[Info-vax] Hard links on VMS ODS5 disks

[John Reagan]

On Monday, August 28, 2023 at 9:26:19 PM UTC-4, Dan Cross wrote:
> In article <45fd1f30-55fb-4691... at googlegroups.com>, 
> John Reagan <xyzz... at gmail.com> wrote: 
> >[big snip]
> >Macro doesn't use the LLVM optimizer (it doesn't use the GEM one either). All the 
> >branching between routines doesn't fit the high-level model. 
> > 
> >Macro does a limited job of pulling address computations out of loops if there are 
> >free registers (and on x86, the answer is "almost never"). 
> > 
> >With GEM, AMACRO/IMACRO gets the benefit of GEM's instruction level peephole 
> >optimizer. That doesn't exist in that form for LLVM. So XMACRO today has several 
> >"branches to branches" and is sloppy with x86 condition codes. However, we think 
> >the microarchitecture and predictive execution takes care of those branches to branches 
> >and the like. 
> > 
> >In general Macro-32 code is optimized by the human while typing it in.
> This begs a question: how useful are those optimizations? I 
> imagine many are based on an execution environment that is not 
> the actual target anymore; is it possible that something that 
> was a clear win on VAX is a pessimization as implemented on 
> x86? 
> 
> - Dan C.
I was mostly talking about doing things like common sub-expression (computing a value
once outside of a loop), doing clever shifts vs divide, etc.  For knowing that some VAX
instructions are "faster" than other VAX instructions stopped being useful when moving
to Alpha.    For instance, no integer divide on Alpha; no actual divide on Itanium; but both
kinds of divide on x86.