[go-nuts] Re: SSA-able and canSSA meaning in Go compiler's SSA backend?

[Agniva De Sarker]

> Here is a link to a talk from a Go developer about adding SSA to the 
compiler

That Go developer is the one who answered OP's question :)

On Thursday, 19 September 2019 17:55:33 UTC+5:30, howar...@gmail.com wrote:
>
> Read this wiki page to understand what the goal is:
> https://en.wikipedia.org/wiki/Static_single_assignment_form
>
> Basically, SSA-form allows certain optimizations that are harder without 
> it, but SSA is also itself hard to apply. SSA examples are often posed in 
> the form of simple variables, but real code has structs and arrays and maps 
> and slices. So they mark things that are worth the effort of applying SSA 
> to and things they've (the compiler writers, that is) have decided are not 
> worth of pushing down to an SSA form.
>
> So, the special purpose it serves is to enable a class of optimizations. 
> And the difference from non-SSA-able values is simply one of cost/benefit. 
> Non-SSA-able values are actually those where implementing SSA was deemed 
> either not possible or too expensive (either in terms of time, or code, or 
> just the compiler author's brainspace).
>
> Here is a link to a talk from a Go developer about adding SSA to the 
> compiler:
> https://about.sourcegraph.com/go/generating-better-machine-code-with-ssa
>
> (P.S. if this ends up posting multiple times, I apologize. It has told me 
> there was an error communicating with the server twice now.)
>

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[go-nuts] Re: SSA-able and canSSA meaning in Go compiler's SSA backend?

[howardcshaw]

Read this wiki page to understand what the goal is:
https://en.wikipedia.org/wiki/Static_single_assignment_form

Basically, SSA-form allows certain optimizations that are harder without 
it, but SSA is also itself hard to apply. SSA examples are often posed in 
the form of simple variables, but real code has structs and arrays and maps 
and slices. So they mark things that are worth the effort of applying SSA 
to and things they've (the compiler writers, that is) have decided are not 
worth of pushing down to an SSA form.

So, the special purpose it serves is to enable a class of optimizations. 
And the difference from non-SSA-able values is simply one of cost/benefit. 
Non-SSA-able values are actually those where implementing SSA was deemed 
either not possible or too expensive (either in terms of time, or code, or 
just the compiler author's brainspace).

Here is a link to a talk from a Go developer about adding SSA to the 
compiler:
https://about.sourcegraph.com/go/generating-better-machine-code-with-ssa

(P.S. if this ends up posting multiple times, I apologize. It has told me 
there was an error communicating with the server twice now.)

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[go-nuts] Re: SSA-able and canSSA meaning in Go compiler's SSA backend?

[howardcshaw]

Read this wiki page to understand what the goal is:
https://en.wikipedia.org/wiki/Static_single_assignment_form

Basically, SSA-form allows certain optimizations that are harder without 
it, but SSA is also itself hard to apply. SSA examples are often posed in 
the form of simple variables, but real code has structs and arrays and maps 
and slices. So they mark things that are worth the effort of applying SSA 
to and things they've (the compiler writers, that is) have decided are not 
worth of pushing down to an SSA form.

So, the special purpose it serves is to enable a class of optimizations. 
And the difference from non-SSA-able values is simply one of cost/benefit. 
Non-SSA-able values are actually those where implementing SSA was deemed 
either not possible or too expensive (either in terms of time, or code, or 
just the compiler author's brainspace).

Here is a link to a talk from a Go developer about adding SSA to the 
compiler:
https://about.sourcegraph.com/go/generating-better-machine-code-with-ssa

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[go-nuts] Re: SSA-able and canSSA meaning in Go compiler's SSA backend?

[howardcshaw]

Read this wiki page to understand what the goal is:
https://en.wikipedia.org/wiki/Static_single_assignment_form

Basically, SSA-form allows certain optimizations that are harder without 
it, but SSA is also itself hard to apply. SSA examples are often posed in 
the form of simple variables, but real code has structs and arrays and maps 
and slices. So they mark things that are worth the effort of applying SSA 
to and things they've (the compiler writers, that is) have decided are not 
worth of pushing down to an SSA form.

So, the special purpose it serves is to enable a class of optimizations. 
And the difference from non-SSA-able values is simply one of cost/benefit. 
Non-SSA-able values are actually those where implementing SSA was deemed 
either not possible or too expensive (either in terms of time, or code, or 
just the compiler author's brainspace).

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[go-nuts] Re: SSA-able and canSSA meaning in Go compiler's SSA backend?

['Keith Randall' via golang-nuts]

On Wednesday, September 18, 2019 at 1:50:27 PM UTC-7, Mohit Verma wrote:
>
> Hi All,
>
> I was reading Go compiler's SSA backend code at 
> cmd/compile/internal/gc/ssa/go 
>  
> & 
> cmd/compile/internal/ssa 
> , 
> and I keep on seeing some code generation logic depending on if a type is 
> "SSA-able". 
> For example, 
> While building SSA tree for an assignment operation, I see comments like 
> this 
> 
> :
>
>
>
>
>
>
> *// We're assigning to a field of an ssa-able value. We need to build a 
> new structure with the new value for the field we're assigning and the old 
> values for the other fields. For instance:// type T struct {a, b, c int}// 
> var T x// x.b = 5// For the x.b = 5 assignment we want to generate x = 
> T{x.a, 5, x.c}*
>  
>
which seems like a sub-optimal way of updating only one element of a 
> struct. 
>

If T is SSAable, then one of the optimization passes decomposes x into its 
constituent parts.
So we effectively rewrite this to:

var x_a int  \
var x_b int   |  from var T x
var x_c int  /
x.a = x.a \
x.b = 5 | from x.b = 5
x.c = x.c  /

The two trivial assignments are then quickly optimized away.

My question is: 
> *What is a SSA-able value/type for Go compiler? What is a SSA-able node in 
> Go's AST representation? How is it different from other non SSA-able 
> values? What special purpose does it serve?*
>
>
First types. An array type with len>1 can't be represented in SSA, because 
there is no way to encode indexing operations.
In addition, as a heuristic we don't SSA large types, as SSAing them 
usually means breaking them up into their component values, and that can 
run the compiler out of registers pretty quickly.
(this is canSSAType)

Then variables. If a variable doesn't have an SSAable type, then we won't 
generate any SSA for it. In addition, if its address is taken we can't SSA 
it, because SSA representation has no way to represent possible side 
effects through pointers. There are a few other cases which also forbid 
making a variable SSAable.
(this is (*state).canSSA)


 

> Can anyone help me understand this/point me to links to existing 
> literature around this?
>
> Thanks!
> Mohit
>

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