[lldb/testsuite] Fix TestInlineStepping on arm64 with newer compilers

[friss]

Summary

TestInlineStepping tests LLDB's ability to step in the presence of
inline frames. The testcase source has a number of functions and some
of them are marked always_inline.

The test is built around the assumption that the inline function will
be fully represented once inlined, but this is not true with the
current arm64 code generation. For example:

void caller() {

always_inline_function(); // Step here

}

When stppeing into caller() above, you might immediatly end up in
the inlines frame for always_inline_function(), because there might
literally be no code associated with caller() itself.

This patch hacks around the issue by adding an asm volatile("nop")
on some lines with inlined calls where we expect to be able to
step. Like so:

void caller() {

asm volatile("nop"); always_inline_function(); // Step here

}

This guarantees there is always going to be one instruction for this
line in the caller.

Any other (better) idea how to deal with this?

[labath]

This doesn't sound too bad. I am not sure if "nop" is a valid opcode on all targets, but if it isn't we could try hiding it in a macro, or replace it with an increment of a volatile/atomic variable or a call to a noinline function (the latter would probably require changes in test expectations.

lldb/test/API/functionalities/inline-stepping/calling.cpp

 {
     caller_trivial_2(); // In caller_trivial_1.
     inline_value += 1; 
 }
 
 void
 caller_trivial_2 ()
 {
-    inline_trivial_1 (); // In caller_trivial_2.
+    asm volatile ("nop"); inline_trivial_1 (); // In caller_trivial_2.
     inline_value += 1;  // At increment in caller_trivial_2.
 }
 
 void
 called_by_inline_trivial ()
 {
     inline_value += 1; // In called_by_inline_trivial.
 }
 
 void
 inline_trivial_1 ()
 {
-    inline_trivial_2(); // In inline_trivial_1.
+    asm volatile ("nop"); inline_trivial_2(); // In inline_trivial_1.
     inline_value += 1;  // At increment in inline_trivial_1.
 }
 
 void
 inline_trivial_2 ()
 {
     inline_value += 1; // In inline_trivial_2.
     called_by_inline_trivial (); // At caller_by_inline_trivial in inline_trivial_2.