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updated apple silicon chapter to reflect changes in the macros
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Perry Kivolowitz
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1 changed files with 38 additions and 17 deletions
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@ -25,7 +25,7 @@ macro as an early form of C++ templated function (kinda but not really).
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Here's an example of an assembly language macro:
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```text
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.macro LD_ADDR xreg, label
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.macro LLD_ADDR xreg, label
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adrp \xreg, \label@PAGE
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add \xreg, \xreg, \label@PAGEOFF
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.endm
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@ -34,7 +34,7 @@ Here's an example of an assembly language macro:
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Here's how it might be used:
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```text
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LD_ADDR x0, fmt
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LLD_ADDR x0, fmt
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```
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This gets expanded to:
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@ -62,6 +62,7 @@ would this be possible since we've seen that addresses are (often) six
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bytes long and our instructions are always 4 bytes long? As we describe
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elsewhere, the above `ldr` instance is actually turned into instructions
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to load an address relative to the address of the current instruction.
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As long as the data we want is relatively close to the `ldr`, this works
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out to a difference in addresses that is small (and so, can be fit into
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a 4 byte instruction).
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@ -93,6 +94,11 @@ address into `x0` forming a complete address.
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In this way, labels can be further away from the current instruction
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than the Linux way.
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Apple does something similar with global variables, perhaps defined in
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C or C++ files. Instead of `PAGE` and `PAGEOFF` they use global
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versions. The macro `GLD_ADDR` is used in this case rather than
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`LLD_ADDR` which works with "locally" defined addresses.
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## How does this help bridge Apple and Linux?
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[Here](./macros.S) is an assembly language file containing the macros
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@ -102,7 +108,7 @@ closer together.
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Notice it has:
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```text
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.macro LD_ADDR xreg, label
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.macro LLD_ADDR xreg, label
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adrp \xreg, \label@PAGE
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add \xreg, \xreg, \label@PAGEOFF
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.endm
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@ -111,7 +117,7 @@ Notice it has:
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but also:
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```text
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.macro LD_ADDR xreg, label
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.macro LLD_ADDR xreg, label
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ldr \xreg, =\label
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.endm
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```
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@ -134,17 +140,17 @@ provided by the standard C pre-processor. I.e.:
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C pre-processor. `clang` on Linux will not by default but can if you
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specify `-x assembler-with-cpp`.
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gcc on Mac OS can be based on clang so on Mac OS it inherits `clang`'s
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behavior. gcc on Linux does not run assembly language files through
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the C pre-processor *if the asm file ends in .s but WILL if the file
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ends in .S* It took the author a long time to find this...
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`gcc` on Mac OS can be based on `clang` so on Mac OS it inherits
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`clang`'s behavior. `gcc` on Linux does not run assembly language files
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through the C pre-processor *if the asm file ends in .s but WILL if the
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file ends in .S*
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## Differences between Apple and Linux
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### Loading label addresses
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This was described above. If you use `LD_ADDR` the macros will adapt for
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you.
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This was described above. If you use `LLD_ADDR` the macros will adapt
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for you.
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### Function labels
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@ -181,6 +187,8 @@ use
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and the macros will adapt.
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You can find documentation on the macros [here](../../macros/README.md).
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## Variadic functions
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Functions like `printf()` are variadic. This means the function can take
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@ -203,17 +211,24 @@ use the stack.
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Apple will put the first parameter in the zero register and then shifts
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immediately to putting all other parameters onto the stack.
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Here is how we overcame this difference:
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We overcome this difference by detecting which environment we are
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building in using `#if` after having first set up for the Linux version.
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By setting up for the Linux version, the Apple version involves just
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pushing registers onto the stack.
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Remember that to print a float or double, they must be copied to `x`
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registers.
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An example:
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```text
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// setting up a two value printf as usual
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LD_ADDR x0, fmt // loads the address of fmt
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LD_ADDR x1, ptr // loads **ptr
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ldr x1, [x1] // dereferences **ptr to make *ptr
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ldr x2, [x1] // dereferences *ptr to get value
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LLD_ADDR x0, fmt // loads the address of fmt
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LLD_PTR x1, ptr // loads **ptr
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ldr x1, [x1] // turns **ptr into *ptr
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ldr x2, [x1] // dereferences *ptr to get value
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# if defined(__APPLE__)
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// if apple, push the second and third argument to stack
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stp x1, x2, [sp, -16]!
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PUSH_P x1, x2
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CRT printf
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add sp, sp, 16
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# else
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@ -237,6 +252,8 @@ To be Apple compatible, in addition to backing up `x30` also back up
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`mov x29, sp`
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We will be converting all sample code to do this over time.
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### More?
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As we discover more differences, they will be described here.
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@ -248,6 +265,10 @@ code. These work the same on both Apple Silicon and Linux. If you want
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these, put `START_PROC` after the label introducing a function. Then,
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put `END_PROC` after the last statement of the function.
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This helps the debuggers understand where a function begins and ends.
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We will transition all sample code to use this over time.
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## A useful link
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[Here](https://gcc.gnu.org/onlinedocs/gcc/Invoking-GCC.html) is an
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