[Mspgcc-users] Tip for reading generated assembly - look out for partial inlining

[David B. <da...@we...>]

I spent a number of hours in panic yesterday while reading through
assembly listings generated by the latest msp430 gcc on my code.  My
issue was with functions like this:

static bool sleeping;
static void checkWake(void) {
	if (sleeping) {
		// Send "wakeup" command on ports
		sleeping = false;
	}
}

When I looked at the generated assembly, it seemed that the compiler was
ignoring the "if (sleeping)" check - the "checkWake" assembly code went
straight to the code for sending the command out on the ports, followed
by setting "sleeping = false".

It turns out the compiler is /not/ ignoring the conditional and
generating wrong code - it is just being smarter than previous gcc
versions.  The "if (sleeping)" part is being inlined into the caller
function, while the body of the conditional is kept in it's own
function.  On closer inspection, the assembly code is labelled
"checkWake.part.0" rather than "checkWake" to make this clear.

In many cases, such partial inlining can improve the speed of the code,
especially for when the conditional is not taken.  It can also improve
the size of the code, especially if the caller function in this case
already has "sleeping" in a register.

But it can certainly lead to confusion while debugging or checking the
generated assembly!

mvh.,

David