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[llvm-mca] add example workflow for source code 

This is copied from Andrea's text in PR36875:
https://bugs.llvm.org/show_bug.cgi?id=36875

As noted there, this is a hack...but it's a good one!
It's important to show potential workflows up-front
with examples, so customers can copy and experiment
with them.

llvm-svn: 329726
This commit is contained in:
Sanjay Patel 2018-04-10 17:49:45 +00:00
parent 90a937e3e1
commit c86033a55a
1 changed files with 20 additions and 0 deletions
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llvm/docs/CommandGuide/llvm-mca.rst
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@ -36,6 +36,26 @@ substring ``LLVM-MCA-END`` marks the end of a code region. For example:
...
# LLVM-MCA-END
Inline assembly directives may also be used from source code to annotate the
assembly text:
.. code-block:: c++
int foo(int a, int b) {
__asm volatile("# LLVM-MCA-BEGIN foo");
a += 42;
__asm volatile("# LLVM-MCA-END");
a *= b;
return a;
}
So for example, you can compile code with clang, output assembly, and pipe it
directly into llvm-mca for analysis:
.. code-block:: bash
$ clang foo.cpp -O2 -target x86_64-unknown-unknown -S -o - | llvm-mca -mcpu=btver2
Multiple regions can be specified provided that they do not overlap. A code
region can have an optional description. If no user defined region is specified,
then :program:`llvm-mca` assumes a default region which contains every