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doc, cmd/internal/obj/arm64: update the directives in the doc

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Adding the usage of PCALIGN directive for arm64, and updating some
details on using some directives defined in the textflag.h file.

Change-Id: I43d363e3337939bab69b856831caf06803a292d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/227801
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
fanzha02 2020-03-31 18:05:50 +08:00 committed by Cherry Zhang
parent e0e0dab411
commit cb00d93431
2 changed files with 65 additions and 0 deletions
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25
doc/asm.html
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@ -437,6 +437,31 @@ This is a wrapper function and should not count as disabling <code>recover</code
(For <code>TEXT</code> items.)
This function is a closure so it uses its incoming context register.
</li>
<li>
<code>LOCAL</code> = 128
<br>
This symbol is local to the dynamic shared object.
</li>
<li>
<code>TLSBSS</code> = 256
<br>
(For <code>DATA</code> and <code>GLOBL</code> items.)
Put this data in thread local storage.
</li>
<li>
<code>NOFRAME</code> = 512
<br>
(For <code>TEXT</code> items.)
Do not insert instructions to allocate a stack frame and save/restore the return
address, even if this is not a leaf function.
Only valid on functions that declare a frame size of 0.
</li>
<li>
<code>TOPFRAME</code> = 2048
<br>
(For <code>TEXT</code> items.)
Function is the top of the call stack. Traceback should stop at this function.
</li>
</ul>
<h3 id="runtime">Runtime Coordination</h3>

40
src/cmd/internal/obj/arm64/doc.go
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@ -45,6 +45,46 @@ instructions and floating-point(scalar) instructions.
AESD V22.B16, V19.B16 <=> aesd v19.16b, v22.16b
SCVTFWS R3, F16 <=> scvtf s17, w6
6. Align directive
Go asm supports the PCALIGN directive, which indicates that the next instruction should be aligned
to a specified boundary by padding with NOOP instruction. The alignment value supported on arm64
must be a power of 2 and in the range of [8, 2048].
Examples:
PCALIGN $16
MOVD $2, R0 // This instruction is aligned with 16 bytes.
PCALIGN $1024
MOVD $3, R1 // This instruction is aligned with 1024 bytes.
PCALING also changes the function alignment. If a function has one or more PCALIGN directives,
its address will be aligned to the same or coarser boundary, which is the maximum of all the
alignment values.
In the following example, the function Add is aligned with 128 bytes.
Examples:
TEXT ·Add(SB),$40-16
MOVD $2, R0
PCALIGN $32
MOVD $4, R1
PCALIGN $128
MOVD $8, R2
RET
On arm64, functions in Go are aligned to 16 bytes by default, we can also use PCALGIN to set the
function alignment. The functions that need to be aligned are preferably using NOFRAME and NOSPLIT
to avoid the impact of the prologues inserted by the assembler, so that the function address will
have the same alignment as the first hand-written instruction.
In the following example, PCALIGN at the entry of the function Add will align its address to 2048 bytes.
Examples:
TEXT ·Add(SB),NOSPLIT|NOFRAME,$0
PCALIGN $2048
MOVD $1, R0
MOVD $1, R1
RET
Special Cases.
(1) umov is written as VMOV.