Optimizer optimizes and breaks hand-written assembly code

[zzarko]

When writing this function, I came to a weird bug where optimizer changes my ASM code and makes something that does not work.

function ChangeByte(value: byte):byte;
var temp: byte;
begin
	asm("
		lda localVariable_ScreenUtil_ChangeByte_ScreenUtil_value
		nop ; added just to fool TRSE's optimization that messes up above LDA instruction
		tay
		lsr
		tya
		ror
		tay
		lsr
		tya
		ror
		and #$CC
		tax

		lda localVariable_ScreenUtil_ChangeByte_ScreenUtil_value
		nop ; added just to fool TRSE's optimization that messes up above LDA instruction
		tay
		asl
		tya
		rol
		tay
		asl
		tya
		rol
		and #$33
		sta localVariable_ScreenUtil_ChangeByte_ScreenUtil_temp
		txa
		ora localVariable_ScreenUtil_ChangeByte_ScreenUtil_temp
	");
end;

I have added two NOPs to stop optimizer from messing up the code. If NOPs are removed, previous LDA instructions are changed to LDY and code does not work as intended anymore.

[AndyHOvine]

Try adding ;keep to the end of the lines you don't want trse to optimise away...

Lda var;keep
Or
Lda var ;keep

Tay;keep

Etc

[zzarko]

@AndyHOvine I just tried to put ";keep", but unfortunately it still changes the instruction to LDY. I'm keeping NOP for now.

[AndyHOvine]

Try it with, or without a space. I can't remember which.

[AndyHOvine]

I've had time to test it, hope this helps :)

	asm ; problem example
		
		lda i
		tax
		lsr
		txa
		ror
		
	end;

produces:

		ldx i ; optimized, look out for bugs
		lsr
		txa
		ror

Whereas,

	asm ; fixed example
		
		lda i
		tax;keep
		lsr
		txa
		ror
		
	end;

produces:

		lda i
		tax;keep
		lsr
		txa
		ror

tax has not been removed and lda is preserved.