Improve assembly test for CMSE ABIs

This ensures the code-gen for these ABIs does not change silently.

Co-authored-by: Folkert <folkert@folkertdev.nl>

Author: tdittr

tests/assembly/cmse.rs

@@ -1,6 +1,9 @@
+//@ revisions: hard soft
 //@ assembly-output: emit-asm
-//@ compile-flags: --target thumbv8m.main-none-eabi --crate-type lib -Copt-level=1
-//@ needs-llvm-components: arm
+//@ [hard] compile-flags: --target thumbv8m.main-none-eabihf --crate-type lib -Copt-level=1
+//@ [soft] compile-flags: --target thumbv8m.main-none-eabi --crate-type lib -Copt-level=1
+//@ [hard] needs-llvm-components: arm
+//@ [soft] needs-llvm-components: arm
 #![crate_type = "lib"]
 #![feature(abi_c_cmse_nonsecure_call, cmse_nonsecure_entry, no_core, lang_items)]
 #![no_core]
@@ -10,14 +13,85 @@ pub trait Sized {}
 pub trait Copy {}
 
 // CHECK-LABEL: __acle_se_entry_point
-// CHECK: bxns
+// CHECK: entry_point:
+//
+// Write return argument (two registers since 64bit integer)
+// CHECK: movs r0, #0
+// CHECK: movs r1, #0
+//
+// If we are using hard-float:
+// * Check if the float registers were touched (bit 3 in CONTROL)
+// hard: mrs     r12, control
+// hard: tst.w   r12, #8
+// hard: beq     .LBB0_2
+//
+// * If touched clear all float registers (d0..=d7)
+// hard: vmov    d0, lr, lr
+// hard: vmov    d1, lr, lr
+// hard: vmov    d2, lr, lr
+// hard: vmov    d3, lr, lr
+// hard: vmov    d4, lr, lr
+// hard: vmov    d5, lr, lr
+// hard: vmov    d6, lr, lr
+// hard: vmov    d7, lr, lr
+//
+// * If touched clear FPU status register
+// hard: vmrs    r12, fpscr
+// hard: bic     r12, r12, #159
+// hard: bic     r12, r12, #4026531840
+// hard: vmsr    fpscr, r12
+// hard: .LBB0_2:
+//
+// Clear all other registers that might have been used
+// CHECK: mov r2, lr
+// CHECK: mov r3, lr
+// CHECK: mov r12, lr
+//
+// Clear the flags
+// CHECK: msr apsr_nzcvq, lr
+//
+// Branch back to non-secure side
+// CHECK: bxns lr
 #[no_mangle]
 pub extern "C-cmse-nonsecure-entry" fn entry_point() -> i64 {
     0
 }
 
+// NOTE for future codegen changes:
+// The specific register assignment is not important, however:
+// * all registers must be cleared before `blxns` is executed
+//     (either by writing arguments or any other value)
+// * the lowest bit on the address of the callee must be cleared
+// * the flags need to be overwritten
+// * `blxns` needs to be called with the callee address
+//     (with the lowest bit cleared)
+//
 // CHECK-LABEL: call_nonsecure
-// CHECK: blxns
+// All arguments are written to (writes r0..=r3 and r12)
+// CHECK: mov r12, r0
+// CHECK: movs r0, #0
+// CHECK: movs r1, #1
+// CHECK: movs r2, #2
+// CHECK: movs r3, #3
+//
+// Lowest bit gets cleared on callee address
+// CHECK: bic r12, r12, #1
+//
+// Ununsed registers get cleared (r4..=r11)
+// CHECK: mov r4, r12
+// CHECK: mov r5, r12
+// CHECK: mov r6, r12
+// CHECK: mov r7, r12
+// CHECK: mov r8, r12
+// CHECK: mov r9, r12
+// CHECK: mov r10, r12
+// CHECK: mov r11, r12
+//
+// Flags get cleared
+// CHECK: msr apsr_nzcvq, r12
+//
+// Call to non-secure
+// CHECK: blxns r12
 #[no_mangle]
 pub fn call_nonsecure(
     f: unsafe extern "C-cmse-nonsecure-call" fn(u32, u32, u32, u32) -> u64,