Dead branch elimination needs to know about the constructs that a block is contained it when determining what to do with its merge instruction. We currently fold branches in block as we see them, which is parent constructs before their children. This causes the struct cfg analysis to crash because it tries to get the parent construct for a block after the parent has been folded.
This can be fixed by folding the branch of the children before the parents.
Fixes#2667.
* Handle nested breaks from switches.
There was a recent decision made to allow branches to the merge node of
a switch even if the switch is not the first enclosing construct. They
can be generated by glslang from break statements in switches.
Dead branch elimination seems to be the only optimization that will
break because of this change, so I will update that optimizations.
The change made are:
- Track switches in structured cfg analysis.
- In Dead branch elimination:
- Look for nested breaks that will require a switch instruction.
- Rewrite, but don't delete, switchs that are required even if it
could be replaced by an unconditional branch.
- When looking for the first break, consider the merge of a switch
as well.
See #2612.
* Fix variable names and comments.
* Add tests for the struct cfg analysis and switches.
* Fix typos in comments.
Fixes#2452
Swaps priority of handling unreachable merge and continues so that the
back-edge is retained in the case a block is both a loop continue and
loop merge
* Handle back edges better in dead branch elim.
Loop header must have exactly one back edge. Sometimes the branch
with the back edge can be folded. However, it should not be folded
if it removes the back edge.
The code to check this simply avoids folding the branch in the
continue block. That needs to be changed to not fold the back edge,
wherever it is.
At the same time, the branch can be folded if it folds to a branch to
the header, because the back edge will still exist.
Fixes#2391.
When looking for a break from a selection construct, we do not realize
that a jump to the continue target of a loop containing the selection
is a break. This causes and infinit loop, or possibly other failures.
Fixes#2004.
When looking for a break from a selection construct, we do not need to
look inside nested constructs. However, if a loop header has an
unconditional branch, then we enter the loop. Entering the loop causes
an infinite loop because we keep going through the loop.
The solution is to look for a merge block, if one exsits, even for block
terminated by an OpBranch.
Fixes#1979.
* Check rules from Execution Mode tables, 2.16.2 and the Vulkan
environment spec
* Allows MeshNV execution model with the following execution modes
* LocalSize, LocalSizeId, OutputPoints and OutputVertices
* Done to not break their validation
* Create structed cfg analysis.
There are lots of optimization that have to traverse the CFG in a
structured order just because it wants to know which constructs a
basic block in contained in. This adds extra complexity to these
optimizations, for causes too much refactoring of older optimizations.
To help with this problem, I have written an analysis that can give this
information.
* Identify branches breaking from loops.
Dead branch elimination does a search for a conditional branch to the
end of the current selection construct. This search assumes that the
only way to leave the construct is through the merge node. But that is
not true. The code can jump to the merge node of a loop that contains
the construct.
The search needs to take this into consideration.
When dead-branch-elim folds a conditional branch, it also deletes the
OpSelectionMerge instruction. If that construct contains a
conditional branch to the merge node, it will not have its own
OpSelectionMerge. When the headers merge instruction is deleted, the
the inner conditional branch will no longer be legal. It will be a
selection to a node that is not a merge node.
We fix this up by moving the OpSelectionMerge to a new location if it is
still needed.
Fixes#1727
* If the pass finds any dead branches it can optimize then at the end of
the pass it reorders basic blocks to ensure they satisfy block ordering
requirements
* Added some new tests
* While investigating this issue, found and fixed a non-deterministic
ordering of dominators
* Now the edges used to construct the dominator tree are sorted
according to posorder traversal indices
CPPreference.com has this description of digits10:
“The value of std::numeric_limits<T>::digits10 is the number of
base-10 digits that can be represented by the type T without change,
that is, any number with this many significant decimal digits can be
converted to a value of type T and back to decimal form, without
change due to rounding or overflow.”
This means that any number with this many digits can be represented
accurately in the corresponding type. A change in any digit in a
number after that may or may not cause it a different bitwise
representation. Therefore this isn’t necessarily enough precision to
accurately represent the value in text. Instead we need max_digits10
which has the following description:
“The value of std::numeric_limits<T>::max_digits10 is the number of
base-10 digits that are necessary to uniquely represent all distinct
values of the type T, such as necessary for
serialization/deserialization to text.”
The patch includes a test case in hex_float_test which tries to do a
round-robin conversion of a number that requires more than 6 decimal
places to be accurately represented. This would fail without the
patch.
Sadly this also breaks a bunch of other tests. Some of the tests in
hex_float_test use ldexp and then compare it with a value which is not
the same as the one returned by ldexp but instead is the value rounded
to 6 decimals. Others use values that are not evenly representable as
a binary floating fraction but then happened to generate the same
value when rounded to 6 decimals. Where the actual value didn’t seem
to matter these have been changed with different values that can be
represented as a binary fraction.
In dead branch elimination, we already recognize unreachable continue
blocks, and update OpPhi instruction accordingly. This change adds an
extra check: if the head block has exactly 1 other incoming edge, then
replace the OpPhi with the value from that edge.
Fixes#1314.
* When handling unreachable merges and continues, do not optimize to the
same IR
* pass did not check whether the unreachable blocks were in the
optimized form before transforming them
* added a test to catch this issue
* If the dead branch elim is performed on a module without structured
control flow, the OpSelectionMerge may not be present
* Add a check for pointer validity before dereferencing
* Added a test to catch the bug
Pass now paints live blocks and fixes constant branches and switches as
it goes. No longer requires structured control flow. It also removes
unreachable blocks as a side effect. It fixes the IR (phis) before doing
any code removal (other than terminator changes).
Added several unit tests for updated/new functionality.
Does not remove dead edge from a phi node:
* Checks that incoming edges are live in order to retain them
* Added BasicBlock::IsSuccessor
* added test
Fixing phi updates in the presence of extra backedge blocks
* Added tests to catch bug
Reworked how phis are updated
* Instead of creating a new Phi and RAUW'ing the old phi with it, I now
replace the phi operands, but maintain the def/use manager correctly.
For unreachable merge:
* When considering unreachable continue blocks the code now properly
checks whether the incoming edge will continue to be live.
Major refactoring for review
* Broke into 4 major functions
* marking live blocks
* marking structured targets
* fixing phis
* deleting blocks
Re-formatted the source tree with the command:
$ /usr/bin/clang-format -style=file -i \
$(find include source tools test utils -name '*.cpp' -or -name '*.h')
This required a fix to source/val/decoration.h. It was not including
spirv.h, which broke builds when the #include headers were re-ordered by
clang-format.
Expands dead branch elimination to eliminate dead switch cases. It also
changes dbe to eliminate orphaned merge blocks and recursively eliminate
any blocks thereby orphaned.
- DeadBranchElim: Make sure to mark orphan'd merge blocks and continue
targets as live.
- Add test with loop in dead branch
- Add test that orphan'd merge block is handled.
Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/776
