[FaultMaps][Docs] Document the ImplicitNullChecks pass.

[oota-llvm.git] / docs / Extensions.rst

diff --git a/docs/Extensions.rst b/docs/Extensions.rst
index f499bdd57e6ef823af9abfaa9e07b2ab67e9f977..c8ff07c2b0ca20fcbbfa117559e785be2e8828bc 100644 (file)
--- a/docs/Extensions.rst
+++ b/docs/Extensions.rst
@@ -76,7 +76,7 @@ the target.  It corresponds to the COFF relocation types
 
 Syntax:
 
 
 Syntax:
 

-   ``.linkonce [ comdat type [ section identifier ] ]``
+   ``.linkonce [ comdat type ]``

 
 Supported COMDAT types:
 
 
 Supported COMDAT types:
 


@@ -95,16 +95,6 @@ Supported COMDAT types:
    Duplicates are discarded, but the linker issues an error if any duplicates
    do not have exactly the same content.
 
    Duplicates are discarded, but the linker issues an error if any duplicates
    do not have exactly the same content.
 

-``associative``
-   Links the section if a certain other COMDAT section is linked. This other
-   section is indicated by its section identifier following the comdat type.
-   The following restrictions apply to the associated section:
-
-   1. It must be the name of a section already defined.
-   2. It must differ from the current section.
-   3. It must be a COMDAT section.
-   4. It cannot be another associative COMDAT section.
-

 ``largest``
    Links the largest section from among the duplicates.
 
 ``largest``
    Links the largest section from among the duplicates.
 


@@ -118,10 +108,6 @@ Supported COMDAT types:
   .linkonce
     ...
 
   .linkonce
     ...
 

-  .section .xdata$foo
-  .linkonce associative .text$foo
-    ...
-

 ``.section`` Directive
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ``.section`` Directive
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 


@@ -144,8 +130,8 @@ MC supports passing the information in ``.linkonce`` at the end of
   .long 1
 
 Note that in the combined form the COMDAT symbol is explicit. This
   .long 1
 
 Note that in the combined form the COMDAT symbol is explicit. This

-extension exits to support multiple sections with the same name in
-different comdats:
+extension exists to support multiple sections with the same name in
+different COMDATs:

 
 
 .. code-block:: gas
 
 
 .. code-block:: gas


@@ -159,3 +145,90 @@ different comdats:
   .globl Symbol2
   Symbol2:
   .long 1
   .globl Symbol2
   Symbol2:
   .long 1

+
+In addition to the types allowed with ``.linkonce``, ``.section`` also accepts
+``associative``. The meaning is that the section is linked  if a certain other
+COMDAT section is linked. This other section is indicated by the comdat symbol
+in this directive. It can be any symbol defined in the associated section, but
+is usually the associated section's comdat.
+
+   The following restrictions apply to the associated section:
+
+   1. It must be a COMDAT section.
+   2. It cannot be another associative COMDAT section.
+
+In the following example the symobl ``sym`` is the comdat symbol of ``.foo``
+and ``.bar`` is associated to ``.foo``.
+
+.. code-block:: gas
+
+       .section        .foo,"bw",discard, "sym"
+       .section        .bar,"rd",associative, "sym"
+
+
+ELF-Dependent
+-------------
+
+``.section`` Directive
+^^^^^^^^^^^^^^^^^^^^^^
+
+In order to support creating multiple sections with the same name and comdat,
+it is possible to add an unique number at the end of the ``.seciton`` directive.
+For example, the following code creates two sections named ``.text``.
+
+.. code-block:: gas
+
+       .section        .text,"ax",@progbits,unique,1
+        nop
+
+       .section        .text,"ax",@progbits,unique,2
+        nop
+
+
+The unique number is not present in the resulting object at all. It is just used
+in the assembler to differentiate the sections.
+
+Target Specific Behaviour
+=========================
+
+Windows on ARM
+--------------
+
+Stack Probe Emission
+^^^^^^^^^^^^^^^^^^^^
+
+The reference implementation (Microsoft Visual Studio 2012) emits stack probes
+in the following fashion:
+
+.. code-block:: gas
+
+  movw r4, #constant
+  bl __chkstk
+  sub.w sp, sp, r4
+
+However, this has the limitation of 32 MiB (±16MiB).  In order to accommodate
+larger binaries, LLVM supports the use of ``-mcode-model=large`` to allow a 4GiB
+range via a slight deviation.  It will generate an indirect jump as follows:
+
+.. code-block:: gas
+
+  movw r4, #constant
+  movw r12, :lower16:__chkstk
+  movt r12, :upper16:__chkstk
+  blx r12
+  sub.w sp, sp, r4
+
+Variable Length Arrays
+^^^^^^^^^^^^^^^^^^^^^^
+
+The reference implementation (Microsoft Visual Studio 2012) does not permit the
+emission of Variable Length Arrays (VLAs).
+
+The Windows ARM Itanium ABI extends the base ABI by adding support for emitting
+a dynamic stack allocation.  When emitting a variable stack allocation, a call
+to ``__chkstk`` is emitted unconditionally to ensure that guard pages are setup
+properly.  The emission of this stack probe emission is handled similar to the
+standard stack probe emission.
+
+The MSVC environment does not emit code for VLAs currently.
+