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doc/asm: document go_asm.h only works in the runtime package

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Fixes #33054

Change-Id: I687d45e092d721a6c22888cc7ddbe420c16a5af9
GitHub-Last-Rev: a7208c89a0
GitHub-Pull-Request: golang/go#33069
Reviewed-on: https://go-review.googlesource.com/c/go/+/185917
Reviewed-by: Rob Pike <r@golang.org>
This commit is contained in:
Pure White 2019-08-11 05:37:29 +00:00 committed by Rob Pike
parent 3928915ec7
commit 2754118731
1 changed files with 13 additions and 3 deletions
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doc/asm.html
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@ -590,27 +590,37 @@ Here follow some descriptions of key Go-specific details for the supported archi
<p>
The runtime pointer to the <code>g</code> structure is maintained
through the value of an otherwise unused (as far as Go is concerned) register in the MMU.
A OS-dependent macro <code>get_tls</code> is defined for the assembler if the source includes
a special header, <code>go_asm.h</code>:
An OS-dependent macro <code>get_tls</code> is defined for the assembler if the source is
in the <code>runtime</code> package and includes a special header, <code>go_tls.h</code>:
</p>
<pre>
#include "go_asm.h"
#include "go_tls.h"
</pre>
<p>
Within the runtime, the <code>get_tls</code> macro loads its argument register
with a pointer to the <code>g</code> pointer, and the <code>g</code> struct
contains the <code>m</code> pointer.
There's another special header containing the offsets for each
element of <code>g</code>, called <code>go_asm.h</code>.
The sequence to load <code>g</code> and <code>m</code> using <code>CX</code> looks like this:
</p>
<pre>
#include "go_tls.h"
#include "go_asm.h"
...
get_tls(CX)
MOVL g(CX), AX // Move g into AX.
MOVL g_m(AX), BX // Move g.m into BX.
</pre>
<p>
Note: The code above works only in the <code>runtime</code> package, while <code>go_tls.h</code> also
applies to <a href="#arm">arm</a>, <a href="#amd64">amd64</a> and amd64p32, and <code>go_asm.h</code> applies to all architectures.
</p>
<p>
Addressing modes:
</p>