[SNES] Is CMP #$00 necessary in this situation?

[Mauron]

Code Select Expand

LDA $CC0000,x
CMP #$00
BNE $0E

From my understanding, LDA will set the z flag if the value loaded is 0, and bne will branch if the z flag is clear, so the CMP is unnecessary.

I just wanted to make sure I'm right on this.

[justin3009]

Nope, I'm pretty sure it's completely unnecessary to have it there.  If it's supposed to break when the value isn't 00, then just remove the CMP and you're all set.

[Bregalad]

It is completely useless. Unless there is some kind of self-mod code going on, I see no reason why to do that.

[furrykef]

Probably the programmer was either new to 6502/65816 or was used to architectures like x86 and Z80 which don't set flags on register loads.

[magno]

Probably the programmer was either new to 6502/65816 or was used to architectures like x86 and Z80 which don't set flags on register loads.

Don't think so... It is far more likely that that was the result of some higher level language compiler. If the compiler finds

Code Select Expand

if ( var != 3 )

it converts it to

Code Select Expand

LDA $CC0000,x
CMP #$03
BNE $0E

And systematically does the same whatever the compared value.

Anyway, double-check that there isn't any branch or jump just before CMP #$00; it there is, removing that instruction will corrupt the game. This is an example:

Code Select Expand


LDA $CC0000,x
here_2
CMP #$00
BNE here_1
<do whatever...>
here_1
LSR A
BRA here_2


[Bregalad]

If the compiler finds [....] systematically does the same whatever the compared value.

Nope, peepholes optimizers are supposed to remove this kind of things.

Although bad code produced by a (badly configured ?) compiler could be a valid explaination.

[Revenant]

Nope, peepholes optimizers are supposed to remove this kind of things.

Although bad code produced by a (badly configured ?) compiler could be a valid explaination.

Most/all of EA's SNES sports games were written using a compiler, and it routinely did stupid things like explicitly allocating 0 bytes on the stack for functions without any arguments or local variables, so I wouldn't be surprised to see something like the OP either.

Then again, my first assumption was that this is a bit of code Mauron wrote themselves for feedback, rather than something found in an existing ROM (and if I had time, I'd let bgrep check for me  )

[Mauron]

Then again, my first assumption was that this is a bit of code Mauron wrote themselves for feedback, rather than something found in an existing ROM (and if I had time, I'd let bgrep check for me  )

It was taken out of Chrono Trigger's AI routines. It would show up when checking to display a message.

[magno]

Nope, peepholes optimizers are supposed to remove this kind of things.

I've checked several games' source code and you can trust me: the ASM code is barely optimized, maybe programmer's fault or maybe compiler's fault. My guess is that compilers weren't capable of getting the most optimized machine code, specially when using macros.

[lytron]

It was taken out of Chrono Trigger's AI routines. It would show up when checking to display a message.

AFAIK it is wrong to expect that Squaresoft's games are all masterly programmed (just because they are masterly concepted). I was told that FF6 was "more like a big hack" with many things programmed that they work somehow, not necessarily the fastest or most clearly arranged way. There were different programmers, and I expect them to be of different quality. Some did the tasks nobody else could do, and some did where they could damage the project the least... 

[Revenant]

AFAIK it is wrong to expect that Squaresoft's games are all masterly programmed (just because they are masterly concepted).

I find this is true of big-name software in general, not just Square games

[DougRPG]

CMP sets the C flag, and LDA don´t. So there is no way to tell if this CMP is useless unless you check the next intructions following the branches.

And, like was already said, you need to be sure there is no branch falling in this CMP (may be very difficult to find out).

[Bregalad]

CMP #$00 always set the C flag, and SEC is more efficient in doing so.

AFAIK it is wrong to expect that Squaresoft's games are all masterly programmed (just because they are masterly concepted)

Well, this is interesting, I never thought of it this way, but I think you're right. Some shitty games could have been masterfully programmed and vice-versa.

At least I know some early NES Square games have really stranges programming quicks and are programmed in a very non-standard way for the console (i.e. the games don't use "true" interrupts, sprites have hard-wired OAM positions and the engine relying on them, stuff like that).

[Mauron]

I double checked the branching, and there's nothing relevant going on - just a skip of the JSL if the value is 0.

The carry flag is not used for a while afterward, where it's used in a different CMP instruction.

Thanks for all the help.