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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> <p>
The runtime pointer to the <code>g</code> structure is maintained 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. 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 An OS-dependent macro <code>get_tls</code> is defined for the assembler if the source is
a special header, <code>go_asm.h</code>: in the <code>runtime</code> package and includes a special header, <code>go_tls.h</code>:
</p> </p>
<pre> <pre>
#include "go_asm.h" #include "go_tls.h"
</pre> </pre>
<p> <p>
Within the runtime, the <code>get_tls</code> macro loads its argument register 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 with a pointer to the <code>g</code> pointer, and the <code>g</code> struct
contains the <code>m</code> pointer. 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: The sequence to load <code>g</code> and <code>m</code> using <code>CX</code> looks like this:
</p> </p>
<pre> <pre>
#include "go_tls.h"
#include "go_asm.h"
...
get_tls(CX) get_tls(CX)
MOVL g(CX), AX // Move g into AX. MOVL g(CX), AX // Move g into AX.
MOVL g_m(AX), BX // Move g.m into BX. MOVL g_m(AX), BX // Move g.m into BX.
</pre> </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> <p>
Addressing modes: Addressing modes:
</p> </p>