[M3devel] "please confirm my understanding"

[Tony Hosking]

On Nov 29, 2007, at 8:28 PM, Jay wrote:

> Thanks! It helps to know I'm understanding things correctly.
>
> The backend isn't bad really, it's written by someone who very much  
> knew what they were doing, which is necessary.
>
>  > end interfaces. For example, it only ever stacks integer things as
>  > 32 or 64 bits, even if the integers have a memory representation  
> that
>
> Understood.
>
> Actually, btw, the front/middleend could do more easy  
> optimizations, like some of the ones I already commited, and  
> really, all the constant propagation that m3x86.m3 does, via its  
> stack stuff and remembering if something is "immediate". I should  
> look into that some time.......

I'd be leery of complicating the front/middle-ends with optimizations  
given that backends can do them (ie, gcc/m3x86).

>
>
>  - Jay
>
>
> > From: hosking at cs.purdue.edu
> > Date: Thu, 29 Nov 2007 10:59:26 -0500
> > To: jay.krell at cornell.edu
> > CC: m3devel at elegosoft.com
> > Subject: Re: [M3devel] "please confirm my understanding"
> >
> >
> > On Nov 29, 2007, at 6:15 AM, Jay wrote:
> >
> > > please confirm my understanding
> > >
> > > I don't feel great being a lone reader/changer of code esp. that I
> > > didn't write and that lacks
> > > sufficient documentation for my small brain/confidence. (I'm not
> > > stupid, but this isn't trivial stuff either.)
> >
> > If you are talking about the native x86 (non-gcc) backend then I
> > concur that it is pretty unreadable. Hence my lack of enthusiasm in
> > working on it myself.
> >
> > > I am bound to make mistakes.
> >
> > We all do!
> >
> > > In the context of Modula-3 code gen interfaces, and the word
> > > interfaces, which apparently are identical/analogous/whatever:
> > >
> > > There is:
> > >
> > > Shift (aka, my bad naming, "generic" shift)
> >
> > Yes, shift by n>0 is shift left. n<0 is shift right.
> >
> > > LeftShift
> > > RightShift
> > >
> > > LeftShift and RightShift allow only positive shift counts, 0 .. 31
> > > or 0 .. 63 (given
> > > particular word sizes and depending on types..)
> > > The "front end" checks constants against these.
> > > The "front end" inserts checks for variables against these.
> > > The back end can assume 0 .. 31 or 0 .. 63.
> >
> > Yes, depending on word-size.
> >
> > > It looks like the x86 back end does redundant masks against 31, at
> > > least when shifting by a constant,
> > > only in its own data, not in the codegen. Just a small waste of
> > > time in the compiler, no negative affect on codegen.
> > >
> > > "generic" Shift accepts any number for a shift count.
> > > Negative numbers mean a right shift.
> > > Positive numbers mean a left shift.
> > > If the shift count is >31|63 or <-31|63, the result is not an
> > > error, but zero.
> >
> > Yes.
> >
> > > In all cases, right shifts are unsigned, zero filling.
> >
> > Yes.
> >
> > > There is no shift operation that propagates the sign bit.
> > > In the Modula-3 language even. Perhaps, yes, integer division. But
> > > the frontend does not optimize
> > > there and turn multiplication or division into shifts, I think,
> > > haven't read much of the frontend. The backend could.
> >
> > Correct. I think the thinking was that a decent back-end would turn
> > integer division by a power of 2 into an arithmetic right shift (the
> > gcc backend certainly does).
> >
> > > Rotate is presumably similar, but I have not looked at it.
> > > (LeftRotate, RightRotate, Rotate...)
> >
> > Weird thing with these is the way the high/low bits are rotated
> > around, so sign can change!
> >
> > > m3-sys\m3back\src\m3x86.m3 implements left_shift, and right_shift,
> > > very redundanty code, could be combined.
> > >
> > > The layering is /roughly/
> > > m3x86 on top
> > > stackx86 in the middle
> > > codex86 at the bottom
> > >
> > > However m3x86 does call through to codex86 directly.
> > > They are largely one layer. There is no strict division.
> > > "stack" is a simple way to implement some optimizations and  
> stategies
> > > around constant propagation and register allocation.
> > >
> > > It is a stack of "operands", which may be constants, globals, or
> > > locals.
> > > If something is needed in a register, a register will be freed up.
> > > If something is later needed and the register hasn't been spilled,
> > > it will be
> > > reused. If not, not.
> > >
> > > Presently the operands are assumed to be of word size, 32 bits,
> > > register size.
> > > That is largely a convenience, and very convenient.
> > > I will expand it to allow two word operands.
> > > It could be expanded to arbitrarily, at least to "fill" "all" the
> > > registers, for some
> > > not particularly interesting but theoretically useful scenarios --
> > > passing "medium"
> > > sized structs around by value or such. There are ABI concerns
> > > though, this could
> > > only be amongst local functions that aren't visible/used
> > > externally (aka "custom calling conventions").
> >
> > You can make some assumptions about how the front-end uses the  
> middle-
> > end interfaces. For example, it only ever stacks integer things as
> > 32 or 64 bits, even if the integers have a memory representation  
> that
> > is 8 bits, etc.
> >
> > > The "generic" shift (vs. left_shift, right_shift) is  
> implemented in
> > > stackx86
> > > while left_shift, right_shift are in m3x86. All three functions  
> are
> > > very similar,
> > > could be combined, and there is no good reason for some to be in
> > > one layer
> > > vs. the other and it's a just a pointless inconsistency.
> > >
> > > Granted, "generic" shift is a good bit different, it has to check
> > > the sign and the magnitude,
> > > whereas left and right can just blindly do the operation with no
> > > comparisons or branches.
> > >
> > > Some of this I gather from attempting to compile code with shifts
> > > -- esp. the frontend checking of stuff, which then lead me to look
> > > at word.i3
> > > which declares subranges for the parameters.
> > >
> > > Is Bill Kalsow around?
> >
> > Good question. Farshad Nayeri may know where he ended up.
> >
> > > Or the other original authors?
> >
> > I know Michel Dagenais had a student who adapted the native x86
> > backend for use on Linux with PM3. I don't remember the student's  
> name.
> >
> > > Everyone just figured stuff out from the code, comments, and
> > > "literature" (I ordered another copy of Nelson's good book. :) )
> > >
> > >
> > > - Jay
> > >
> > >
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> >
>
>
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