Converting bits from one array to another?

Question

I am building a library for this vacuum fluorescent display. Its a very simple interface and I have all the features working.

The problem I am having now is that I am trying to make the code as compact as possable, but the custom character loading is not intuitive. That is the bitmap for the font maps to completely different bits and bytes to the display itself. From the IEE VFD datasheet, when you scroll down you see that the bits are mapped all over the place.

The code I have so far works like so:

// input the font bitmap, the bit from that line of the bitmap and the bit it needs to go to
static unsigned char VFD_CONVERT(const unsigned char* font, unsigned char from, unsigned char to) {
return ((*font >> from) & 0x01) << to;
//return (*font & (1 << from)) ? (1<<to) : 0;
}
// macros to make it easyer to read and see
#define CM_01 font+0, 4
#define CM_02 font+0, 3
#define CM_03 font+0, 2
#define CM_04 font+0, 1
#define CM_05 font+0, 0

// One of the 7 lines I have to send
   o = VFD_CONVERT(CM_07,6) | VFD_CONVERT(CM_13,5) | VFD_CONVERT(CM_30,4) | VFD_CONVERT(CM_23,3) | VFD_CONVERT(CM_04,2) | VFD_CONVERT(CM_14,1) | VFD_CONVERT(CM_33,0);
send(o);

This is oviously not all the code. You can see the rest over my Google code repository but it should give you some idea what I am doing.

So the question I have is if there is a better way to optimize this or do the translation?

Changing the return statement on VFD_CONVERT makes GCC go crazy (-O1, -O2, -O3, and -Os does it) and expands the code to 1400 bytes. If I use the return statement with the inline if, it reduces it to 800 bytes. I have been going though the asm generated statements and current I am tempted to just write it all in asm as I am starting to think the compiler doesn't know what it is doing. However I thought maybe its me and I don't know what I am doing and so it confuses the compiler.

As a side note, the code there works, both return statements upload the custom character and it gets displayed (with a weird bug where I have to send it twice, but that's a separate issue).

Answer 1

First of all, you should file a bug report against gcc with a minimal example, since -Os should never generate larger code than -O0. Then, I suggest storing the permutation in a table, like this

const char[][] perm = {{ 7, 13, 30, 23,  4, 14, 33}, ...

with special values indicating a fixed zero or one bit. That'll also make your code more readable.