This chapter documents the ongoing work in defining a macro suite that allows coding AARCH64 programs once with the ability to build correctly on Apple Silicon and Linux machines without change.
The work is ongoing and subject to change.
There are limits to what these macros can do. Variadic functions such as
printf() must be handled via parallel code paths (i.e. use of #if).
For widest compatibility, end your assembly language files in capital S
rather than small s. This forces gcc to make use of the C preprocessor
as there is no command line option to make it do so. clang (and a
gcc derived from it) may or may not have a command line option to
force the invocation of the preprocessor but ending your file names
in capital S is universally appropriate.
A main difference unified by the macros is Apple's prepending of
underscores to labels defined by libraries such as the CRT and certain
other symbols like main.
So, main will not be found by the linker on Apple systems and _main
will be an error on Linux systems.
The macros adjust for this.
There are some exceptions to the prepending rule on Apple such as making
use of FILE * stdin. On Linux this would be stdin. On Mac OS you
would expect _stdin but you'd be wrong... instead Apple uses
___stdinp. Why? Because Apple.
There is an assumption here that labels created by you do not have prepended underscores. This can be a problem if this isn't the case. The solution may be to add a parallel set of macros that either do prepend or do not. This is an open question which we hope to get user input to resolve.
Functions such as printf() do not have fixed signatures. That is, they
may accept a variable number of parameters of varying types. Linux and
Apple Silicon handle these functions quite differently.
This is explained at length in the chapter on variadic functions.
First, we describe a number of macros which are the same on both Apple and Linux. These macros don't converge Apple and Linux. They're just nice to have.
AASCIZ label, string
This macro invokes .asciz with the string set to string and the
label set to label. In addition, this macro ensures that the string
begins on a 4-byte-aligned boundary.
These macros save some repetitive typing. For example:
PUSH_P x29, x30
resolves to:
stp x29, x30, [sp, -16]!
Place START_PROC after the label introducing a function.
Place END_PROC after the last ret of the function.
These resolve to: .cfi_startproc and .cfi_endproc respectively.
Handy more readable macros for determining minima and maxima. Note that
the macro performs a cmp which subtracts src_b from src_a
(discarding the results) in order to set the flags to be interpreted by
the following csel.
Thank you to u/TNorthover for nudge to add the cmp directly into the macro.
Signature:
MIN src_a, src_b, dest
The smaller of src_a and src_b is put into dest.
Signature:
MAX src_a, src_b, dest
The larger of src_a and src_b is put into dest.
Makes a label available externally.
Signature:
GLABEL label
An underscore is prepended.
If you create your own function without an underscore, just call it as usual.
If you need to call a function such as those found in the C runtime library, use this macro in this way:
CRT strlen
An underscore is prepended on the Mac.
Put MAIN on a line by itself. Notice there is no colon.
An underscore is prepended on the Mac.
The externally defined errno is accessed via a CRT function which
isn't seen when coding in C and C++. The function is named differently
on Mac versus Linux. To get the address of errno use:
ERRNO_ADDR
This macro makes the correct CRT call and leaves the address of errno
in x0.
Loads the address of a label and then dereferences it where, on Apple the label is in the global space and on Linux is a relatively close label.
Signature:
GLD_PTR xreg, label
When this macro finishes, the specified x register contains what 64 bit value lives at the specified label.
Loads the address of the label into the specified x register. No dereferencing takes place. On Apple machines, the label will be found in the global space.
Signature:
GLD_ADDR xreg, label
When this macro completes, the address of the label is in the x register.
Similar to GLD_ADDR this macro loads the address of a "local" label.
Signature:
LLD_ADDR xreg, label
When this macro completes, the address of the label is in the x register.
Signature:
LLD_DBL xreg, dreg, label
When this macro completes, a double that lives at the specified local label will sit in the specified double register.
Note: No underscore is prepended.
See this sample program for an example.
Signature:
LLD_FLT xreg, sreg, label
When this macro completes, a float that lives at the specified local label will sit in the specified single precision register.
Note: No underscore is prepended.
See this sample program for an example.