Reading the direction off a subtract
An addition written the wrong way round still gives the right answer. A subtraction does not, so the encoding gcc picks carries more information.
The two-operand form takes the destination as the left side. Here is x - y:
0 sub r0, r1
2 bx lr
r0 = r0 - r1. Nothing surprising. Now the other direction, y - x:
0 sub r0, r1, r0
2 bx lr
r0 = r1 - r0. ARM's full instruction set has a reverse subtract, rsb, for exactly this case. Thumb dropped it, so gcc uses the three-operand sub instead and lets the destination appear again as the right-hand source.
That gives you a rule you will use constantly:
sub rD, rN, rM subtracts rM from rN. The middle register is the minuend.- When
rD and rM are the same register, the compiler is subtracting the value it has been building from something else. Whatever is in the middle came from further left in the source expression.
The same reading works when one side is a partial result rather than an argument. sub r1, r3, r1 takes whatever r1 has been accumulating and subtracts it from the value sitting in r3, and everything after it operates on that difference.
Work through your target one line at a time, keeping track of what r0 holds after each, and the expression falls out in source order.
Your task
Write func_08037a44 to reproduce the target assembly.