Following up on prior RFC
(https://lists.llvm.org/pipermail/llvm-dev/2020-September/145357.html)
we can now improve above our highly-optimized basic-block-sections
binary (e.g., 2% for clang) by applying path cloning. Cloning can
improve performance by reducing taken branches.
This patch prepares the profile format for applying cloning actions.
The basic block cloning profile format extends the basic block sections
profile in two ways.
1. Specifies the cloning paths with a 'p' specifier. For example, `p 1 4
5` specifies that blocks with BB ids 4 and 5 must be cloned along the
edge 1 --> 4.
2. For each cloned block, it will appear in the cluster info as
`<bb_id>.<clone_id>` where `clone_id` is the id associated with this
clone.
For example, the following profile specifies one cloned block (2) and
determines its cluster position as well.
```
f foo
p 1 2
c 0 1 2.1 3 2 5
```
This patch keeps backward-compatibility (retains the behavior for old
profile formats). This feature is only introduced for profile version >=
1.
This patch introduces a new version for the basic block sections profile
as was requested in D158442, while keeping backward compatibility for
the old version.
The new encoding is as follows:
```
m <module_name>
f <function_name_1> <function_name_2>...
c <bb_id_1> <bb_id_2> <bb_id_3>
c <bb_id_4> <bb_id_5>
...
```
Module name specifier (starting with 'm') is optional and allows
distinguishing profiles for internal-linkage functions with the same
name. If not specified, profile will be applied to any function with the
same name.
Function name specifier (starting with 'f') can specify multiple
function name aliases.
Finally, basic block clusters are specified by 'c' and specify the
cluster of basic blocks, and the internal order in which they must be
placed in the same section.
Basic block sections profiles are ingested based on the function name. However, conflicts may occur when internal linkage functions with the same symbol name are linked into the binary (for instance static functions defined in different modules). Currently, these functions cannot be optimized unless we use `-funique-internal-linkage-names` (D89617) to enforce unique symbol names.
However, we have found that `-funique-internal-linkage-names` does not play well with inline assembly code which refers to the symbol via its symbol name. For example, the Linux kernel does not build with this option.
This patch implements a new feature which allows differentiating profiles based on the debug info filenames associated with each function. When specified, the given path is compared against the debug info filename of the matching function and profile is ingested only when the debug info filenames match. Backward-compatibility is guaranteed as omitting the specifiers from the profile would allow them to be matched by function name only. Also specifiers can be included for a subset of functions only.
Reviewed By: shenhan
Differential Revision: https://reviews.llvm.org/D146770
D68049 created options for basic block sections: -fbasic-block-sections=,
-funique-basic-block-section-names. Rename options in llc and lld (--lto-)
to be consistent. Specifically,
+ Rename basicblock-sections to basic-block-sections
+ Rename unique-bb-section-names to unique-basic-block-section-names
Differential Revision: https://reviews.llvm.org/D84462
