My compiler is optimising something out that it shouldn't

Question

My compiler, MPLAB C30 (GCC v3.23) is not compiling this code:

 if(font_info.flags & FONT_LOWERCASE_ONLY) ch = tolower(ch); if(font_info.flags & FONT_UPPERCASE_ONLY) ch = toupper(ch);

It produces no assembly output (somehow optimising it out) and I can't figure out why.

I have defined everything correctly as far as I can see:

#define FONT_LOWERCASE_ONLY 1 #define FONT_UPPERCASE_ONLY 2 struct FontEntry { int id; unsigned char width, height; const char *name; const char *lookup; const char *data; int flags; }; struct FontEntry fonts[NUM_FONTS + 1] = { { 0, 8, 14, "Outlined8x14", &font_lookup_outlined8x14, &font_data_outlined8x14, 0 }, { 1, 8, 8, "Outlined8x8", &font_lookup_outlined8x8, &font_data_outlined8x8, FONT_UPPERCASE_ONLY }, { 2, 8, 8, "Tiny5x5", 0, 0, 0 }, // not yet implemented { -1 } // ends font table };

The function I am using is:

void write_char(char ch, unsigned int x, unsigned int y, int flags, int font) { int i, yy, addr_temp, row, row_temp, xshift; uint16_t and_mask, or_mask, level_bits; struct FontEntry font_info; char lookup; fetch_font_info(ch, font, &font_info, &lookup); // ... }

The definition of fetch_font_info:

int fetch_font_info(char ch, int font, struct FontEntry *font_info, char *lookup) { // First locate the font struct. if(font > SIZEOF_ARRAY(fonts)) return 0; // font does not exist, exit. // Load the font info; IDs are always sequential. *font_info = fonts[font]; // Locate character in font lookup table. (If required.) if(lookup != NULL) { *lookup = font_info->lookup[ch]; if(lookup == 0xff) return 0; // character doesn't exist, don't bother writing it. } return 1; }

What am I doing wrong?

What are you doing with ch after computing it? If you're throwing the value away, then the compiler is correct to optimize-out the code. — R.. GitHub STOP HELPING ICE
– R.. GitHub STOP HELPING ICE, Commented Dec 4, 2010 at 15:43
@R.. The ch bits are manipulated, the position of the character data is located and then read out; the code then goes on to output it on a display device. — Thomas O
– Thomas O, Commented Dec 4, 2010 at 16:05

Andrew White · Accepted Answer · 2010-12-04 15:01:41Z

Since FONT_LOWERCASE_ONLY and FONT_UPPERCASE_ONLY are non 0 then font_info.flags must always be 0 (or not have any 1s in the lower two bits). Compilers can be clever about how they evaluate "constants" even if you don't define them as such.

I see that your fonts array has a few 0s in the flag section so I am betting you have a hard coded reference to one of those entries at compile time.

Yes, I was thinking along those lines too. Something like static const FontEntry font_info = fonts[2]
I'm not so sure. I am using font #1, the one which has FONT_UPPERCASE_ONLY set in the flags field. So it should be compiling it in.

Michael Burr · Accepted Answer · 2010-12-04 17:49:41Z

Is it possible you're acting on the return code from fetch_font_info() in write_char() (it's hard to know since the write_char() you've posted isn't what you're really compiling; some editing has happened)?

If you're acting on the return from fetch_font_info() then the problem might be cuased by the bug in testing the character pointed to by lookup:

if(lookup == 0xff)

should be

if(*lookup == 0xff)

with the bugged test, fetch_font_info() will always return 0 if lookup is non-null.

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