## Summary 16 bit sized subroutine parameters passed by reference gets clobbered under certain conditions when optimization level 3 or higher is active. ## System Information - **Operating system:** embedded - **Processor architecture:** AVR - **Compiler version:** 3.3.1 (11cf24891dee700e81bbb9eeca61f867502f6d7b), also 3.2.2 - **Device:** Microcontroller ## Steps to reproduce Compile and test example project: ## Example Project ``` program testsimavr; var w, r: word; procedure func(var a: word; var r: word); begin r := a; dec(a); inc(r); end; begin w := 1234; func(w, r); writeln('w = ', w, ' (expected w = 1233)'); writeln('r = ', r, ' (expected r = 1235)'); end. ``` ## What is the current bug behavior? ``` $ ~/fpc/gitlab/compiler/avr/pp -n @~/fpc/gitlab/fpc.cfg -Wpavrsim -al -O3 testsimavr.pp $ ~/LazProjs/fp-avrsim-cc/avrsim testsimavr.bin w = 978 (expected w = 1233) r = 1235 (expected r = 1235) ```` ## What is the expected (correct) behavior? ``` $ ~/fpc/gitlab/compiler/avr/pp -n @~/fpc/gitlab/fpc.cfg -Wpavrsim -al -O2 testsimavr.pp $ ~/LazProjs/fp-avrsim-cc/avrsim testsimavr.bin w = 1233 (expected w = 1233) r = 1235 (expected r = 1235) ``` ## Relevant logs and/or screenshots The bug manifests in procedure func, on the line `dec(a)` - specifically when loading the value of variable `a` from its reference: ``` # [8] r := a; movw r30,r24 movw r26,r22 ld r0,Z+ st X+,r0 ld r0,Z st X,r0 .Ll2: # [9] dec(a); ld r18,Z ldd r19,Z+1 ``` The bug is that the instruction `ld r0, Z+` modifies Z, so Z should be reloaded with the reference to `a` before the `ld r18, Z` instruction. With -O2 this does happen: ``` # [8] r := a; movw r30,r24 movw r26,r22 ld r0,Z+ st X+,r0 ld r0,Z st X,r0 .Ll2: # [9] dec(a); movw r30,r24 ld r18,Z ldd r19,Z+1 ``` Since the correct instruction sequence is generated at lower optimization levels it suggests that some optimization is responsible for this bug. It is possible that this optimization check misses that `ld r0, Z+` modifies Z and therefore eliminates the required `movw r30, r24`. Unfortunately the specific optimization responsible for this is not reported by -dDEBUG_AOPTCPU.