[Info-vax] Native compilers

[abrsvc]

On Tuesday, March 8, 2022 at 10:10:52 AM UTC-5, xyzz... at gmail.com wrote:
> On Monday, March 7, 2022 at 7:46:02 PM UTC-5, chris wrote: 
> > On 03/05/22 18:05, John Reagan wrote: 
> > > As a follow-up and something else to focus on... As we continued to do more testing of the native 
> > > Macro-32 compiler, we had various ACCVIOs and LIB$GET_VM errors on programs that used macros. 
> > > It had no clear pattern other than perhaps something to with macro processing. It wasn't clear if it 
> > > was part of the macro body scanning or macro expansion. [We're using DELTA to debug these things 
> > > at the moment, the symbolic debugger is almost ready but it didn't like debugging a full MACRO.EXE 
> > > image.] 
> > > 
> > > At some point it felt to me that perhaps the cross Macro-32 compiler was unable to compile the native 
> > > Macro-32 compiler [such is life in the world of bootstrapping compilers]. We finally found the faulty instruction 
> > > 
> > > MOVL (R7)+,R7 
> > That's interesting. In theory, it's loading what R7 points to into R7, 
> > but the result depends on when the autoincrement takes place, after the 
> > initial (r7) load, or at the copy of R7 in the destination field ?. 
> > Perhaps not all architectures follow the same path hence the bug.
> The VAX is pretty clear. The correct behavior is: 
> - Load 32-bits from memory from address contained in R7, place into temporary 
> - Increment R7 by 4 
> - Store temporary in R7 
> 
> We just deferred the increment until after the store by mistake.
> > > 
> > > MOVAL (R3)+,(R3)+ 
> > Again depends on when the autoincremnt happens in the sequence...
> VAX architecture defines the behavior exactly. The human brain? Not so much. :)
Does is make sense to increment R7 at all?  It seems that this step should be eliminated as it will be overwritten anyway.

The case of the MOVAL (R3)+,(R3)+ is a little more convoluted...

Get the address pointed to by R3 and place in temp, add 4 to R3.
Place temp into address pointed to by R3 then add 4 to r3

The above is correct, I think.  But yes, it tests the mind as to what this is actually doing... and why!