Revert "[Option] Introduce Arg::print(raw_ostream&) and use llvm::dbgs"

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

diff --git a/docs/Extensions.rst b/docs/Extensions.rst
index 52dadc74e3595d6e85691895d32bd155768349a2..c8ff07c2b0ca20fcbbfa117559e785be2e8828bc 100644 (file)
--- a/docs/Extensions.rst
+++ b/docs/Extensions.rst
@@ -24,6 +24,7 @@ LLVM's assemblers allow floating-point constants to be written in C99's
 hexadecimal format instead of decimal if desired.
 
 .. code-block:: gas
 hexadecimal format instead of decimal if desired.
 
 .. code-block:: gas

+

   .section .data
   .float 0x1c2.2ap3
 
   .section .data
   .float 0x1c2.2ap3
 


@@ -36,7 +37,7 @@ X86/COFF-Dependent
 Relocations
 ^^^^^^^^^^^
 
 Relocations
 ^^^^^^^^^^^
 

-The following additional relocation type is supported:
+The following additional relocation types are supported:

 
 **@IMGREL** (AT&T syntax only) generates an image-relative relocation that
 corresponds to the COFF relocation types ``IMAGE_REL_I386_DIR32NB`` (32-bit) or
 
 **@IMGREL** (AT&T syntax only) generates an image-relative relocation that
 corresponds to the COFF relocation types ``IMAGE_REL_I386_DIR32NB`` (32-bit) or


@@ -53,13 +54,29 @@ corresponds to the COFF relocation types ``IMAGE_REL_I386_DIR32NB`` (32-bit) or
     .long (fun@imgrel + 0x3F)
     .long $unwind$fun@imgrel
 
     .long (fun@imgrel + 0x3F)
     .long $unwind$fun@imgrel
 

+**.secrel32** generates a relocation that corresponds to the COFF relocation
+types ``IMAGE_REL_I386_SECREL`` (32-bit) or ``IMAGE_REL_AMD64_SECREL`` (64-bit).
+
+**.secidx** relocation generates an index of the section that contains
+the target.  It corresponds to the COFF relocation types
+``IMAGE_REL_I386_SECTION`` (32-bit) or ``IMAGE_REL_AMD64_SECTION`` (64-bit).
+
+.. code-block:: gas
+
+  .section .debug$S,"rn"
+    .long 4
+    .long 242
+    .long 40
+    .secrel32 _function_name
+    .secidx   _function_name
+    ...

 
 ``.linkonce`` Directive
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 
 
 ``.linkonce`` Directive
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 

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

 
 Supported COMDAT types:
 
 
 Supported COMDAT types:
 


@@ -78,16 +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.
 


@@ -101,6 +108,127 @@ Supported COMDAT types:
   .linkonce
     ...
 
   .linkonce
     ...
 

-  .section .xdata$foo
-  .linkonce associative .text$foo
-    ...
+``.section`` Directive
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+MC supports passing the information in ``.linkonce`` at the end of
+``.section``. For example,  these two codes are equivalent
+
+.. code-block:: gas
+
+  .section secName, "dr", discard, "Symbol1"
+  .globl Symbol1
+  Symbol1:
+  .long 1
+
+.. code-block:: gas
+
+  .section secName, "dr"
+  .linkonce discard
+  .globl Symbol1
+  Symbol1:
+  .long 1
+
+Note that in the combined form the COMDAT symbol is explicit. This
+extension exists to support multiple sections with the same name in
+different COMDATs:
+
+
+.. code-block:: gas
+
+  .section secName, "dr", discard, "Symbol1"
+  .globl Symbol1
+  Symbol1:
+  .long 1
+
+  .section secName, "dr", discard, "Symbol2"
+  .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.
+