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floating point / apple silicon improvements
This commit is contained in:
Perry Kivolowitz
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commit
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4 changed files with 100 additions and 217 deletions
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@ -10,14 +10,12 @@ MAIN
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mov x29, sp
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LLD_ADDR x0, fmt
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LLD_FLT x1, s0, flt
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#if defined(__APPLE__)
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fcvt d0, s0
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fmov x1, d0
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PUSH_R x1
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#if defined(__APPLE__)
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PUSH_R d0
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CRT printf
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add sp, sp, 16
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#else
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fcvt d0, s0
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CRT printf
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#endif
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POP_P x29, x30
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@ -2,17 +2,17 @@
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This book is written to the Linux calling convention as stated early on.
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Unfortunately, this means that even if you own an Apple Silicon machine,
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which is AARCH64, you'd still need a Linux virtual machine. This didn't
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sit well with some on reddit and rightfully so. We undertook to
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develop a way of writing assembly code once and having it work on both
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Mac OS and Linux to the degree possible.
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which is AARCH64, you'd still need a Linux virtual machine.
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We are pleased to present this chapter along with a set of assembly
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language macros that, if used, help a great deal.
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This didn't sit well with some on reddit and rightfully so. We undertook
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to develop a way of writing assembly code once and having it work on
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both Mac OS and Linux to the degree possible.
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[Much of this chapter has been replaced here](./../../macros/).
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There are some things we cannot adapt, such as variadic functions (e.g.
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`printf()`) but we explain how code can be written to be compatible with
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both environments at the expense of some duplicated code.
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both environments at the expense of some minor amount duplicated code.
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## Assembly language macros
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@ -44,95 +44,10 @@ This gets expanded to:
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add x0, x0, fmt@PAGEOFF
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```
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## Loading the address of data
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## Reminder - there is documentation for these macros
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Assuming:
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```text
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.data
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fmt: .asciz "Hello!"
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```
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When we:
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`ldr x0, =fmt`
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we are hoping to put the address of the label `fmt` into `x0`. But how
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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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Apple does not allow instructions of the form:
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`ldr x0, =fmt`
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Instead they take a more general approach of splitting addresses of data
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into two parts:
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1. The *page* on which the label lives - think of this as generating the
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upper bits of the address.
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2. The *offset* on the page where the label actually resides - think of
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this as the lower bits of the address.
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Hence:
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```text
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adrp x0, fmt@PAGE
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add x0, x0, fmt@PAGEOFF
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```
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The first instruction puts the high bits of the label's address in `x0`.
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Then, the second instruction literally adds the low bits of the label's
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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](./apple-linux-convergence.S) is an assembly language file
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containing the macros we're developing to bring Linux and Apple Silicon
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assembly language closer together.
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Notice it has:
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```text
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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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```
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but also:
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```text
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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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Which of these are used is determined by whether or not you are
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assembling on an Apple machine or a Linux machine using features
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provided by the standard C pre-processor. I.e.:
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```text
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# if defined(__APPLE__)
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// apple stuff
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# else
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// not apple stuff
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# endif
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```
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The documentation for the macro suite has been moved
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[here](./../../macros/).
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## How to force the C pre-processor to run on assembly language
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@ -147,52 +62,13 @@ 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 `LLD_ADDR` the macros will adapt
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for you.
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### Function labels
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Apple prepends an underscore, Linux does not. Instead of:
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`bl printf`
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do:
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`CRT printf`
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and the macro will adapt.
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### main
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Like other function labels, Apple wants `_main` while Linux wants
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`main`.
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Simply use:
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`MAIN`
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and the macro will adapt.
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### Globals
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Instead of writing:
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`.global main`
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use
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`GLABEL main`
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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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any number of parameters. The first argument contains some information
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*This is important! Understand this section in order to be able to use
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`printf()`.*
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Functions like `printf()` are variadic. These are functions that can
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take any number of parameters. The first argument contains information
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that tells the function how many parameters were actually given.
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For example:
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@ -205,36 +81,35 @@ expected.
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Apple and Linux handle variadic differently.
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Linux will use the scratch registers first up to `x7`. *Then* it will
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use the stack.
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Linux will use the scratch registers first up to the integer or floating
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point register 7. *Then* it will 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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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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Remember that `%f` **always** expects a double. This is hidden from you
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in C and C++ but is important in assembly language. Use `fcvt` to shift
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from single precision to double.
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An example:
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```text
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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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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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// if not apple, the registers are already set up
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CRT printf
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# endif
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LLD_ADDR x0, fmt
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LLD_FLT x1, s0, flt
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fcvt d0, s0
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#if defined(__APPLE__)
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PUSH_R d0
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CRT printf
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add sp, sp, 16
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#else
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CRT printf
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#endif
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```
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## Other differences
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Binary file not shown.
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@ -1,77 +1,87 @@
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// Macros to permit the "same" assembly language to build on ARM64
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// Linux systems as well as Apple Silicon systems.
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//
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// Perry Kivolowitz
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// A Gentle Introduction to Assembly Language
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/* Macros to permit the "same" assembly language to build on ARM64
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Linux systems as well as Apple Silicon systems.
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See the fuller documentation at:
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https://github.com/pkivolowitz/asm_book/blob/main/macros/README.md
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Perry Kivolowitz
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A Gentle Introduction to Assembly Language
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*/
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.macro GLD_PTR xreg, label
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#if defined(__APPLE__)
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// Apple makes a distinction between loading something close by
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// versus something global. Note the use of GOTPAGE rather then
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// PAGE.
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//
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// Note: this macro dereferences the label getting what is at
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// the label's address.
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.macro GLD_PTR xreg, label // Dereference a global *
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adrp \xreg, _\label@GOTPAGE
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ldr \xreg, [\xreg, _\label@GOTPAGEOFF]
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#else
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ldr \xreg, =\label
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ldr \xreg, [\xreg]
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#endif
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.endm
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.macro GLD_ADDR xreg, label // Get a global address
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#if defined(__APPLE__)
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adrp \xreg, _\label@GOTPAGE
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add \xreg, \xreg, _\label@GOTPAGEOFF
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.endm
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// This macro loads the address of a nearby label.
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.macro LLD_ADDR xreg, label // Load a local address
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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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.macro GLABEL label
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.global _\label
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.endm
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.macro MAIN
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_main:
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.endm
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.macro CRT label
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bl _\label
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.endm
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#else // LINUX
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.macro GLABEL label
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.global \label
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.endm
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.macro MAIN
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main:
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.endm
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.macro CRT label
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bl \label
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.endm
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// This macro treats label as a pointer and dereferences it.
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// That is, it puts into the xreg what is found at the address
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// of the label.
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.macro GLD_PTR xreg, label // Dereference a global *
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#else
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ldr \xreg, =\label
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ldr \xreg, [\xreg]
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#endif
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.endm
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// This macro loads the address of a nearby label.
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.macro LLD_ADDR xreg, label
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#if defined(__APPLE__)
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adrp \xreg, \label@PAGE
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add \xreg, \xreg, \label@PAGEOFF
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#else
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ldr \xreg, =\label
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#endif
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.endm
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.macro LLD_DBL xreg, dreg, label
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#if defined(__APPLE__)
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adrp \xreg, \label@PAGE
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add \xreg, \xreg, \label@PAGEOFF
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ldur \dreg, [\xreg]
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// fmov \dreg, \xreg
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#else
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ldr \xreg, =\label
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ldur \dreg, [\xreg]
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#endif
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.endm
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.macro LLD_FLT xreg, sreg, label
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#if defined(__APPLE__)
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adrp \xreg, \label@PAGE
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add \xreg, \xreg, \label@PAGEOFF
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ldur \sreg, [\xreg]
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#else
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ldr \xreg, =\label
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ldur \sreg, [\xreg]
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#endif
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.endm
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.macro GLABEL label
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#if defined(__APPLE__)
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.global _\label
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#else
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.global \label
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#endif
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.endm
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.macro MAIN
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#if defined(__APPLE__)
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_main:
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#else
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main:
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#endif
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.endm
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.macro CRT label
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#if defined(__APPLE__)
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bl _\label
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#else
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bl \label
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#endif
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.endm
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.macro START_PROC // after starting label
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.cfi_startproc
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