Add a test case for the edge case that triggers this. Thanks to Chandler for bringing this to my attention.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142794 91177308-0d34-0410-b5e6-96231b3b80d8
introduce no-return or unreachable heuristics.
The return heuristics from the Ball and Larus paper don't work well in
practice as they pessimize early return paths. The only good hitrate
return heuristics are those for:
- NULL return
- Constant return
- negative integer return
Only the last of these three can possibly require significant code for
the returning block, and even the last is fairly rare and usually also
a constant. As a consequence, even for the cold return paths, there is
little code on that return path, and so little code density to be gained
by sinking it. The places where sinking these blocks is valuable (inner
loops) will already be weighted appropriately as the edge is a loop-exit
branch.
All of this aside, early returns are nearly as common as all three of
these return categories, and should actually be predicted as taken!
Rather than muddy the waters of the static predictions, just remain
silent on returns and let the CFG itself dictate any layout or other
issues.
However, the return heuristic was flagging one very important case:
unreachable. Unfortunately it still gave a 1/4 chance of the
branch-to-unreachable occuring. It also didn't do a rigorous job of
finding those blocks which post-dominate an unreachable block.
This patch builds a more powerful analysis that should flag all branches
to blocks known to then reach unreachable. It also has better worst-case
runtime complexity by not looping through successors for each block. The
previous code would perform an N^2 walk in the event of a single entry
block branching to N successors with a switch where each successor falls
through to the next and they finally fall through to a return.
Test case added for noreturn heuristics. Also doxygen comments improved
along the way.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142793 91177308-0d34-0410-b5e6-96231b3b80d8
two more subtle routines to the bottom and expand on their cautionary
comments a bit. No functionality or actual interface change here.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142789 91177308-0d34-0410-b5e6-96231b3b80d8
a single class. Previously it was split between two classes, one
internal and one external. The concern seemed to center around exposing
the weights used, but those can remain confined to the implementation
file.
Having a single class to maintain the state and analyses in use will
also simplify several of the enhancements I want to make to our static
heuristics.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142783 91177308-0d34-0410-b5e6-96231b3b80d8
to bring it under direct test instead of merely indirectly testing it in
the BlockFrequencyInfo pass.
The next step is to start adding tests for the various heuristics
employed, and to start fixing those heuristics once they're under test.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142778 91177308-0d34-0410-b5e6-96231b3b80d8
to get important constant branch probabilities and use them for finding
the best branch out of a set of possibilities.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142762 91177308-0d34-0410-b5e6-96231b3b80d8
block frequency analyses. This differs substantially from the existing
block-placement pass in LLVM:
1) It operates on the Machine-IR in the CodeGen layer. This exposes much
more (and more precise) information and opportunities. Also, the
results are more stable due to fewer transforms ocurring after the
pass runs.
2) It uses the generalized probability and frequency analyses. These can
model static heuristics, code annotation derived heuristics as well
as eventual profile loading. By basing the optimization on the
analysis interface it can work from any (or a combination) of these
inputs.
3) It uses a more aggressive algorithm, both building chains from tho
bottom up to maximize benefit, and using an SCC-based walk to layout
chains of blocks in a profitable ordering without O(N^2) iterations
which the old pass involves.
The pass is currently gated behind a flag, and not enabled by default
because it still needs to grow some important features. Most notably, it
needs to support loop aligning and careful layout of loop structures
much as done by hand currently in CodePlacementOpt. Once it supports
these, and has sufficient testing and quality tuning, it should replace
both of these passes.
Thanks to Nick Lewycky and Richard Smith for help authoring & debugging
this, and to Jakob, Andy, Eric, Jim, and probably a few others I'm
forgetting for reviewing and answering all my questions. Writing
a backend pass is *sooo* much better now than it used to be. =D
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142641 91177308-0d34-0410-b5e6-96231b3b80d8
AsmParser. This patch adds validation for target data layout strings upon
construction of TargetData objects. An attempt to construct a TargetData object
from a malformed string will trigger an assertion.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142605 91177308-0d34-0410-b5e6-96231b3b80d8
Add a Value named "NAME" to each Record. This will be set to the def or defm
name when instantiating multiclasses. This will replace the #NAME# processing
hack once paste functionality is in place.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142518 91177308-0d34-0410-b5e6-96231b3b80d8
Add a Record constructor that takes the Record name as an Init. This
is more work toward paste functionality.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142508 91177308-0d34-0410-b5e6-96231b3b80d8
Record names may not be fully resolved at this point so ask for the
record name as a string explicitly. This avoids a potential assert.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142502 91177308-0d34-0410-b5e6-96231b3b80d8
Allow template arg names to be Inits. This is further work to
implement paste as it allows template names to participate in paste
operations.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142500 91177308-0d34-0410-b5e6-96231b3b80d8
Add a couple of utility functions to take a variable name and qualify
it with the namespace of the enclosing class and/or multiclass. This
is inpreparation for making template arg names first-class Inits.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142498 91177308-0d34-0410-b5e6-96231b3b80d8
Make the VarInit name an Init itself. We need this to implement paste
functionality so we can reference variables whose names are not yet
completely resolved.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142497 91177308-0d34-0410-b5e6-96231b3b80d8
Add accessors to get Record values by Init name. This lets us look up
Record values whose names are not yet fully resolved. More work
toward paste.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142496 91177308-0d34-0410-b5e6-96231b3b80d8
Add a utility to get the name init and get the string representation
of the name. This will be used for paste functionality.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142495 91177308-0d34-0410-b5e6-96231b3b80d8
Add a couple of utility functions to get at the name init and return
the name init as a string. This will be used for paste functionality.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142494 91177308-0d34-0410-b5e6-96231b3b80d8
