Optimizer breaks the code with accessing data via pointer (6510)

[zzarko]

I caught a weird bug, but I am unsure which optimizer option triggers it. I have tried to make a more minimal example, but was unsuccessful, so I trimmed down my current code. Most probably not all of it is needed, but I have no idea what exactly triggers the bug.

The error appears in Stars.tru file, starting in line 33:

	// something weird...
	j -= 1;
	// due to wrong optimization, this code does not work without NOP !!!
	//asm("	nop");	// introduced to fool TRSE optimizer, uncomment to get a working code
	if ScreenUtil::ptr[j] = Config::BlankChar then begin
		ScreenUtil::ptr[j] := Config::StarIdx1;
	end;

Without NOP, this is the code that was generated:

; // something weird...
	; Test Inc dec D
	dec Stars_j
	; LineNumber: 37
	; Binary clause Simplified: EQUALS
	; Load pointer array
	tay ; optimized y, look out for bugs L22 ORG 	ldy Stars_j
	lda (ScreenUtil_ptr),y
	; Compare with pure num / var optimization
	cmp #$fe;keep

Instead of loading Y with current value of variable J, some previous content of A is moved to Y and then completely wrong data is accessed. If NOP line is uncommented, correct sequence is generated:

; // something weird...
	; Test Inc dec D
	dec Stars_j
	; LineNumber: 36
	nop
	; LineNumber: 37
	; Binary clause Simplified: EQUALS
	; Load pointer array
	ldy Stars_j
	lda (ScreenUtil_ptr),y
	; Compare with pure num / var optimization
	cmp #$fe;keep

I am unsure, but seems that origin may be the same as for this bug: #880

ErrorOpt.zip