diff --git a/tests/c/fp_in_out.c b/tests/c/fp_in_out.c
new file mode 100644
index 000000000..ab43e0502
--- /dev/null
+++ b/tests/c/fp_in_out.c
@@ -0,0 +1,57 @@
+// Run-time:
+//   env-var: YKD_SERIALISE_COMPILATION=1
+//   env-var: YK_LOG=4
+//   stderr:
+//     yk-jit-event: start-tracing
+//     5.100000
+//     5.200000
+//     yk-jit-event: stop-tracing
+//     4.100000
+//     4.200000
+//     yk-jit-event: enter-jit-code
+//     3.100000
+//     3.200000
+//     2.100000
+//     2.200000
+//     yk-jit-event: deoptimise
+
+// Check that passing floats to/from a function works correctly.
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <yk.h>
+#include <yk_testing.h>
+
+__attribute__((yk_outline))
+float f_f(float x) {
+  return x + 1.1;
+}
+
+__attribute__((yk_outline))
+double f_d(double x) {
+  return x + 1.2;
+}
+
+int main(int argc, char **argv) {
+  YkMT *mt = yk_mt_new(NULL);
+  yk_mt_hot_threshold_set(mt, 0);
+  YkLocation loc = yk_location_new();
+
+  int res = 9998;
+  int i = 4;
+  NOOPT_VAL(loc);
+  NOOPT_VAL(res);
+  NOOPT_VAL(i);
+  while (i > 0) {
+    yk_mt_control_point(mt, &loc);
+    fprintf(stderr, "%f\n", f_f((float) i));
+    fprintf(stderr, "%f\n", f_d((double) i));
+    i--;
+  }
+  NOOPT_VAL(res);
+  yk_location_drop(loc);
+  yk_mt_shutdown(mt);
+  return (EXIT_SUCCESS);
+}
diff --git a/ykrt/src/compile/jitc_yk/codegen/x64/lsregalloc.rs b/ykrt/src/compile/jitc_yk/codegen/x64/lsregalloc.rs
index 529d5c6fd..91695d1ef 100644
--- a/ykrt/src/compile/jitc_yk/codegen/x64/lsregalloc.rs
+++ b/ykrt/src/compile/jitc_yk/codegen/x64/lsregalloc.rs
@@ -825,15 +825,13 @@ impl LSRegAlloc<'_> {
             match cnstr {
                 RegConstraint::InputIntoReg(_, reg)
                 | RegConstraint::InputIntoRegAndClobber(_, reg)
+                | RegConstraint::InputOutputIntoReg(_, reg)
                 | RegConstraint::OutputFromReg(reg)
                 | RegConstraint::Clobber(reg) => avoid.set(*reg),
                 RegConstraint::Input(_)
                 | RegConstraint::InputOutput(_)
                 | RegConstraint::Output
                 | RegConstraint::Temporary => {}
-                RegConstraint::InputOutputIntoReg(_, _) => {
-                    panic!();
-                }
             }
         }
 
@@ -870,11 +868,11 @@ impl LSRegAlloc<'_> {
                 },
                 RegConstraint::InputIntoReg(_, _)
                 | RegConstraint::InputIntoRegAndClobber(_, _)
+                | RegConstraint::InputOutputIntoReg(_, _)
                 | RegConstraint::Clobber(_) => {
                     // OPT: do the same trick as Input/InputOutput
                 }
                 RegConstraint::Output | RegConstraint::OutputFromReg(_) => (),
-                RegConstraint::InputOutputIntoReg(_op, _reg) => unreachable!(),
                 RegConstraint::Temporary => (),
             }
         }
@@ -888,20 +886,19 @@ impl LSRegAlloc<'_> {
                 RegConstraint::Input(op)
                 | RegConstraint::InputIntoReg(op, _)
                 | RegConstraint::InputIntoRegAndClobber(op, _)
+                | RegConstraint::InputOutputIntoReg(op, _)
                 | RegConstraint::InputOutput(op) => {
                     let reg = match x {
                         RegConstraint::Input(_) | RegConstraint::InputOutput(_) => {
                             self.assign_empty_fp_reg(asm, iidx, avoid)
                         }
                         RegConstraint::InputIntoReg(_, reg)
-                        | RegConstraint::InputIntoRegAndClobber(_, reg) => {
+                        | RegConstraint::InputIntoRegAndClobber(_, reg)
+                        | RegConstraint::InputOutputIntoReg(_, reg) => {
                             // OPT: Not everything needs spilling
                             self.spill_fp_if_not_already(asm, *reg);
                             *reg
                         }
-                        RegConstraint::InputOutputIntoReg(_, _) => {
-                            unreachable!()
-                        }
                         RegConstraint::Output
                         | RegConstraint::OutputFromReg(_)
                         | RegConstraint::Temporary
@@ -934,14 +931,12 @@ impl LSRegAlloc<'_> {
                             self.fp_regset.unset(reg);
                             RegState::Empty
                         }
-                        RegConstraint::InputOutput(_) => {
+                        RegConstraint::InputOutput(_) | RegConstraint::InputOutputIntoReg(_, _) => {
                             debug_assert!(!found_output);
                             found_output = true;
                             RegState::FromInst(iidx)
                         }
-                        RegConstraint::InputOutputIntoReg(_, _)
-                        | RegConstraint::Output
-                        | RegConstraint::OutputFromReg(_) => {
+                        RegConstraint::Output | RegConstraint::OutputFromReg(_) => {
                             unreachable!()
                         }
                         RegConstraint::Temporary => todo!(),
@@ -971,7 +966,6 @@ impl LSRegAlloc<'_> {
                     avoid.set(*reg);
                     out[i] = Some(*reg);
                 }
-                RegConstraint::InputOutputIntoReg(_, _) => unreachable!(),
                 RegConstraint::Temporary => {
                     let reg = self.assign_empty_fp_reg(asm, iidx, avoid);
                     self.fp_regset.unset(reg);