There's always an edge case - Program all the things!

22 October 2013

This post is a follow-up to Your fla- Your flash is now clean. An updated, improved version of the modified PSTR() macro is at the bottom of this page.

I solicited feedback in the Arduino forums to my de-duplicating PSTR() macro replacement described in last Sunday’s Your fla- Your flash is now clean post. It didn’t take long until someone had found an edge case where my construct broke the build.

Darryl points out that the following (perfectly reasonable) use of the updated PSTR() macro (via the F() macro) makes the compiler barf:

Serial.println(F("%"));

This innocuous line causes the compiler to say this:

PSTR.ino: In function 'void setup()':
PSTR.ino:15:6: error: invalid 'asm': invalid %-code

Uhh. Well, darn.

In hindsight, that’s not so very surprising, though. Given "%", the updated PSTR() macro expands to something like this:

PGM_P ptr;
asm volatile
(
  ".pushsection .progmem.data, \"SM\", @progbits, 1" "\n\t"
  "PSTR%=: .string \"%\""  /* oops! */               "\n\t"
  ".popsection"                                      "\n\t"
  "ldi %A0, lo8(PSTR%=)"                             "\n\t"
  "ldi %B0, hi8(PSTR%=)"                             "\n\t"
  : "=d" (ptr)
);

Can you see it? The %\" next to the .string directive looks a lot like those %A0 and %B0 and %= placeholders, except it doesn’t. It’s a good thing Darryl didn’t use a string like “£2×50%=£1”, or he’d have ended up with gibberish instead of a proper compiler error.

The good news is that GCC’s asm directive only attempts to interpret “%-codes” if any outputs are declared.

Of course, I do need outputs to get the string address out. But I can split the asm statement into two consecutive ones, the first one defining the string, the second one getting its address.

There’s a problem with that, too, though, because now I can’t use the %= construct to generate a unique label for the string. (And %= would resolve to different values in those two asm blocks anyway.) But fortunately, the assembler has a concept of local labels that seems to work even in my case.

Long story short, here’s an updated version that works even in the F("%") case:

#define PSTR(str) \
  (__extension__({ \
    PGM_P ptr;  \
    asm volatile \
    ( \
      ".pushsection .progmem.data, \"SM\", @progbits, 1" "\n\t" \
      "0: .string " #str                                 "\n\t" \
      ".popsection"                                      "\n\t" \
    ); \
    asm volatile \
    ( \
      "ldi %A0, lo8(0b)"                                 "\n\t" \
      "ldi %B0, hi8(0b)"                                 "\n\t" \
      : "=d" (ptr) \
    ); \
    ptr; \
  }))

Side note: I tested this with some statements like these:

Serial.print(F("%"));
Serial.print(F("%%"));
Serial.print(F("%%%"));

and it turns out the linker actually cleverly merges those three strings into each other. In the resulting ELF file, only “%%%” remains, and “%” and “%%” end up pointing into that string at the appropriate offset. Nice!

arduino 4