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Are AlignmentNops actually used?

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Boris Shingarov

Are AlignmentNops actually used?

In my -- admittedly very limited (Squeak ReaderImages simulating ARMv7)
-- tests I've never seen #concretizeAlignmentNops do anything. It does
get called, but it never emits any code. This is because
machineCodeSize=0, so the

0 to: machineCodeSize - 1 by: 4 do:
[:p| self machineCodeAt: p put: 16rE1A00000].

gets no iterations.

Are there cases when this gets called with nonzero machineCodeSize, or
is it a candidate for deletion?

Eliot Miranda-2

Re: Are AlignmentNops actually used?

Hi Boris,

> On Sep 17, 2020, at 1:59 AM, Boris Shingarov <[hidden email]> wrote:
>
> In my -- admittedly very limited (Squeak ReaderImages simulating ARMv7) -- tests I've never seen #concretizeAlignmentNops do anything. It does get called, but it never emits any code. This is because machineCodeSize=0, so the
>
> 0 to: machineCodeSize - 1 by: 4 do:
> [:p| self machineCodeAt: p put: 16rE1A00000].
>
> gets no iterations.
>
> Are there cases when this gets called with nonzero machineCodeSize, or is it a candidate for deletion?

Yes it is used. In images using embedded blocks (EncoderForV3PlusClosures instruction set) and with x86 or x86_64 (variable-sized instructions with byte granularity), AlignmentNops are used to pad to align the fake method header embedded in machine code for each CogBlockMethod in a CogMethod.

__,,,^..^,,,__
Best, Eliot