Shoddy Compilers, Part 2

Some may remember the original shoddy compilers post from December. This is basically more of the same.

I mean, people. With this code:

int foo(void) {
  const int i = 42;
  return i + 3;

What compiler doesn't just optimise the body away to "return 45" when I ask for optimised code? Well, one which also doesn't optimise away the test in do {...} while(0), that is the answer.

This compiler has a positive plethora of optimisation options: you can optimise for either speed or size, from either level 0 to level 9. Set to size optimisation level 9, the generated code goes something like:

Grow stack by 2 bytes
Put 42 in R16
Put 0 in R17
Store R16 via Z (stack pointer)
Store R17 via Z+
Put 45 in R16
Shrink stack by 2 bytes
Return (R16/R17 pair is the return value)

Nothing quite like 6 wasted instructions out of 7 on a <4MHz processor... When people moan about GCC not doing the right thing, they don't know the meaning of "broken compiler". Amusingly the compiler didn't generate the code to clear R17 to 0, so it either noticed that had already happened, or the code generator is even worse than I thought. I'm not sure what case to believe in.

NP: When It Falls, Zero 7.

21:19 Saturday, 18 Feb 2012 [#] [computers] ( comments)