_signatures(func: &mut Function, isa: &TargetIsa) {
-    isa.legalize_signature(&mut func.signature, true);
-    func.signature.compute_argument_bytes();
+    legalize_signature(&mut func.signature, true, isa);
     for sig_data in func.dfg.signatures.values_mut() {
-        isa.legalize_signature(sig_data, false);
-        sig_data.compute_argument_bytes();
+        legalize_signature(sig_data, false, isa);
     }
 
     if let Some(entry) = func.layout.entry_block() {
@@ -46,6 +44,20 @@ pub fn legalize_signatures(func: &mut Function, isa: &TargetIsa) {
     }
 }
 
+/// Legalize the libcall signature, which we may generate on the fly after
+/// `legalize_signatures` has been called.
+pub fn legalize_libcall_signature(signature: &mut Signature, isa: &TargetIsa) {
+    legalize_signature(signature, false, isa);
+}
+
+/// Legalize the given signature.
+///
+/// `current` is true if this is the signature for the current function.
+fn legalize_signature(signature: &mut Signature, current: bool, isa: &TargetIsa) {
+    isa.legalize_signature(signature, current);
+    signature.compute_argument_bytes();
+}
+
 /// Legalize the entry block parameters after `func`'s signature has been legalized.
 ///
 /// The legalized signature may contain more parameters than the original signature, and the
diff --git a/lib/codegen/src/legalizer/libcall.rs b/lib/codegen/src/legalizer/libcall.rs
index 70982da3cb..fb1ba6e313 100644
--- a/lib/codegen/src/legalizer/libcall.rs
+++ b/lib/codegen/src/legalizer/libcall.rs
@@ -3,6 +3,7 @@
 use ir;
 use ir::{get_libcall_funcref, InstBuilder};
 use isa::TargetIsa;
+use legalizer::boundary::legalize_libcall_signature;
 use std::vec::Vec;
 
 /// Try to expand `inst` as a library call, returning true is successful.
@@ -21,7 +22,10 @@ pub fn expand_as_libcall(inst: ir::Inst, func: &mut ir::Function, isa: &TargetIs
     let funcref = get_libcall_funcref(libcall, func, inst, isa);
     func.dfg.replace(inst).call(funcref, &args);
 
-    // TODO: ask the ISA to legalize the signature.
+    // Ask the ISA to legalize the signature.
+    let fn_data = &func.dfg.ext_funcs[funcref];
+    let sig_data = &mut func.dfg.signatures[fn_data.signature];
+    legalize_libcall_signature(sig_data, isa);
 
     true
 }