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[docs] Typos and whitespace fixed in LTO docs.

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While reading the LTO docs I fixed few small typos and whitespace issues.

Patch by: Jonas Devlieghere <jonas@devlieghere.com>

Differential Revision: https://reviews.llvm.org/D27196

llvm-svn: 288171
This commit is contained in:
Mehdi Amini 2016-11-29 18:00:31 +00:00
parent 7d5e5aa9e9
commit 6e479ee202
1 changed files with 21 additions and 21 deletions
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docs/LinkTimeOptimization.rst
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@ -9,7 +9,7 @@ Description
=========== ===========
LLVM features powerful intermodular optimizations which can be used at link LLVM features powerful intermodular optimizations which can be used at link
time. Link Time Optimization (LTO) is another name for intermodular time. Link Time Optimization (LTO) is another name for intermodular
optimization when performed during the link stage. This document describes the optimization when performed during the link stage. This document describes the
interface and design between the LTO optimizer and the linker. interface and design between the LTO optimizer and the linker.
@ -21,7 +21,7 @@ intermodular optimization, in the compiler tool chain. Its main goal is to let
the developer take advantage of intermodular optimizations without making any the developer take advantage of intermodular optimizations without making any
significant changes to the developer's makefiles or build system. This is significant changes to the developer's makefiles or build system. This is
achieved through tight integration with the linker. In this model, the linker achieved through tight integration with the linker. In this model, the linker
treates LLVM bitcode files like native object files and allows mixing and treats LLVM bitcode files like native object files and allows mixing and
matching among them. The linker uses `libLTO`_, a shared object, to handle LLVM matching among them. The linker uses `libLTO`_, a shared object, to handle LLVM
bitcode files. This tight integration between the linker and LLVM optimizer bitcode files. This tight integration between the linker and LLVM optimizer
helps to do optimizations that are not possible in other models. The linker helps to do optimizations that are not possible in other models. The linker
@ -34,7 +34,7 @@ Example of link time optimization
The following example illustrates the advantages of LTO's integrated approach The following example illustrates the advantages of LTO's integrated approach
and clean interface. This example requires a system linker which supports LTO and clean interface. This example requires a system linker which supports LTO
through the interface described in this document. Here, clang transparently through the interface described in this document. Here, clang transparently
invokes system linker. invokes system linker.
* Input source file ``a.c`` is compiled into LLVM bitcode form. * Input source file ``a.c`` is compiled into LLVM bitcode form.
@ -64,7 +64,7 @@ invokes system linker.
int foo1(void) { int foo1(void) {
int data = 0; int data = 0;
if (i < 0) if (i < 0)
data = foo3(); data = foo3();
data = data + 42; data = data + 42;
@ -121,12 +121,12 @@ Alternative Approaches
In this model, a new, separate, tool or library replicates the linker's In this model, a new, separate, tool or library replicates the linker's
capability to collect information for link time optimization. Not only is capability to collect information for link time optimization. Not only is
this code duplication difficult to justify, but it also has several other this code duplication difficult to justify, but it also has several other
disadvantages. For example, the linking semantics and the features provided disadvantages. For example, the linking semantics and the features provided
by the linker on various platform are not unique. This means, this new tool by the linker on various platform are not unique. This means, this new tool
needs to support all such features and platforms in one super tool or a needs to support all such features and platforms in one super tool or a
separate tool per platform is required. This increases maintenance cost for separate tool per platform is required. This increases maintenance cost for
link time optimizer significantly, which is not necessary. This approach link time optimizer significantly, which is not necessary. This approach
also requires staying synchronized with linker developements on various also requires staying synchronized with linker developments on various
platforms, which is not the main focus of the link time optimizer. Finally, platforms, which is not the main focus of the link time optimizer. Finally,
this approach increases end user's build time due to the duplication of work this approach increases end user's build time due to the duplication of work
done by this separate tool and the linker itself. done by this separate tool and the linker itself.
@ -136,12 +136,12 @@ Multi-phase communication between ``libLTO`` and linker
The linker collects information about symbol definitions and uses in various The linker collects information about symbol definitions and uses in various
link objects which is more accurate than any information collected by other link objects which is more accurate than any information collected by other
tools during typical build cycles. The linker collects this information by tools during typical build cycles. The linker collects this information by
looking at the definitions and uses of symbols in native .o files and using looking at the definitions and uses of symbols in native .o files and using
symbol visibility information. The linker also uses user-supplied information, symbol visibility information. The linker also uses user-supplied information,
such as a list of exported symbols. LLVM optimizer collects control flow such as a list of exported symbols. LLVM optimizer collects control flow
information, data flow information and knows much more about program structure information, data flow information and knows much more about program structure
from the optimizer's point of view. Our goal is to take advantage of tight from the optimizer's point of view. Our goal is to take advantage of tight
integration between the linker and the optimizer by sharing this information integration between the linker and the optimizer by sharing this information
during various linking phases. during various linking phases.
@ -152,33 +152,33 @@ The linker first reads all object files in natural order and collects symbol
information. This includes native object files as well as LLVM bitcode files. information. This includes native object files as well as LLVM bitcode files.
To minimize the cost to the linker in the case that all .o files are native To minimize the cost to the linker in the case that all .o files are native
object files, the linker only calls ``lto_module_create()`` when a supplied object files, the linker only calls ``lto_module_create()`` when a supplied
object file is found to not be a native object file. If ``lto_module_create()`` object file is found to not be a native object file. If ``lto_module_create()``
returns that the file is an LLVM bitcode file, the linker then iterates over the returns that the file is an LLVM bitcode file, the linker then iterates over the
module using ``lto_module_get_symbol_name()`` and module using ``lto_module_get_symbol_name()`` and
``lto_module_get_symbol_attribute()`` to get all symbols defined and referenced. ``lto_module_get_symbol_attribute()`` to get all symbols defined and referenced.
This information is added to the linker's global symbol table. This information is added to the linker's global symbol table.
The lto* functions are all implemented in a shared object libLTO. This allows The lto* functions are all implemented in a shared object libLTO. This allows
the LLVM LTO code to be updated independently of the linker tool. On platforms the LLVM LTO code to be updated independently of the linker tool. On platforms
that support it, the shared object is lazily loaded. that support it, the shared object is lazily loaded.
Phase 2 : Symbol Resolution Phase 2 : Symbol Resolution
--------------------------- ---------------------------
In this stage, the linker resolves symbols using global symbol table. It may In this stage, the linker resolves symbols using global symbol table. It may
report undefined symbol errors, read archive members, replace weak symbols, etc. report undefined symbol errors, read archive members, replace weak symbols, etc.
The linker is able to do this seamlessly even though it does not know the exact The linker is able to do this seamlessly even though it does not know the exact
content of input LLVM bitcode files. If dead code stripping is enabled then the content of input LLVM bitcode files. If dead code stripping is enabled then the
linker collects the list of live symbols. linker collects the list of live symbols.
Phase 3 : Optimize Bitcode Files Phase 3 : Optimize Bitcode Files
-------------------------------- --------------------------------
After symbol resolution, the linker tells the LTO shared object which symbols After symbol resolution, the linker tells the LTO shared object which symbols
are needed by native object files. In the example above, the linker reports are needed by native object files. In the example above, the linker reports
that only ``foo1()`` is used by native object files using that only ``foo1()`` is used by native object files using
``lto_codegen_add_must_preserve_symbol()``. Next the linker invokes the LLVM ``lto_codegen_add_must_preserve_symbol()``. Next the linker invokes the LLVM
optimizer and code generators using ``lto_codegen_compile()`` which returns a optimizer and code generators using ``lto_codegen_compile()`` which returns a
native object file creating by merging the LLVM bitcode files and applying native object file creating by merging the LLVM bitcode files and applying
various optimization passes. various optimization passes.
@ -212,7 +212,7 @@ their object files and the standard linker tool.
``lto_module_t`` ``lto_module_t``
---------------- ----------------
A non-native object file is handled via an ``lto_module_t``. The following A non-native object file is handled via an ``lto_module_t``. The following
functions allow the linker to check if a file (on disk or in a memory buffer) is functions allow the linker to check if a file (on disk or in a memory buffer) is
a file which libLTO can process: a file which libLTO can process:
@ -254,7 +254,7 @@ The attributes of a symbol include the alignment, visibility, and kind.
Once the linker has loaded each non-native object files into an Once the linker has loaded each non-native object files into an
``lto_module_t``, it can request ``libLTO`` to process them all and generate a ``lto_module_t``, it can request ``libLTO`` to process them all and generate a
native object file. This is done in a couple of steps. First, a code generator native object file. This is done in a couple of steps. First, a code generator
is created with: is created with:
.. code-block:: c .. code-block:: c
@ -267,19 +267,19 @@ Then, each non-native object file is added to the code generator with:
lto_codegen_add_module(lto_code_gen_t, lto_module_t) lto_codegen_add_module(lto_code_gen_t, lto_module_t)
The linker then has the option of setting some codegen options. Whether or not The linker then has the option of setting some codegen options. Whether or not
to generate DWARF debug info is set with: to generate DWARF debug info is set with:
.. code-block:: c .. code-block:: c
lto_codegen_set_debug_model(lto_code_gen_t) lto_codegen_set_debug_model(lto_code_gen_t)
Which kind of position independence is set with: which kind of position independence is set with:
.. code-block:: c .. code-block:: c
lto_codegen_set_pic_model(lto_code_gen_t) lto_codegen_set_pic_model(lto_code_gen_t)
And each symbol that is referenced by a native object file or otherwise must not And each symbol that is referenced by a native object file or otherwise must not
be optimized away is set with: be optimized away is set with: