Control Structure Analysis

Hi,

I have an experimental patch that implements an algorithm to detect control structures in the CFG, such as If-Then, If-Then-Else, Self-Loop etc. This analysis is kind of an extension of interval/region analysis to detect more than just loops. The algorithm and data structures used in this implementation are based on the description given in “Advanced Compiler Design and Implementation” – Steven. S. Muchnik (Section 7.7 “Structural Analysis”). The result of the analysis is a tree called the control-structure-tree of the control flow graph of the function begin analysis.

Typical uses for this analysis are:

  1. Perform control flow transformations based on the control structure tree. Some control transformations are easier to reason about using this tree.
  2. IR to source transformations. Detecting source level constructs in the CFG is usually useful in generating high level code from an already reduced IR.
  3. Similar to the uses of interval/region analysis, control structure analysis also can be used to speed up data flow analyses.

The code, although tested and used internally, needs further work:

  1. Make it an actual LLVM analysis.
  2. Provide a richer set of APIs for updates.
  3. Improve analysis time.

I’m not sure how useful such an analysis would be to the community. If people find it to be useful, I’m open to working on it further and submitting it. Any feedback will be appreciated.

http://reviews.llvm.org/D17031

Hi,

I have an experimental patch that implements an algorithm to detect
control structures in the CFG, such as If-Then, If-Then-Else, Self-Loop
etc. This analysis is kind of an extension of interval/region analysis
to detect more than just loops. The algorithm and data structures used
in this implementation are based on the description given in "Advanced
Compiler Design and Implementation" -- Steven. S. Muchnik (Section 7.7
"Structural Analysis"). The result of the analysis is a tree called the
control-structure-tree of the control flow graph of the function begin
analysis.

Typical uses for this analysis are:
1. Perform control flow transformations based on the control structure
tree. Some control transformations are easier to reason about using this
tree.
2. IR to source transformations. Detecting source level constructs in
the CFG is usually useful in generating high level code from an already
reduced IR.
3. Similar to the uses of interval/region analysis, control structure
analysis also can be used to speed up data flow analyses.

The code, although tested and used internally, needs further work:
1. Make it an actual LLVM analysis.
2. Provide a richer set of APIs for updates.
3. Improve analysis time.

I'm not sure how useful such an analysis would be to the community. If
people find it to be useful, I'm open to working on it further and
submitting it. Any feedback will be appreciated.

Do you know the StructurizeCFGPass? We use it in AMDGPU to simplify the control flow which allows a simpler lowering pass. GPUs run "waves" of "threads", where the physical instruction pointer is shared by all threads of a wave, even though threads can take different paths through control structures. An exec mask is used to disable threads that should currently be inactive (e.g. while running through the "wrong" half of an if-else).

I haven't thought about it at all, but it's conceivable that a thorough analysis could help produce better code in some complicated cases.

Cheers,
Nicolai

You might also have a look at RegionInfo, which provides similar information.

Best,
Tobias